Micro-Tomographic Atlas of the Mouse Skeleton

Micro-Tomographic Atlas of the Mouse Skeleton

Itai Bab, Carmit Hajbi-Yonissi, Yankel Gabet, Ralph Müller

Micro-Tomographic Atlas of the Mouse Skeleton With 196 Figures

Itai Bab Professor and Chief, Bone Laboratory The Hebrew University of Jerusalem P.O. Box 12272 Jerusalem 91120 Israel

Yankel Gabet Bone Laboratory The Hebrew University of Jerusalem P.O.Box 12272 Jerusalem 91120 Israel

Carmit Hajbi-Yonissi Bone Laboratory The Hebrew University of Jerusalem P.O. Box 12272 Jerusalem 91120 Israel

Ralph Müller Professor of Biomechanics ETH Zurich Wolfgang-Pauli-Strasse 10 8093 Zurich Switzerland

Library of Congress Control Number: ISBN-10: 0-387-39254-8 ISBN-13: 978-0-387-39254-7

e-ISBN-10: 0-387-39258-0 e-ISBN-13: 978-0-387-39258-5

Printed on acid-free paper. © 2007 Springer Science+Business Media, LLC All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. 9 8 7 6 5 4 3 2 1 springer.com

KYO

Preface At the present time, the laboratory mouse has become a central tool for skeletal studies, mainly because of the extensive use of genetic manipulations in this species. Naturally, this widespread use of mice in developmental, bone, joint, tooth, and neurological research calls for detailed anatomical knowledge of the mouse skeleton as a reference for experimental design and phenotyping under a variety of experimental conditions, including genetic manipulations (e.g., transgenic and knockout mice). Several general treatises on the normal anatomy of the mouse and rat have been published in the previous century. In the absence of adequate technologies, these books describe only the external anatomical features of the different parts of the skeleton. In general, images in these atlases are camera lucida-based line drawings rather than accurate three-dimensional images. Furthermore, so far a systematic two- and three-dimensional description of the internal anatomy of bones, as well as the three-dimensional relationship exhibited in joints, are not available. Recently, microcomputed tomography (µCT) has emerged as a central tool for the descriptive and quantitative analysis of skeletal anatomy. A publication analysis using PubMed indicates an increase in the number of studies reporting µCT skeletal anatomical surveys from less than ten before 2000 to several hundreds thereafter. Employing up-to-date µCT systems, two- and three-dimensional images of the external and internal skeletal anatomy are attainable at resolutions as high as 6 micrometers. Moreover, morphometric software has been developed for the accurate quantification of anatomical structures such as the cortical and trabecular compartments of bone. Unlike histomorphometry, which in the vast majority of cases is based on a rather small number of sections, µCT morphometry employs a detailed three-dimensional anatomical reconstruction of the whole structural component being investigated. Another major advantage of µCT is its nondestructive nature, namely, skeletal specimens subjected to µCT scanning and morphometric analysis can be further analyzed by complementary histological and biomechanical methods. In view of the vast employment of genetically manipulated mice and the emergence of µCT as a central tool in bone research, it appears appropriate for the timely publication of a comprehensive reference for skeletal microanatomy. The present treatise is based on skeletal specimens obtained from sexually mature (10-week-old) male C57-Bl/6J mice. This mouse strain was chosen because of its prevalent use for genetic manipulations. The animals were sacrificed by anesthetic overdose and the skeleton surgically stripped of soft tissues, fixed in phosphate-buffered formalin, and then kept in 70% ethanol to avoid decalcification. Scanning in the µCT apparatus (µCT 40, Scanco Medical AG, Bassersdorf, Switzerland) was performed with specimens immersed in 70% ethanol. The first two parts of the present treatise describe the axial and appendicular skeleton, respectively, with distinct chapters each devoted to a specific bone or group of bones (depending on structural complexity) and their corresponding joints. In general, images of the bones are tri-dimensional, with two-dimensional images or relatively thin three-dimensional slices used to highlight particular microanatomical details such as foramina, canals, and tuberosities. In some cases, these microanatomical details have not been reported previously; consequently, their presence has been confirmed using multiple specimens. Because of the great variations in skeletal anatomy between mouse strains, genders and ages, the third part comprises a comparative morphometric analysis of the femur and a representative lumbar vertebra (L3), the most frequently used experimental skeletal sites. The quantitative data provide reference comparative values separately for the cortical and trabecular compartments as well as average values for these osseous components at the whole femur level. The emergence of µCT technology has made it possible to generate new, detailed anatomical insights that are more complete than any of the so-far-available anatomical references for lower mammals. The descriptive and quantitative data provided should have value not only for the experimental skeletal biologist but also for a general interpretation of the evolutionary development of the mammalian series.

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PREFACE

A couple of new attractive µCT applications have been introduced after completing the manuscript for this book. One is determination of the bone material (mineral) density that for the first time allows accurate spatial assessment of changes in bone calcification. The second is an in-vivo µCT scanner that enables the rapid repeated acquisition of high-resolution images of whole anesthetized animals, thus facilitating longitudinal studies in the same animals as well as a simple approach to the study of blood vessels using conventional contrasting materials.

Contents

Part A Axial Skeleton Section I: Skull 1.

2.

Nose, Palate and Upper Jaw, Cranium and Tympanic Bulla .................................................................................... Nose and Palate.................................................................................................................................................... Cranium and Tympanic Bulla .............................................................................................................................. Hyoid, Mandible, and Temporo-Mandibular Joint ...................................................................................................

5 5 13 27

Section II: Vertebral Column 1. 2. 3. 4. 5.

Cervical Vertebrae ..................................................................................................................................................... Thoracic Vertebrae..................................................................................................................................................... Lumbar Vertebrae ...................................................................................................................................................... Sacrum....................................................................................................................................................................... Caudal Vertebrae........................................................................................................................................................

41 67 73 79 87

Section III: Thorax 1.

Sternum, Sternal-Rib Joint, Ribs and Rib-Vertebral Joints.......................................................................................

95

Part B Appendicular Skeleton Section I: Rostral Appendage 1. 2. 3. 4. 5.

Clavicle...................................................................................................................................................................... Scapula ...................................................................................................................................................................... Humerus and Shoulder Joint ..................................................................................................................................... Forearm (Ulna, Radius, and Elbow Joint)................................................................................................................. Manus ........................................................................................................................................................................

113 117 123 131 145

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CONTENTS

Section II: Caudal Appendage 1. 2. 3. 4.

Pelvic Girdle.............................................................................................................................................................. 153 Femur and Hip Joint.................................................................................................................................................. 161 Tibio-Fibular Complex and Knee Joint..................................................................................................................... 171 Hindfoot..................................................................................................................................................................... 183

Part C Murine Comparative Microanatomy 1. 2.

Strain Differences...................................................................................................................................................... Gender and Age Differences .....................................................................................................................................

191 195

Index .................................................................................................................................................................................

201

PA RT A

Axial Skeleton

SECTION I

Skull

1 Nose, Palate and Upper Jaw, Cranium and Tympanic Bulla

The skull was scanned at 10-µm voxel resolution. It extends ~21.0 mm from the tip of the nasal bone rostrally to the posterior-most aspect of the external occipital crest (Figs. 1 and 2). Its maximal latero-lateral dimension, ~10.3 mm, is between the lateral aspects of the squamosal bones (Figs. 3 and 4). The skull is a single solid unit made of individual bones fused together along sutures, thus forming a puzzle-like structure. Parts of the individual bones join to form the functional structures of the skull, e.g., nose, cranium, upper jaw, orbit, zygomatic arch, and base of the skull. To describe the skull, we have divided it into a rostral part, which includes the nose and upper jaw, and a caudal part, which comprises the cranium and tympanic bulla. NOSE AND PALATE

The rostral part of the skull consists of five bones: the premaxillae, maxillae, palatine bones, nasal bones, and ethmoid (Figs. 1–12). Other than the ethmoid, these bones are symmetrically paired on either side of the midline. Rostrally, the premaxillae define the lateral and ventral portions of the nasal cavity, the rostral part of the hard palate, and the rostral part of the upper jaw that accommodates the upper incisor teeth (Figs. 1, 3–5, and 8). Ventrally, the premaxillae accommodate the rostral third of the rostral (anterior) palatine foramen. At the midline, the premaxillary rostro-caudal dimension is ~5 mm (Fig. 4). The rostro-caudal dimension of most of its ventral and lateral aspects is ~2.9 mm. At its dorsal part, the lateral aspect has a caudal tail measuring ~0.3 mm latero-medially and ~3.3 mm rostro-caudally. The latero-medial dimension of the premaxilla varies both dorso-ventrally and rostro-caudally. In general, it is maximal at the level of the incisal socket, where it measures ~1.0 mm (Figs. 5, 10, and 11). Rostral of the maxilla, the premaxilla presents a rather bulky appearance. Its lateral wall, at the level of the incisal root, is ~130 µm thick. It is thicker dorsal of the socket (~0.35 mm) and at the hard palate, where it measures ~0.8 mm in either the latero-medial and dorso-ventral dimensions (Fig. 5). The medial wall of the incisal 5

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PART A, SECTION I, CHAPTER 1

Figure 1. Lateral view of mouse skull. Top: whole skull. Middle: internal view from plane through left maxillary zygomatic process and tympanic bulla. Bottom: internal view from plane immediately right of midline.

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Figure 2. Sagittal slices, 200 μm thick, of mouse skull. Top: mid-sagittal plane. Bottom: plane through upper molar teeth and tympanic bulla.

socket and the floor of the nasal cavity are 50–110 µm thick bony plates (Fig. 5). On either side of the midline, the premaxillae give rise to the bony leaves that comprise the vomer (the ventral bony component of the nasal septum). The leaves are ~0.45 µm in diameter. They extend 0.4–1.4 mm from the floor of the nose dorsally (Fig. 5). Their rostro-caudal dimension is ~4.2 mm. In the midline, ~1.0 mm caudal of the incisor teeth, the midline suture opens to form the incisive foramen, a ~0.4 mm in diameter opening of the incisive canal to the oral cavity (Figs. 4 and 5). On either side of the vomer leaves, the nasal floor also gives rise to the ventral (inferior) conchae. The conchae are bony leaves with a lateral convexity that togeth-

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PART A, SECTION I, CHAPTER 1

Figure 3. Dorsal view of mouse skull.

er with the vomer form a partially closed spindle, ellipsoid-shaped pneumatic space (Figs. 1, 4, 5, and 10). The lateral wall of the incisal socket gives rise to another leaf, the middle concha (Figs. 1 and 5). From the frontal plane passing through the rostral end of the caudal tail and further caudally, the incisal socket "hangs" from the tail thus occupying a dorso-lateral position away from the maxilla. It is connected to the tail by a lateral and medial bony leaves that define a pneumatized space measuring ~2.7 and 0.9 mm in the dorso-ventral and latero-medial dimensions (Fig. 5). The lateral part of the maxilla extends ~4.6 mm from the dorsal part of the premaxilla to the ventral end of the alveolar process. The dimension of the entire lateral part of the maxilla, from the premaxilla rostrally to the basisphenoid caudally is ~7.8 mm (Fig. 1). It includes (i) the lateral wall of the nasal cavity caudal and ventral of the premaxilla and rostro-ventral of the frontal bone, (ii) the maxillary zygomatic process, and (iii) the alveolar process that accommodates the molar teeth. The maxillary lateral nasal wall is bordered rostrally and dorsally by the premaxilla and caudo-dorsally by the lateral part of the frontal bone. Rostral of the molar teeth the wall is a ~25 µm thick bony plate located 85–250 µm lateral of the incisal socket. The gap between the socket and wall forms the infraorbital foramen and canal (Fig. 5). Caudal of the socket and dorsal of the molar teeth, the wall delineates the lateral border of the maxillary sinus. The sphenopalatine foramen opens into the sinus through the maxillary nasal wall. It measures ~0.45 and ~1.4 mm in the dorso-ventral and rostro-caudal dimensions, respectively (Figs. 1 and 5). Caudal to the center of the third molar tooth, the lateral nasal wall fuses with the presphenoidal wing (see below this chapter) to form the lateral wall of the nasopharynx (Fig. 6) and the ventral outer aspect of the orbit (Fig. 1). Here the wall is 50–80 µm thick (Fig. 6).

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Figure 4. Basal aspect of mouse skull.

The base of the zygomatic process is ~3.2 and ~0.66 mm in the dorso-ventral and rostro-caudal dimensions, respectively (Fig. 1). Its dorso-caudal part forms a suture with the lacrimal bone (Figs. 1 and 3). It extends ~1.15 mm rostro-laterally to form the infraorbital fissure and then folds caudo-latero-ventrally to become the rostral part of the zygomatic arch (Figs. 1, 4, and 8). This part of the zygomatic process is ~5.9 mm long. Its medio-lateral dimension is ~0.45 mm (Fig. 4). The dorso-ventral dimension in the rostral half of this part is ~1 mm. In its caudal half, which overlaps the zygomatic bone, this dimension is ~0.4 mm (Fig. 1). The alveolar process is a caudal continuation of the base of the zygomatic process. It bulges laterally and ventrally from the maxillary surface ~0.15 mm in either direction (Figs. 1 and 5). Dorsal to the turn, where the zygomatic process becomes the alveolar process, is the ~1.4 mm diameter spheno-palatine foramen, which leads into the maxillary sinus (Figs. 1 and 5). The alveolar process is bordered dorsally by the lateral part of the frontal bone and caudally by the basisphenoid. It consists of lateral and medial ridges connected dorsal of the apex of the molar tooth roots by a ~20 µm thick bony plate (Figs. 1, 2, 5, and 6). The lateral ridge extends ~0.8 mm lateral and ~0.6 mm dorsal of the lateral nasal wall. A ~0.35 mm wide crevice separates the ridge from the wall (Figs. 1, 5, and 6). The medial ridge is triangular in shape, with the vertex pointing ventro-laterally. Rostrally the triangle is made of a bulk of compact bone. Its height is ~0.22 mm. Further caudally it increases in size to ~0.5 mm and consists of trabecular bone surrounded by a 60–95 µm compact cortical plate (Figs. 5 and

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PART A, SECTION I, CHAPTER 1

Figure 5. Frontal, 10 µm slices through mouse skull (0–6.6 mm from rostral reference frontal plan). Numbers in slice panels indicate distances in mm from reference plane through anterior aspect of first incisor. Upper panel shows slice position (yellow lines); black line indicates position of reference plane.

6). The roots of the molar teeth are between the lateral and medial ridges, surrounded by a 28–100 µm wide periodontal ligament space (Figs. 5, 6, and 10). At the hard palate, the premaxillo-maxillary suture presents a bulky structure in which components of the two bones are interlaced (Figs. 2, 5, and 10). In a frontal plane, the suture is triangular with its base and vertex forming part of the nasal floor and lateral edge of the rostral (anterior) palatine foramen, respectively. The triangle base is ~1 mm long and the height is ~0.88 mm (Figs. 2 and 5). The rostro-caudal dimension of the suture is ~1.67 mm (Fig. 10). The remainder of the palate is formed by the maxillae and palatine bone. Caudally, the palate is bordered by the basisphenoid bones. The angle between the right and left palate-basisphenoid sutures is ~100° with the vertex pointing rostrally (Fig. 4). The maxillo-basisphenoid suture is also a bulky, interlaced structure measuring ~0.42 mm latero-medially, ~1.22 mm rostro-caudally and ~1 mm dorso-ventrally (Figs. 2, 6, and 10). The palatine bones are between the maxillae rostrally and laterally. Each palatine bone measures ~0.56 mm latero-medially and ~0.73 mm rostro-caudally (Fig. 4). At the midline, the maxillary rostro-caudal dimension between the premaxillae and palatine bones is ~5 mm. The rostro-caudal dimension of the palatine bones is ~0.75 mm. Lateral of the palatine bones, the maxillary rostro-caudal dimension between the premaxilla and basisphenoid is ~5.85 mm (Fig. 4). The maxillary palate is 0.15–0.21 mm thick. The thickness of the palatine bone is ~0.26 mm (Fig. 2). The rostro-caudal and latero-medial dimensions of the rostral (anterior) palatine foramen are ~5 and ~0.86 mm, respectively (Fig. 4). The caudal (posterior) palatine foramen is an expansion of the maxillo-palatine suture. It is ~0.7 mm rostro-caudally and

NOSE, PALATE AND UPPER JAW, CRANIUM AND TYMPANIC BULLA

11

Figure 5 (cont'd)

~0.85 mm latero-medially (Figs. 4 and 5). The maxillary sinus is a large pneumatized space dorsal of the alveolar ridge and second and third molar teeth (Figs. 2 and 5). The nasal bones comprise the roof of the nasal cavity. They are ~7 mm in the rostro-caudal dimension and ~1.35 mm from the midline to their lateral border (Figs. 1, 3, 5, 8, 9, and 12). In their rostral 1 mm the nasal bones have rostral and lateral free borders (not connected to other bones) (Fig. 1). Further caudally, their lateral end forms a suture with the premaxillary bone (Figs. 1, 3, 5, and 8). Their caudal end is fused with the frontal bones and spine (Figs. 2, 3, 6, and 8). In their rostral part, the nasal bones consist of a single, ~100 µm thick, bony plate. At their medial end, on either side of the midline, the plates become wider (~0.3 mm) and split to form diploë and a midline crest (Figs. 2 and 5). Moving caudally, the crest increases in its dorso-ventral dimension (maximum of ~0.63 mm) and forms the dorsal part of the nasal septum (Figs. 2, 5, 9, and 12). At the naso-frontal-ethmoidal junction the crest articulates, from above downward, with the spine of the frontal bone and the perpendicular plate of the ethmoid (Figs. 2, 5, and 6). Near the junction, the diploë widen (40–200 µm), with thinner (~80 µm) dorsal and thicker (~150 µm) ventral compact plates. The plates are connected by ~40 µm thick medial and lateral plates and a few trabeculae of similar thickness. Laterally, approximately two thirds to three quarters the latero-medial dimension away from the midline, there is another thickening and diploë formation that give rise to the dorsal (superior) nasal concha (Figs. 1, 5, 8, and 9).

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PART A, SECTION I, CHAPTER 1

Figure 6. Frontal, 10 µm slices through mouse skull (7.4–12.8 mm from rostral reference frontal plan). Numbers in slice panels indicate distances in mm from reference plane through anterior aspect of first incisor. Upper panel shows slice position (yellow lines); black line indicates position of reference plane.

The ethmoid is a single bone that forms the rostro-ventral part of the base of the cranium and caudo-dorsal part of the roof of the nose (Figs. 1, 2, 5–9, 11, and 13). It consists of four parts: a cribriform plate situated in a diagonal, rostro-dorsal to caudo-ventral plane, which separates the cranium from the nasal cavity (Figs.1, 2, 5, 7–9, and 13); two lateral masses or labyrinths, located rostro-ventral of the cribriform plate that occupy the caudal portion of the nasal cavity; and a perpendicular plate that comprises the caudal part of the nasal septum (Figs. 1, 2, 5–9, and 11). The cribriform plate is a thin (~30 µm) riddled bony lamina, with pores measuring 50–170 µm in diameter. At its lateral ends it is fused to the inner lateral aspects of the frontal bones (Figs. 5, 7–9). The pores are substantially more crowded in its caudal two thirds (Figs. 8 and 9). The ethmoidal labyrinths consist of multiple pneumatized spaces separated by 10–25 µm thick bony leaves. The lateral walls of the labyrinths consist of the lateral aspects of the maxilla ventrally, and frontal bones dorsally (Figs. 1, 2, 5–9, and 11). The perpendicular plate arises at the midline from the cribriform plate. Rostral of the naso-fronto-ethmoidal junction, the plate is partially fused to the frontal spine and midline crest of the nasal bone (Figs. 2, 5, 6, 8, and 9). The perpendicular plate consists mainly of a single, ~40 µm thick leaf that is occasionally split into two leaves. Its dorso-ventral and rostro-caudal dimensions are ~2 and 1.4 mm, respectively (Figs. 2, 5, 6, 8, and 9).

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Figure 6 (cont'd)

CRANIUM AND TYMPANIC BULLA

Rostral of the interparietal bone, the calvarial bones comprise symmetrical pairs separated by the sagittal suture. The frontal bones lie between the nasal and premaxillary bones rostrally, the parietal bones and basisphenoids caudally and the maxillae, ventrally (Figs. 1–3, 5–7, and 12). The coronal suture separates the frontal and parietal bones (Figs. 3 and 12). Each of the frontal bones has two plate-like structures, horizontal and vertical. The largest rostro-caudal dimension of the horizontal plates, ~7 mm, is at the sagittal suture (Fig. 3). Their broadest latero-medial dimension is ~3.15 mm, between the sagittal suture and the convergence of the coronal, temporal and parietal sutures (Figs. 3 and 12). The vertical plate of the frontal bone includes the rostro-dorsal aspect of the orbit (Fig. 1). The medial ends of the vertical plate are folded inwardly (medially), where they are separated near the midline by crista galli of the ethmoid (Figs. 1, 6, and 8). Its maximal dimensions, rostro-caudally at the border with the horizontal plate and dorso-ventrally rostral of the optic foramen, are both ~4.20 mm (Fig. 1). Along their rostral third, the two frontal bones are separated by the frontal spine, a distinct osseous component measuring ~3 mm in the rostro-caudal dimension and ~375 µm latero-laterally. The rostral ~370 µm of the spine are locat-

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PART A, SECTION I, CHAPTER 1

Figure 7. Internal, rostral view of mouse skull from frontal plane through second molar teeth.

ed between the nasal bones (Figs. 2, 3, and 5). Its maximal dorso-ventral dimension, immediately caudal to the naso-fronto-ethmoidal junction, is ~740 µm. Most of the spine contains diploë (Fig. 5). The horizontal plates are composed of two cortical, compact bone layers separated by diploë. The cortical layers are 25–74 µm thick. They occasionally fuse to form a single, ~77 µm thick layer. The vertical plates are made of a single, 50–154 µm thick layer (Figs. 5 and 6). Caudal of the frontal bones are the parietal bones (Figs. 1–3, 12, and 14). Most of the parietal bones is made of the horizontal plate (Fig. 3). In addition, they have a small vertical plate in their caudal half (Figs. 1 and 14). Their maximal dimension, ~79 mm, is between the convergence of the coronal, temporal, and parietal sutures rostrally and the anterior lambdoid suture, posteriorly (Fig. 3). Latero-medially, the maximal dimension of the horizontal plate is ~47 mm (Fig. 3). The vertical plate measures ~1.5 and 3.2 mm ventro-dorsally and rostro-caudally, respectively (Fig. 1). Caudally, the parietal and interparietal bones are separated by the anterior lambdoid suture (Figs. 1, 3, and 12). Laterally and ventrally the parietal suture connects the parietal and squamosal bones (Figs. 1, 3, and 14). Like the frontal bones, the parietal bones are made of two cortical layers, 30–85 µm thick, which occasionally fuse to form a single 89–165 µm thick layer (Fig. 6). The interparietal bone is a single unit between the parietal bones rostrally and occipital bone caudally and ventrally from which it is separated by the posterior lambdoid suture (Figs. 1, 3, and 12). Latero-laterally the interparietal bone measures ~7.8 mm. The midsagittal dimension is ~3.2 mm (Fig. 3). The cortical layers are 38–100 µm thick; the maximal width of the diploë is ~150 µm (Fig. 2). The occipital bone forms the caudal wall of the calvaria and caudal portion of the base of the skull. It houses the foramen magnum and articulates with the atlas, the first cervical vertebra (Figs. 1–4, 8, 12–14). Laterally, the caudal aspect of the occipital bone borders the horizontal plate of the parietal bone. Further ventrally it borders the squamosal bone. Its periotic capsule then fuses with the tympanic bulla (Figs. 1, 13, and 14). Most of the caudal aspect is cap-shaped with the lateral part of the occipital bones and periotic capsules flanking at its lateral and ventro-lateral ends (Figs. 1, 13, and 14). The latero-lateral perimeter of the cap-shaped portion is ~75 mm long (Fig. 8). The occipital crest is a triangular eminence in the middle of the cap-shaped structure. The triangle vertex is at the posterior lambdoid suture. The base, ~1.15 mm ventral to the vertex, is ~1 mm long (Figs. 1, 3, 12, and 13). The periotic capsules are lateral of the condyles. Ventrally they have the paroccipital process (Figs. 1 and 13). Between the condyle and bulla, the capsule measures ~1 mm. The dorso-ventral dimension is ~2.1 mm with the paroccipital process further extending ~0.5 mm ventrally (Fig. 13). The caudo-ventral part of the occipital bone is occupied by the foramen magnum, a ~1.9 mm opening connecting the cranium with the vertebral neural canal (Figs. 4, 8, and 13). The foramen magnum is surrounded by a ~0.9 mm wide belt-like thickening (Fig. 13). Laterally, on each side of the foramen magnum, at the transition from the caudal to the ventral aspect, the belt-like thickening

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Figure 8. Internal, dorsal view of floor of mouse cranial cavity and of nasal cavity from horizontal planes dorsal of superior semicircular duct (left) and ventral border of the cribriform plate of ethmoid.

protrudes ~0.4 mm to form the occipital condyle, a ~1.71 mm (caudo-ventrally) by ~1.15 mm (medio-laterally) eminence that articulates with the atlas (Figs. 1, 4, and 13). Rostro-lateral to the condyles are the funnel -like, ~0.33 mm in diameter, external openings of the hypoglossal canals (Figs. 4 and 8). The basilar part of the occipital bone is a trapezoid, having its long ~4 mm base between the posterior lacerated foramina and immediately rostral of the external openings of the hypoglossal canals. The short ~1.75 mm rostral trapezoid base is along the suture connecting the occipital bone with the basisphenoid (Figs. 4 and 8). The rostro-caudal dimension of the basilary part, from the rostral edge of the foramen magnum to the basisphenoid, is ~3.25 mm (Fig. 4). Along the midsagittal line is the pharyngeal tubercle, a ~0.18 mm long midline ridge that runs from the short base caudally (Fig. 4) and protrudes ~0.35 mm from the external occipital surface (Fig. 8). On either side, the basilar part of the occipital bone forms the medial edge of the carotid canal rostrally and the posterior lacerated foramen posteriorly. Between these foramina the occipital bone is fused with the

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PART A, SECTION I, CHAPTER 1

Figure 9. Internal, dorsal view of nasal cavity from horizontal planes through ventral border of the cribriform plate of ethmoid (left), middle of cribriform plate of ethmoid (center), and dorsal border of the cribriform plate of ethmoid (right).

Figure 10. Two-dimensional horizontal slice through roots of upper incisor and molar teeth.

NOSE, PALATE AND UPPER JAW, CRANIUM AND TYMPANIC BULLA

Figure 11. Two-dimensional horizontal slice through dental papilla of upper incisor teeth.

Figure 12. Internal, ventral view of mouse cranium.

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PART A, SECTION I, CHAPTER 1

Figure 13. Caudal view of mouse skull.

tympanic bulla. (Figs. 4, 8, and 14). Like the rest of the calvarial bones, the occipital cap is made of two thin layers that fuse occasionally into a single ~50 µm thick leaf. The layers become thicker and further apart at the occipital condyles where they are ~200 µm thick and separated by a trabecular network (Fig. 2). The remainder of the lateral aspect of the calvaria, in addition to the frontal, parietal, and occipital bones, includes the lateral wing of the basisphenoid, the zygomatic and squamosal bones, and the lateral aspect of the tympanic bullae (Fig. 1). The squamosal bone occupies most of the caudal half of lateral aspect of the skull. It consists of a main rostral wing that is bordered dorsally by the parietal bone, rostrally by the lateral part of the frontal bone and ventrally by the basisphenoid. Caudally, the ventral part of the rostral wing forms the rostral edge of the postglenoid foramen. In its dorsal part, the rostral wing continues caudally to become the post-tympanic hook, which forms the dorsal edge of the postglenoid foramen (Figs. 1 and 8). The maximal rostro-caudal dimension of the squamosal bone, between the temporal suture and caudal end of the hook is ~8 mm. Between the suture and the rostral edge of the postglenoid foramen it measures ~4.4 mm. The respective rostro-caudal and dorso-ventral dimensions of the hook are ~4.5 and ~1.2 mm (Fig. 1). The squamosal zygomatic process forms the caudal quarter of the zygomatic arch. It has an elongated base, ~4.45 and ~0.64 mm in the rostro-caudal and dorso-ventral dimensions, respectively. It extends ~3 mm in a rostro-latero-ventral direction to join the zygomatic bone. Its respective medio-lateral and dorso-caudal dimensions near the fusion with the zygomatic bone are ~0.27 and ~0.68 mm (Figs. 1 and 4). Medially, its ventral surface forms the glenoid fossa for articulation of the mandibular condyle (Figs. 1 and 4). Rostro-caudally the fossa is roughly flat and measures ~2.8 mm (Fig. 1). Latero-medially it is convex with a ~1.2 mm radius (Fig. 6). Most of the flat parts of the squamosal bones, dorsal and ventral of the base of the zygomatic process, consist of 37–92 µm thick bony layers, separated by a 16–50 µm wide space. At the glenoid fossa these layers combine to form a ~0.3 mm thick bulk. The base of the zygomatic process consists of a trabecular network enveloped by a ~0.11 mm thick cortex (Fig. 6). The zygomatic bone connects the maxillary and squamosal zygomatic processes. It is ~6.4 mm long. Its rostral part forms a ~3.55 mm fusion with the caudal end of the maxillary zygomatic process whereby the zygomatic portion of the fusion is positioned dorsal to the maxillary component. The fusion with squamosal zygomatic process is ~1.27 mm and the zygomatic component is ventral to the squamosal bone (Fig. 1). The basisphenoid bone is a symmetrical osseous unit that extends from the frontal and squamosal bones on one side of the skull, through the base of the skull to the frontal and squamosal bones on the contralateral side of the skull (Figs. 1, 4, and 8). The lateral aspect of the basisphenoids is ventral to the frontal and squamosal bones and caudal to the maxilla. It forms the caudo-ventral aspect of the orbit (Fig. 1). Its rostro-caudal dimension, between the maxilla and postglenoid fora-

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Figure 14. Overview of mouse left tympanic bulla. Upper left: 2D frontal slice through second turn of cochlea. Upper right: lateral internal view from sagittal plane through middle ear, vestibule and labyrinth. Lower left: dorsal view from a horizontal plane through labyrinth. Lower right: dorsal view from a horizontal plane through cochlea and Eustachian tube.

men, is ~4.6 mm. Its maximal dorso-ventral dimension, between the frontal bone and base of the skull, is ~1.6 mm. Its wider part is toward the rostral border. Caudally, it tapers toward the postglenoid foramen (Figs. 1 and 8). Together with the palatine and presphenoid bones (rostral of the basisphenoid) the ventral part of the basisphenoid bone forms the middle (rostro-caudal) portion of the base of the skull (the rostral and caudal parts of the base comprise the premaxillo-maxillary and occipital bones, respectively). The central (latero-lateral) part of this portion consists of the sella turcica, an inwardly folded plateau from the base of the skull into the cranium (Figs. 4, 6, and 8). The sella is flanked on either side by a wing. The wing is separated from the sella by the rostral lacerated foramen rostrally and medial pterygoid process caudally. Further laterally is the lateral pterygoid process. Rostrally in the palatine bone, the pterygoid processes are separated by the pterygoid fossa. Caudally, in the basisphenoid, they are separated by the scaphoid fossa (Figs. 4 and 8). The basi- and presphenoid bones fuse at the sella turcica. Further laterally, beyond the rostral lacerated foramen, the basisphe-

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noid fuses with the palatine bone to form wings and all four pterygoid processes (Figs. 4 and 8). The rostro-caudal dimension of the sella is ~3.18 mm, with ~2.7 mm included in the basisphenoid bone (Figs. 2, 4, and 8). In its rostral part, the plateau of the sella is ~1.13 mm dorsal to the base of the skull. Moving caudally, the sella gradually merges with the base (Fig. 4). Its narrowest latero-lateral dimension, at the presphenoid, is ~0.35 mm. It is ~1.65 mm between the right and left Eustachian tubes, near the suture with the occipital bone (Figs. 4 and 8). The wing forms the rostral border of the petrotympanic fissure. Its maximal rostro-caudal dimension along the petrotympanic fissure is ~3 mm. Its medio-lateral dimension is ~2.67 mm, between the medial pterygoid process and the base of the lateral aspect of the basisphenoid (Fig. 4). The medial pterygoid process protrudes ventro-medially ~0.56 and ~1 mm in its respective rostral and caudal parts (Fig. 6). It has a small, ~0.33 mm long rostral part that belongs to the palatine bone. Its remaining ~2.85 mm rostro-caudal dimension is part of the basisphenoid (Fig. 4). Its medio-lateral dimension varies from ~85 µm near its connection with the wing to ~260 µm toward its ventral end (Fig. 6). In its rostral part, the medial pterygoid process is separated from the sella turcica by the rostral lacerated foramen, which is ~2 and ~0.5 mm rostro-caudally and medio-laterally, respectively (Figs. 4, 6, and 8). Immediately lateral to the medial pterygoid process, ~1 mm rostral of its caudal end, is the ~0.43 mm diameter inter-pterygoid foramen (Figs. 4 and 6). The lateral pterygoid process protrudes ventro-laterally ~1 mm from the pterygoid and scaphoid fossae (Figs. 1, 4, and 6). It tapers toward its ventral end. Its rostral part that belongs to the presphenoid is ~2.5 mm in a rostro-caudal dimension. The posterior, basisphenoidal part, is ~1.5 mm long (Fig. 4). The average medio-lateral dimension of the lateral pterygoid process is 0.43 mm (Fig. 6). The long and short diameters of the foramen ovale are ~0.8 and ~0.4 mm, respectively. However, foramina on either side, including their dimensions, appear asymmetrical and these measurements may vary considerably (Figs. 4 and 8). Also, although in general the foramen ovale is medial to the lateral pterygoid process, it may also extend laterally beyond the process (Fig. 4). On either side of the midline the presphenoid bone forms a bridge between the basisphenoidal part of the sella turcica caudally and the ethmoid bone rostrally. On either side of its rostral part there is a wing mostly occupied by the optic foramen (Fig. 8). Rostrally, the wing is sutured to the maxilla. Caudally it is embraced by the palatine bone medially and by the lateral aspect of the basisphenoid laterally (Figs. 1 and 8). The midline dimension of the presphenoid is ~2 mm. The mediolateral and rostro-caudal dimensions of each wing are ~1.31 and ~0.8 mm, respectively. The optic foramen, ~0.6 mm in diameter, is located halfway between both the midline and the lateral end of the wing and rostral and caudal borders of the wing (Fig. 8). The bones at the base of the skull rostral of the basi- and presphenoid bones constitute symmetrical pairs connected by a midline suture (Fig. 4). Each of the palatine bones, the most caudal of these paired bones, is composed of horizontal and perpendicular plates. The palatine bones comprise the bony part of the separation between the naso- and oropharynx. Thus, ventral to the sella turcica, between the medial pterygoid processes, the horizontal plates of the palatine bones have a free border at the transition between the naso- and oropharyngeal spaces (Figs. 2, 4, and 8). Further laterally, the horizontal plate is sutured to the basisphenoid. This suture runs laterally to cross the lateral pterygoid foramen and rostrally to meet the maxillary alveolar process caudal of the third molar tooth. From this point medially to the midline the horizontal plate is sutured to the maxilla (Fig. 4). The rostro-caudal dimension of the horizontal plate is ~2.1 mm. The border between the midline and medial pterygoid process is ~0.5 mm. The medial part of the suture with the basisphenoid, which is in a rostro-caudal direction, is ~0.9 mm. The medio-lateral and lateral (in a rostro-caudal direction) parts of this suture are ~1.3 and ~1.87 mm. The border with the maxilla has two parts. The lateral part, which runs in a rostro-medial to latero-caudal direction is ~1.1 mm long. From the medial end of the lateral part, the medial part runs ~1 mm rostrally and then 0.65 mm medially, to join the midline suture (Fig. 4). The perpendicular plates connect between the horizontal plates and the presphenoid bone (Figs. 5 and 8). They have respective rostral and caudal free borders with the nasal cavity and rostral lacerated foramen (Fig. 8). Together with the horizontal plates and presphenoid they comprise the bony part of the nasopharynx (Figs. 5 and 8). The latero-lateral nasopharyngeal dimension, between the two plates is ~1.2 mm. It is 0.7 mm dorso-ventrally (Figs. 5 and 8). The latero-caudal component of the base of the skull is occupied by the tympanic bulla. Externally, the bulla has a pear, or water skin shape, and in addition has (i) the arcuate eminence which protrudes dorsally from the dorsal surface of the bulla, (ii) the superior ampulla at the lateral root of the superior semicircular canal of the labyrinth, which forms an arch protruding dorsally near the bulla's latero-caudal end, and (iii) the common limb of the labyrinth extending caudally from the vestibule at the caudo-medial aspect of the bulla (Figs. 1, 8, and 14). The wider part of the water skin forms the latero-

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Figure 15. Ventral view of mouse left external and middle ear (upper image) and ventro-lateral view of auditory ossicles.

caudal aspect of the bulla and houses the bony part of the external and middle ear. It then tapers rostro-medially toward the narrow part of the water skin near the caudal part of the basisphenoid bone (Figs. 1, 4, 8, and 14). The latero-caudal aspect of the bulla measures ~3.5 and ~2.4 mm rostro-caudally and dorso-ventrally, respectively (Fig. 1). At the medial end of the bulla, which accommodates the Eustachian tube, it is ~1.4 mm rostro-caudally and ~0.8 mm dorso-ventrally (Figs. 1 and 14). The arcuate eminence is a ~0.17 mm wide and ~3 mm long ridge that rises ~0.61 mm from the dorsal surface of the bulla and divides it into more or less equal rostro-lateral and medio-caudal parts, including the external aspect of the superior semicircular canal of the labyrinth (Figs. 1, 8, and 14). The external diameter of the superior semicircular canal and common limb is ~0.18 and ~0.375 mm, respectively. The canal rises ~1.5 mm from the dorsal surface of the bulla (Figs. 1 and 8). The ~0.31 by 0.6 mm internal acoustic foramen opens caudo-dorso-medially between the arcuate eminence and posterior lacerated foramen (Figs. 8 and 14). The bulla is bridged to the rest of the skull at three small locations. Rostrally it is fused to the basisphenoid bone at the caudal border of the scaphoid fossa immediately caudal to the inter-pterygoid foramen (Fig. 4). Posteriorly it is connected to the paroccipital processes of the periotic capsule of the occipital bone (Figs. 1 and 8). Its lateral connection is via a ~0.22 mm thick bony plate that bridges the roof of the external acoustic meatus with the squamosal bone. The plate measures ~0.5 mm in a rostro-caudal dimension. It is ~0.75 mm between the squamosal bone ventro-medially and the bulla (Figs. 1, 8, and 14).

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Figure 16. Internal, caudal view of mouse left tympanic bulla from frontal plane through first turn of cochlea. Lower panel: 2D image through same plane demonstrating details of auditory ossicles anchorage.

The bulla houses the bony part of the external acoustic meatus, the middle ear, and the internal ear. The external acoustic meatus is a ~0.5 mm long and ~1.9 mm in diameter canal that extends from the lateral aspect of the bulla medially where it ends in the tympanic ridge which anchors the tympanic membrane (Figs. 1, 4, 14–18). The ridge is a circular eminence bulging ~0.11 mm from the surface of the external auditory meatus (Figs. 14–16). The middle ear is a cavity medial of the external acoustic meatus. It occupies the latero-ventral portion of the bulla medial of the tympanic ridge, whereas the internal ear, consisting of the cochlea, labyrinth and vestibule, occupies its rostro-medial portion (Figs. 13–16). The maximal latero-medial, rostro-caudal, and dorso-ventral dimensions of the middle ear are ~1.17, ~2.25, and ~1.75 mm, respectively (Figs. 14–19). The middle ear accommodates the auditory ossicles that transmit acoustic vibrations from the tympanic membrane to the oval window of the vestibule (Figs. 14–19). The largest of the ossicles is the malleus. It has a body, handle, head, and process. The body lies diagonally from the rostro-lateral roof of the middle ear in a ventro-medial direction toward the ventral cochlear wall (Figs. 15, 16, and 18). It is ~1.0 mm long, ~0.5 mm wide. Its center is ~10 µm thick, surrounded by a ~65 µm margin. The malleus is anchored to the cochlear wall via the handle, a ~0.3 mm long, ~0.25 mm wide, and ~0.2 mm thick ventro-medial process. The gap between the handle and the wall is less than 15 µm (Fig. 15). The malleolar

NOSE, PALATE AND UPPER JAW, CRANIUM AND TYMPANIC BULLA

Figure 17. Internal, ventral view of mouse left tympanic bulla from horizontal plane through base of stapes (left) and ventral end of oval window (right).

Figure 18. Internal, ventral view of mouse left tympanic bulla from horizontal planes through center (left) and ventral aspect (right) of round window.

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PART A, SECTION I, CHAPTER 1

Figure 19. Internal, lateral view of mouse left ear showing the relationship between middle ear (M), cochlea, and vestibule (V). Sv, scala vestibuli; St, scala tympani; Bt, basal cochlear turn; It, intermediate cochlear turn; At, apical cochlear turn; Iam, internal acoustic meatus. The malleus is colored red and the stapes orange. Numbers in image panels indicate distances in mm from reference plane through lateral aspect of external ear. Upper panel shows slice position (yellow lines); black line indicates position of reference plane.

NOSE, PALATE AND UPPER JAW, CRANIUM AND TYMPANIC BULLA

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Figure 20. Digital replica of mouse left internal ear space. C, cochlea; Cl, common limb; La, lateral ampulla; Lsd, lateral semicircular duct; Pa, posterior ampulla; Psd, posterior semicircular duct; RW, round window; Sa, superior ampulla; Ssd, superior semicircular duct; U, utricle; V, vestibule.

process is ~0.8 mm long. It is triangular in shape, with the base emerging from the malleolar body and vertex at the tympanic membrane. Like the body, it is less than 10 µm in the center and thicker in its periphery (Figs. 15 and 18). The head of the malleus is a round, ~0.3 mm in diameter projection protruding ~0.11 mm from the body in a dorso-medial direction. Its center forms a socket that articulates with the body of the incus (Figs. 15–18). The body of incus is ~0.3, ~0.38, and ~0.32 mm in the rostro-caudal, latero-medial, and dorso-ventral dimensions, respectively (Fig. 15). The short limb of the incus is a process that projects ~0.3 mm from the body in a general caudal direction. Its caudal tip forms an anchorage with the rostral wall of the labyrinth (Fig. 16). The long limb of incus projects from the body ~0.33 mm in a ventro-caudal direction. At its tip it bends medio-caudally to form the ~80 µm long lenticular process that articulates with the head of the stapes (Figs. 15 and 17). The stapes head is button-shaped, ~160 µm in diameter and ~60 µm in the latero-medial dimension. The anterior and posterior limbs of stapes project ~0.44 mm directly from the head in a general medial direction, thus forming

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a symmetrical arch (Fig. 15). Their medial ends are fused with the base of stapes, a plate-like structure that fits closely into the oval window of the vestibule. The respective rostro-caudal, dorso-ventral and latero-medial dimensions of the base are ~440, ~160 and ~50 µm (Fig. 17). The internal ear consists of the cochlea, rostrally and labyrinth, caudally. The osseous spaces of the two units are connected via the vestibular space (Figs. 17–20). The cochlea consists of a single bony tube that spirals two and a half turns around the internal acoustic meatus (Figs. 18–20). The longitudinal axis of the spiral is oriented in a rostro-ventral to caudo-dorsal direction. The rostro-caudal dimension of the cochlea is ~1.77 mm (Figs. 18 and 20). Similar to a snail, the diameter of the basal (caudal) turn (inner diameter of ~1.32 mm) is larger than that of the apical (rostral) turn (inner diameter of ~0.93 mm). The spiral tube is walled by the otic capsule, a 32–130 µm thick compact bone partition, which also comprises the outer case that accommodates the cochlea. The space for the spiral ganglion is at the root of the spiral lamina (Figs. 14, 18, and 19). Throughout its entire length, the tube space is divided into the scala tympani and scala vestibuli by the 15–20 µm thick osseous spiral lamina. The transition between the scalae, the helicotrema is at the tip of the apical turn (Figs. 14, 16, 18, and 19). The scala vestibuli opens into the vestibule at the vestibular wall opposite the oval window (Figs. 17–19). The scala tympani continues caudal of the promontory toward the round window where the cochlea is separated from the middle ear by the secondary tympanic membrane (Fig. 18). The promontory is a ridge that bulges ~100 µm from the latero-ventral aspect of the vestibular wall into the middle ear and separates between the oval and round windows (Figs. 17–19). It has a medial extension that separates between the vestibule and scala tympani (Fig. 18). The vestibule, at the rostral aspect of the labyrinth, is roughly an elliptical space that links the labyrinth with the middle ear (through the oval window) and with the cochlea (through the scala vestibuli). Its rostro-caudal, latero-medial, and dorsoventral dimensions are ~1.00, ~0.61, and ~0.44 mm, respectively (Figs. 17–19). The vestibule has two recesses, an elliptical one that houses the utricle and a spherical recess that contains the saccule (Fig. 20). The labyrinth as a whole is shown in Figure 20. It includes the superior, posterior, and lateral semicircular ducts and their respective ampullae and the common limb. The common limb is a 225 µm in diameter and ~0.75 mm long, straight duct that projects dorso-laterally from the vestibule. The semicircular ducts are bow-shaped. They are narrowest at the bow center (50–100 µm) and largest at their origin (~200 µm). The superior and posterior ducts branch from the dorso-caudal end of the common limb in a dorsal and caudal direction, respectively. The lateral duct originates from the vestibule lateral of the posterior ampulla. The other end of the ducts connects to the vestibule via the ampullae. The superior and lateral ampullae (connecting the respective superior and lateral ducts) join the vestibular utricle; the lateral ampulla joins the main vestibular chamber.

2 Hyoid, Mandible, and Temporo-Mandibular Joint

The hyoid was scanned at 10-µm voxel resolution. It is an elongated bone consisting of a body, greater horn, and lesser horn (Figs. 1–5). Its position is rostro-ventral to the larynx, visible as the calcified thyroid and cricoid cartilages (Fig. 1). It is not articulated with any other bone. The hyoid is supported by the muscles of the neck and in turn supports the root of the tongue and the floor of the mouth. The body is bow-shaped with ventral convexity (Figs. 1, 4, and 5). It measures ~2.8 mm in its latero-lateral dimension (Figs. 1, 3–5). In the midline it is ~0.25 mm in the rostro-caudal plane (Figs. 1 and 3) and ~0.45 mm dorso-ventrally (Figs. 4 and 5). It becomes progressively thicker from the midline towards the lateral ends where its respective rostro-caudal and dorso-ventral dimensions are ~0.55 and 0.7 mm (Figs. 1 and 5). The horns on either side are connected to the body and to each other by partially ankylosed diarthrodial joints (Figs. 6 and 7), identified as gap lines on the external hyoid surface (Figs. 1–7). The greater horns project ~1.8 mm dorso-laterally from lateral borders of the body (Fig. 2). At the joint with the body they are somewhat triangular measuring ~0.45 × 0.45 mm (Fig. 6), and they become progressively flattened towards their dorso-lateral ends to measure ~0.2 mm (Fig. 3). The lesser horns project ~0.8 mm mainly in a dorsal direction (Fig. 2). Their dorso-ventral and medio-lateral dimensions are ~0.55 mm and ~0.1 mm, respectively (Figs. 2 and 5). All parts of the hyoid are devoid of trabecular bone; it consists almost entirely of 50–100-µm thick cortex (Figs. 6 and 7). The mandible was scanned at 6 µm. It extends from the incisal edge rostrally to the caudal-most aspect of the condylar process (~12 mm maximal dimension; Fig. 8). Its maximal dorso-ventral dimension (~5 mm) is between the levels of the tip of the coronoid process and ventral aspect of the angular process (Fig. 8). The anatomical hallmarks on the lateral (buccal) aspect of the mandible are the mental foramen, located anterior to the alveolar molar ridge, the masseteric tuber, and the masseteric ridge. A few minute foramina (~50 µm in diameter) open to the buccal mandibular surface, especially in the coronoid and angular processes (Fig. 8). Hallmarks on the medial (lingual) mandibular aspect are the mandibular foramen, pterygoid fossa, and symphysis menti (Fig. 9). Two midsize foramina (~150 µm in diameter) are present in the lower border of the body of the mandible below the distal root of the first molar tooth. We have termed these "inframandibular foram27

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PART A, SECTION I, CHAPTER 2

Figure 1. Ventral view of mouse hyoid.

Figure 2. Lateral view of mouse hyoid.

HYOID, MANDIBLE, AND TEMPORO-MANDIBULAR JOINT

Figure 3. Dorsal view of mouse hyoid.

Figure 4. Rostral view of mouse hyoid.

Figure 5. Caudal view of mouse hyoid.

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Figure 6. Internal view of mouse hyoid. Horizontal 3D slice through diarthrodial joints between body and horns.

Figure 7. Internal view of mouse hyoid; horizontal plane through the body. Top: 2D horizontal slice and through the same plane.

HYOID, MANDIBLE, AND TEMPORO-MANDIBULAR JOINT

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Figure 8. Lateral view of mouse mandible.

ina." They connect the incisal socket with the extra mandibular soft tissue milieu (Figs. 9 and 10). Minute foramina (~50 µm in diameter) are also scattered on the medial aspect of the mandibular body (Fig. 9). A dorsal view of the mandible highlights the occlusal surface of the three molar teeth and their periodontal ligament space. The molar alveolar ridge extends diagonally in a caudo-medial direction from a frontal plane passing somewhat rostral to the first molar tooth (Fig. 11). Several foramina (~100 µm in diameter) are present along the lateral aspect of the alveolar ridge (Fig. 11). Sagittal µCT slices through the first molar tooth demonstrate the mandibular canal extending immediately ventral to the molar roots from the region of the mental foramen (rostral to the first molar tooth) caudally into the condylar process. The canal reaches its smallest diameter (~80 µm) ventral of the distal root of the first molar and its larger diameter at a region ventral to the coronoid process (Fig. 12). The periodontal ligament space varies in width from 13 to 125 µm (Figs. 12 and 13). The alveolar septa are 50–200 µm thick with ~35-µm diameter transeptal foramina (Figs. 12 and 13). The spatial relationship between the major mandibular structures changes considerably along the rostro-caudal axis (Fig. 14). At the level of the mesial root of the first molar tooth, the mandibular canal is situated between this root dorsally and the lateral aspect of the incisor tooth and socket ventrally. Because of the diagonal rostro-caudal orientation of the alveolar ridge, which contains the molar and incisal sockets, the sagittal planes of the mandibular canal and the ridge diverge. Still, posterior to the third molar tooth, the sagittal planes of the incisal sockets and mandibular canal reconverge because of a dorso-lateral extension of the incisal socket at the level of the incisal dental papilla and dental sac

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Figure 9. Medial view of mouse mandible.

(Figs. 13 and 14). Closer to the mandibular foramen, situated at the base of the condylar process, the sagittal plane of the mandibular canal is situated dorsal of the medial aspect of the incisal socket and the canal opens medially at the mandibular foramen (Fig. 14). The temporomandibular joint at its closed and open positions is illustrated in Figure 15. When the jaws are closed, the center of the mandibular condyle head is positioned opposite the center of the glenoid fossa (see Chapter 1 for a description of the glenoid fossa). In a frontal plane through the center of the condyle head, its perimeter parallels that of the fossa. The distance between the head and concavity of the fossa is ~0.4 mm (Fig. 15). This gap is occupied by the cartilaginous articular disc. At maximal mouth opening the condyle moves rostrally, with the caudal portion of the condylar articular surface positioned opposite the rostral end of the glenoid fossa (Fig. 15).

HYOID, MANDIBLE, AND TEMPORO-MANDIBULAR JOINT

Figure 10. Ventral view of mouse mandible. Top panel is 2D sagittal slice through the first and second molar teeth demonstrating the path of the inframandibular foramina.

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Figure 11. Rostral view of mouse mandible.

HYOID, MANDIBLE, AND TEMPORO-MANDIBULAR JOINT

Figure 12. Internal view of mouse mandible. Mid- (first) molar 2D (upper panel) and 3D (lower panel) sagittal slices.

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Figure 13. Internal view of mouse mandible. Vertical 2D slice through roots of molar teeth and dorso-lateral extension of incisal socket.

HYOID, MANDIBLE, AND TEMPORO-MANDIBULAR JOINT

Figure 14. Frontal slices through mouse mandible. Numbers in slice headings indicate distances in mm from reference frontal plane through rostral edge of first molar. Red circles, perimeter of mandibular canal. Upper panel shows slice position (transparent yellow). Black line indicates position of reference line.

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Figure 15. Mouse temporomandibular joint from closed to open mouth. Left: closed position. The zygomatic arch was removed digitally to expose the condylar position. Free cut ends of arch are stained red. Center: 2D image of frontal plane through center of the condylar head at closed position. Right: open position.

SECTION II

Vertebral Column

1 Cervical Vertebrae

The cervical vertebral column consists of three atypical (C1, C2, C6) and four typical (C3, C4, C5, C7) vertebrae. All seven vertebrae were scanned at 10-µm voxel resolution. The first vertebra, C1 or atlas, is ring shaped, comprised of the neural arch and related structures. The outer dorso-ventral dimension of the atlas, from the dorsal end of the posterior tubercle to the ventral end of the anterior tubercle is ~4.2 mm (the terms "posterior" and "anterior" here were adopted from human anatomy). The latero-lateral dimension, between the lateral ends of the transverse processes is ~5.34 mm (Figs. 1 and 2). The inner diameter of the neural arch, which comprises the neural canal, is ~2.5 mm. The neural arch consists of the dorsal and ventral arches (analogous to the posterior and anterior arches in humans). The arches are separated on either side by the transverse processes (Figs. 1 and 2). The posterior tubercle is a bulge extending ~0.2 mm dorsally from the outer aspect of the dorsal arch at the midline. The dimensions of its base are ~1.5 mm latero-laterally and 0.8 mm rostro-caudally (Figs. 1–3). The anterior tubercle bulges ventrally ~0.7 mm from the ventral surface of the ventral arch. Its latero-lateral and rostro-caudal dimensions are ~1.15 and ~0.6 mm, respectively (Figs. 1, 2, and 4). The borders between the transverse process and the ventral and dorsal arches are marked by the transverse foramen and foramen for the first cranial nerve (CI), respectively (Fig. 5 and 6). Either foramen has a minimal diameter of ~0.2 mm and connects the neural canal with the extravertebral environment (Figs. 1, 2, 5, and 6). The two foramina are connected by a short canal that traverses in a ventro-dorsal direction within the transverse process (Fig. 6). The articular facet for the skull, a ~1.75 × 1.00 mm shallow concavity, comprises the rostral aspect of the transverse process, which articulates with the occipital condyle of the skull (Fig. 1). The lateral part of the caudal aspect of the transverse process consists of the articular facet for the axis (C2). It is also a shallow concavity, ~75 µm in ventro-dorsal and 25 µm in latero-medial dimension, which articulates with the transverse process of the axis (Fig. 2). A caudal view of the atlas (Fig. 2) shows calcified patches with a trabecular bone-like structure attached to the central aspect of the ventral arch and to the medial aspect of the transverse process, adjacent to the articular facet for 41

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Figure 1. Rostral view of mouse atlas.

the axis. These patches are part of the insertion of the transverse ligament that connects the atlas with the axis (Figs. 2 and 7). A cross-section of the atlas in a transverse plane demonstrates a trabecular content throughout the arches, processes, and tubercles. The trabecular bone is surrounded by a ~15-µm thick cortical envelope (Fig. 7). The outer dorso-ventral dimension of the axis, from the dorsal-most aspect of the dorsal arch to the ventral-most aspect of the ventral arch, is ~3.5 mm. The latero-lateral dimension, between the lateral ends of the transverse processes, is ~4.2 mm (Figs. 8 and 9). The diameter of the neural canal is ~2.1 mm. The axis is the first vertebra that has a spine that extends dorsally from the dorsal arch, postzygapophyses on the lateral aspects of the dorsal arch, and a body that occupies the major part of the ventral arch. These components are typical of most vertebrae. At its thinnest part, the dorsal arch is ~0.25 mm thick. The thickest part, which occupies its central 2 mm, comprises "the base of the spine" and is ~0.55 mm dorso-ventrally and ~1.0 mm rostro-caudally (Figs. 8–10). The spine (or spinous process), a conical tubercle, extends ~0.33 mm from the center of the dorsal arch in a rostro-dorsal direction (Figs. 8, 10, and 12). The postzygapophyses extend from the dorsal arch ~0.18 mm in a latero-dorso-caudal direction. They have a latero-dorso-caudal flat round surface, ~0.55 mm in diameter (Figs. 8, 9, and 12). The transverse processes connect between the postzygapophyses and the body. Either process occupies ~1.1 mm of the neural arch perimeter, and extends slightly in a rostral direction to form the flat, ~1.1 × 0.6 mm articular

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Figure 2. Caudal view of mouse atlas.

surface with the atlas (Figs. 8, 11, and 12). On the opposite side, the process extends ~1.2 mm in a ventro-caudo-lateral direction (Figs. 8–12). It is thicker (~0.7 mm) at its center and near the neural arch, and becomes gradually thinner toward its periphery (~0.04 mm) (Fig. 11). The transverse foramen, ~0.25 mm in diameter, traverses the center of the transverse process in a general rostro-caudal direction (Figs. 9 and 12). From a caudal view, the vertebral body is a trapezium-like structure, ~0.75 and ~0.88 mm in ventro-dorsal and rostro-caudal dimensions, respectively, that occupies ~1.5 mm of the neural arch perimeter (Figs. 9–11). The odontoid process is a tooth-like structure bulging ~1.1 mm from the body rostrally and inwardly into the neural canal (Figs. 8, 11, and 12). The ~0.4-mm diameter tip of the odontoid process extends ~0.4 mm from an irregular plate-like structure. This structure is ~0.06 mm thick and 1.1 mm in diameter and is connected caudally to the broader part of the process (Figs. 8, 11, and 12). A cross-section through the axis in a transverse plane demonstrates a trabecular content throughout the arches, processes, and body. The trabecular bone is surrounded by a ~15-µm thick cortical envelope (Fig. 13). On either side of the midline, the cortex of the body is perforated by an intravertebral foramen (Figs. 9 and 13), apparently connected to the extravertebral foramen through the intertrabecular spaces (Fig. 13).

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Figure 3. Dorsal view of mouse atlas.

The relationship between the atlas and axis is shown in Figures 14–20. The axial ventral arch lies dorsal to the atlar ventral arch. The rostral half of the axial body protrudes into the atlar domain of the neural canal and the ventral aspect of the axial body articulates with dorsal (inner) aspect of the atlar ventral arch. The dorsal arches of both vertebrae have an overall parallel perimeter. The typical cervical vertebrae, represented here by C5, have the same basic structures as the axis (Figs. 21–26). However, they do not have an odontoid process, a unique feature of the axis. The rostral aspect of their body shows a latero-lateral concavity, which is pronounced by the uncinate processes that bulge rostrally on either side of the body (Figs. 21, 23, 24, 28–30). The spinous processes are more pronounced and pointed than those of the axis (Figs. 21–23, 25, and 26). In addition to the postzygapophyses, they also have prezygapophyses protruding rostrally from the arch. The transverse foramen opens between the transverse processes and the zygapophyses (Fig. 22). C6 closely resembles a typical cervical vertebra but has in addition the Chassaignacs's (carotid) tubercle extending rostro-ventrally and slanting caudally from the transverse process (Figs. 27–32). Its thin medio-lateral dimension is ~0.21 mm (Figs. 27–30). Its rostro-ventral dimension is almost 0.7 mm, similar to the rostro-ventral dimension of the vertebral body (Figs. 30 and 31). The transverse foramen passes between the transverse process and Chassaignacs's tubercle (Fig. 28). The tubercle, which is almost entirely devoid of trabecular bone (Fig. 32), serves for insertion of the longus colli muscle. Figure 33 demonstrates the relationship between C5, C6, and C7. Each two vertebrae form three joints: two ventro-lateral joints between the postzygapophysis of the rostral vertebra and prezygapophysis of the caudal vertebra and a third joint between the bodies of adjacent vertebrae which are separated by the intervertebral disc.

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Figure 4. Ventral view of mouse atlas.

Figure 5. Lateral (left) view of mouse atlas.

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Figure 6. Rostral, internal view of left transverse process of mouse atlas at the level of the transverse process, demonstrating the connecting canal between the foramen for CI and the transverse foramen.

Figure 7. Rostral, internal view of the mouse atlas from a transverse plane at a level immediately rostral to the transverse process. Upper image: 2D frontal slice through the same plane demonstrating trabecular, cortical and transverse ligament details.

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Figure 8. Rostral view of mouse axis.

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Figure 9. Caudal view of mouse axis.

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Figure 10. Dorsal view of mouse axis.

Figure 11. Ventral view of mouse axis.

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Figure 12. Lateral (left) view of mouse axis.

Figure 13. Rostral, internal view of the mouse axis from a frontal, somewhat diagonal plane at a level of the transverse process. Upper image: 2D frontal slice through the same plane demonstrating trabecular and cortical details.

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Figure 14. Rostral view of mouse atlas and axis. C1 and C2 designate atlas and axis, respectively.

Figure 15. Caudal view of mouse atlas and axis. C1 and C2 designate atlas and axis, respectively.

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Figure 16. Dorsal view of mouse atlas and axis. C1 and C2 designate atlas and axis, respectively.

Figure 17. Dorsal view of mouse ventral half of atlar-axial domain of the neural canal.

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Figure 18. Ventral view of mouse atlas and axis.

Figure 19. Lateral (left) view of mouse atlas and axis. C1 and C2 designate atlas and axis, respectively.

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Figure 20. Medial view of lateral segment of atlar-axial domain of the mouse neural canal. C1 and C2 designate atlas and axis, respectively.

Figure 21. Rostral view of mouse C5.

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Figure 22. Caudal view of mouse C5.

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Figure 23. Dorsal view of mouse C5.

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Figure 24. Ventral view of mouse C5.

Figure 25. Lateral (left) view of mouse C5.

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Figure 26. Rostral, internal view of the mouse C5 from a frontal, somewhat diagonal plane at a level of the transverse process. Upper image: 2D frontal slice through the same plane demonstrating trabecular and cortical details.

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Figure 27. Rostral view of mouse C6.

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Figure 28. Caudal view of mouse C6.

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Figure 29. Dorsal view of mouse C6.

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Figure 30. Ventral view of mouse C6.

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Figure 31. Lateral (left) view of mouse atlas and axis.

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Figure 32. Rostral, internal view of the mouse C6 from a frontal, somewhat diagonal plane at a level of the transverse process and Chassaignacs's tubercle. Upper image: 2D frontal slice through the same plane demonstrating trabecular and cortical details.

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Figure 33. Relationship between C5–C7 mouse cervical vertebrae. Left: ventral view. Middle: lateral view. Right: dorsal view.

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2 Thoracic Vertebrae

The thoracic vertebral column consists of thirteen (T1–T13) vertebrae all sharing similar anatomical features. T2 was scanned at 10-µm voxel resolution and exhibited as a representative thoracic vertebra (Figs. 1–7). The outer dorso-ventral dimension of T2, from the dorsal end of the spinous process to the ventral surface of the body is ~2.5 mm (Figs. 1, 2, and 5). This dimension is greater by ~0.45 than that of the other thoracic vertebrae because of the exceptionally long spinous process of T2, which extends dorso-caudally ~0.9 mm (see Fig. 2 in Chapter 1, Part A, Section III). The latero-lateral dimension, between the lateral ends of the transverse processes, is ~3.25 mm (Figs. 1 and 2). The inner dorso-ventral and laterolateral diameters of the neural canal are ~1.52 and ~1.8 mm, respectively (Figs. 1 and 2). The special features of all thoracic vertebrae are the articular surfaces for the ribs. There are two such surfaces for every rib. One is the demi-facet for the head of the rib, a ~0.3 × 0.175 mm concavity on the rostro-ventro-lateral aspect of the neural arch, between the vertebral body and the prezygapophysis (Figs. 1, 4, and 8). The second is the articular facet for the tubercle of the rib, a flat, ~0.55 mm diameter surface that comprises the rostro-ventro-lateral aspect of the transverse process (Figs. 1, 4, 5, and 8). The thoracic vertebrae are the first part of the vertebral column, demonstrating the typical architecture of the load-bearing trabecular network, which consists of longitudinal trabecular plates connected to each other and to the cortex by interconnecting trabeculae. The cortical and trabecular thicknesses are similar (Fig. 7).

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Figure 1. Rostral view of mouse T2.

Figure 2. Caudal view of mouse T2.

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Figure 3. Dorsal view of mouse T2.

Figure 4. Ventral view of mouse T2.

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Figure 5. Lateral (left) view of mouse T2.

Figure 6. Rostral, internal view of mouse T2 demonstrating the intra- and extravertebral foramina.

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Figure 7. Internal view of the mouse T2 vertebral body from a midsagittal plane. Upper image: 2D slice through the same plane demonstrating trabecular and cortical details.

Figure 8. Relationship between T3–T5 mouse thoracic vertebrae. Left: ventral view. Middle: lateral view. Right: dorsal view.

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3 Lumbar Vertebrae

The lumbar vertebral column consists of six vertebrae (L1–L6), all sharing similar anatomical features. L3 was scanned at 10-µm voxel resolution and selected as a representative lumbar vertebra (Figs. 1–6). The outer dorso-ventral dimension of L3, from the dorsal end of the spinous process to the ventral surface of the body, is ~4.4 mm (Figs. 1, 2, and 6). The laterolateral dimension between the lateral ends of the prezygapophyses is similar (Figs. 1–4). The inner dorso-ventral and laterolateral diameters of the neural canal are ~1.6 and ~2.55 mm, respectively (Figs. 1, 2, and 6). The special features of all lumbar vertebrae are the anapophyses and a second extravertebral foramen. The anapophyses are ~1.1-mm long processes, ~0.32 mm in diameter. They extend on each side caudally from the caudal border of the neural arch between the transverse process and postzygapophysis (Figs. 1, 2, 4–6, and 8). As in other vertebrae, the bodies show primary ossification centers, consisting of epiphyseal growth engines, as well as secondary ossification centers (Fig. 7).

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Figure 1. Rostral view of mouse L3.

Figure 2. Caudal view of mouse L3.

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Figure 3. Dorsal view of mouse L3.

Figure 4. Ventral view of mouse L3.

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Figure 5. Lateral (left) view of mouse L3.

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Figure 6. Rostral, internal view of mouse L3 demonstrating the intra- and extravertebral foramina.

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Figure 7. Internal view of the mouse L3 vertebral body from a ventral plane. Upper image: 2D slice through the same plane demonstrating trabecular and cortical details. Inset: high magnification of central area of 2D slice showing details of primary and secondary ossification centers.

Figure 8. Relationship between L2–L4 mouse lumbar vertebrae. Left: ventral view. Middle: lateral view. Right: dorsal view.

4 Sacrum

The sacrum consists of four (S1–S4) fused vertebrae. It was scanned at 10-µm voxel resolution. The fused components are the vertebral bodies (Figs. 1, 2, 5, and 6) and zygapophyses (Figs. 2, 3, and 6). In addition, a thin (~40 µm) elongated plate (~2.75 × 0.3 mm) connects the lateral aspect of the transverse processes of S2 and S3 (Figs. 1–3). The vertebral bodies are ~2.5 mm long (Fig. 2). They have a general triangular prism-like shape (Fig. 1). On a cross-section, the triangle sides measure ~1.5 mm (Figs. 6, 7). Each body has two intravertebral foramina opening toward the neural canal (Figs. 6 and 7) and one extravertebral foramen at the midline (Figs. 1 and 7). The epiphyses of adjacent bodies and zygapophyses are fused (Figs. 1, 2, 4–6), with the fusion marked externally by the transverse line (Figs. 1 and 2). The spine of S1 is tapered, extending ~1.3 mm from the neural arch. From a lateral view, the S2–S4 spines have a squarish appearance. S2 has the longest spine (~1.4 mm) and S4 the shortest (~1.1 mm). The transverse processes vary in shape. The transverse processes of S1 form the alae of sacrum. They are substantially larger than the other transverse processes, measuring ~1.6 mm in a medio-lateral dimension (Figs. 1 and 3). Their rostro-caudal and dorso-ventral dimensions near the neural arch are ~1.75 and ~0.95 mm, respectively, expanding to ~3.5 × 1.85 mm at the lateral edge (Figs. 1, 2, 3, and 6). Their lateral aspect, which comprises the articular surface with the ilium, is a slightly concave surface measuring ~2.65 and ~1.85 mm in the rostro-caudal and dorso-ventral dimensions, respectively (Fig. 2). The transverse processes of S2 extend 1.4 mm from the neural arch. Like S1, they expand toward their lateral edge, measuring ~2.0 mm in the rostro-caudal dimension and only ~1.0 mm at the neural arch. Their dorso-ventral dimension is ~0.4 mm (Figs. 1-3). The transverse processes of S3 and S4 extend laterally to a similar extent as those of S2, but they are inclined in a rostro-lateral angle (Figs. 1, 3, and 4). Their maximal dorso-ventral dimension is 0.35 mm (Fig. 2). They are ~0.7 mm at the neural arch and their lateral expansion is minimal (Figs. 1, 3, and 4). Cross-sectionally, the contour of the sacral neural canal is roughly triangular, with the triangle sides measuring ~0.55 mm. The external diameter of the neural arch is ~0.65 mm (Figs. 6 and 7). By comparison to other vertebrae, all components of the sacral vertebrae contain a dense trabecular network. However, none of the components shows a particular orientation of the trabeculae (Figs. 4–7). 79

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Figure 1. Ventral view of mouse sacrum.

Figure 2. Lateral (left) view of mouse sacrum.

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Figure 3. Dorsal view of mouse sacrum.

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Figure 4. Internal view of mouse sacrum from a ventral plane. Upper image: 2D slice through the same plane demonstrating trabecular and cortical details, including fusion of vertebral bodies.

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Figure 5. Internal view of mouse sacrum from a sagittal plane. Upper image: 2D slice through the same plane demonstrating trabecular and cortical details, including fusion of vertebral bodies and zygapophyses.

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Figure 6. Rostral view of mouse S1. Upper right image: 2D slice through a plane immediately rostral to the prezygapophysis demonstrating trabecular and cortical details.

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Figure 7. Internal view of mouse S3 from a frontal plane through the caudal end of the transverse processes. Upper image: 2D slice through the same plane demonstrating trabecular and cortical details, including vertebral foramina.

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5 Caudal Vertebrae

The caudal vertebral column consists of 28 (CA1–CA28) vertebrae exhibiting progressive decrease in size as well as gradual diminution of typical vertebral features such as the spinous process, transverse processes and zygapophyses and eventually their disappearance (Figs. 1–3). The first three caudal vertebrae retain all features, which show close resemblance to S4, but are smaller in size. The spinous process, transverse processes, and zygapophyses of CA3 are ~25% compared to those of S4 (Figs. 1–4). The trend of diminution continues in CA4, where the neural canal and postzygapophyses are missing and the spinous process is of minimal size (Figs. 5–9). The fusion of the body and arch of CA4 result in a cylindrical structure typical of vertebrae distal to CA3 (Figs. 1, 2, 5, and 6). This cylindrical body has an internal structure typical of other vertebral bodies (Fig. 10). The transverse processes and prezygapophyses are completely missing from CA11 and onward (Fig. 1), while a minimal spinous process can still be identified in CA17 (Fig. 2). The proximal 15 caudal vertebral bodies (or fused body and arch) have more or less the same dimensions, measuring ~4 mm in length and ~0.8 mm in diameter. From CA16 and distally there is a gradual decrease in body size, with CA18 being ~0.8 mm long and ~0.15 mm in diameter.

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Figure 1. Ventral view of mouse tail.

Figure 2. Lateral (left) view of mouse tail

Figure 3. Dorsal view of mouse tail.

Figure 4. Relationship between CA1–CA3 mouse caudal vertebrae. Left: ventral view. Middle: lateral view. Right: dorsal view.

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Figure 5. Rostral view of mouse CA4.

Figure 6. Caudal view of mouse CA4.

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Figure 7. Dorsal view of mouse CA4.

Figure 8. Lateral (left) view of mouse CA4.

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Figure 9. Ventral view of mouse CA4.

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Figure 10. Internal view of mouse CA4 fused body and neural arch from a ventral plane. Upper image: 2D slice through the same plane demonstrating trabecular and cortical details.

SECTION III

Thorax

1 Sternum, Sternal-Rib Joints, Ribs and Rib-Vertebral Joints

The whole thorax was scanned at 30-µm voxel resolution. The thoracic cage is comprised of ventral and dorsal axes, the sternum, and the thoracic vertebral column (Part A, Section II, Chapter 2), respectively. Rostrally they are connected directly by seven true ribs. Additional six caudal false ribs are articulalted directly only with thoracic vertebrae but not to the sternum. The ventral end of ribs 8–10 is connected to the sternum indirectly via a chain of consecutive intercostal joints extending from the joint between ribs 7 and 8 rostrally to the joint between ribs 9 and 10 caudally. The last three caudal ribs are called "floating ribs" because they do not form ventral connections to the skeleton (Figs. 1–4). The narrowest part of the thoracic cage is at the level of the first rib where it measures ~6 mm latero-laterally and ~5 mm dorso-ventrally (Fig. 3). The largest part is at the level of rib 13, where the latero-lateral and dorso-ventral dimensions are ~20 mm each (Figs. 1 and 2). The sternum consists of six components. Most rostrally is the manubrium, which is followed caudally by three sternebrae, the xiphisternum, and the xiphoid cartilage (Figs. 5–8). It was scanned at 10-µm voxel resolution. In a latero-lateral plane these components are linearly aligned (Figs. 5 and 8). In a ventro-dorsal plane they form a bow-like shape with a main ventral convexity. At the rostral and caudal ends of the bow there are small dorsal convexities comprised of the manubrium and xiphisternum, respectively (Fig. 6 and 8). These components are separated from each other by 0.1–0.2-mm gaps. Rostrocaudally the manubrium is ~3.8 mm long. From the dorsal and ventral views it has a "heart"-like appearance connected to a "stem". Each of the "heart leaves" extends ~1.3 mm from the midline. The "heart apex" is approximately halfway toward the caudal end of the manubrium (Figs. 5 and 7). The stem is ~0.58 mm in a latero-lateral dimension and ~0.77 mm ventro-dorsally (Figs. 5–8). From a dorsal view the "heart leaves" appear slightly concave in their center and convex in their periphery, forming the insertion for the sterno-thyroid muscles. Each of the muscle tendons contains a sesamoid bone measuring ~0.42 × 0.27 mm (Fig. 7). The corresponding rostro-ventral aspect of the "leaves" forms the insertion for the sternocleidomastoid muscles (Fig. 5). The respective rostro-caudal and dorso-ventral dimensions of the first sternebra are ~1.9 and 0.6 mm (Fig. 6). Latero-laterally it measures ~0.7 mm near the rostral end, progressively increasing to 0.85 mm toward 95

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Figure 1. Ventral overview of mouse thoracic cage.

Figure 2. Lateral overview of mouse thoracic cage.

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Figure 3. Rostro-dorsal overview of mouse thoracic cage.

the caudal end (Figs. 5 and 8). The second, shortest sternebra is ~1.77 mm rostro-caudally. Its latero-lateral dimensions are ~0.88, 0.73, and 1.1 mm at the rostral end, center, and caudal end, respectively (Figs. 5–7). Its dorso-ventral dimension is 0.54 mm (Fig. 6). The third (caudal) sternebra is the largest of the three sternebrae. Its rostro-caudal and dorso-ventral dimensions are ~2.8 and 0.65 mm, respectively. On each side, approximately halfway between the rostral and caudal ends, there is a ridge, extending ~0.3 mm on the lateral aspect, which encompasses the articular cavity for the fifth rib (Figs. 5–7). The respective latero-lateral dimensions rostrally and caudally of these "articular" ridges are ~0.9 mm and ~1.1 mm. In the midline, between the articular ridges, is a small rounded, 0.4 mm in diameter elevation, which extends ~0.07 mm from the ventral surface and contains the ventral sternebral foramen (Figs. 5, 6, and 8). The xiphisternum is the largest part of the sternum. It is ~4.2 mm rostro-caudally and ~0.42 mm dorso-ventrally. Latero-laterally, its rostral, narrow part is ~1.2 mm, expanding to ~1.6 mm toward its caudal end (Figs. 5–8). Visible caudal of the xiphisternum are the calcified masses of the xiphoid cartilage (Figs. 5–8). The internal structure is quite uniform in all the bony parts of the sternum. It consists of a ~3880-µm thick cortical envelope, and a rather dense cancellous network comprised of trabeculae of approximately the same thickness as the cortex (Figs. 5 and 8). Each of the ribs consists of an osseous dorsal segment and a ventral segment (Figs. 1, 2, 4, 9–14) made of calcified cartilage. The osseous segment is composed primarily of cortical bone (Fig. 9). The two segments are connected by the costal dorso-ventral joint (Figs. 2, 4, 9–14). The average cross-sectional dimensions of the dorsal segment are approximately twice compared to the ventral segment (Figs. 1, 2, and 4). The perimeter of the bow-shaped dorsal segment ranges between ~3.5 mm at the first rib (Figs. 3, 10, 11) and ~9.5 mm at rib 9 (Fig. 4). Moving caudally, it becomes shorter, again reaching ~6.5 mm at rib 13 (Fig. 4). In its center, the dorsal segment is flattened, measuring 0.4–0.6 mm in the centripetal dimension. Its dorso-caudal dimension is ~0.15 mm (Figs. 3, 10–12, and 15). Toward its dorsal and ventral ends (costovertebral and dorsoventral joints) the osseous segment becomes rounded, measuring 0.4–0.6 mm in diameter (Figs. 12 and 15). The dorsal end of the bow-shaped osseous segment is the rounded head of rib. Approximately 1.5 mm from the head of ribs 1–10 is a ~0.6mm protuberance called the tubercle of rib (Figs. 4, 10–15).

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Figure 4. Caudal overview of mouse thoracic cage.

The cartilaginous segment of ribs 1–7 connects the osseous segment with articular cavities, ~0.35 mm in diameter, on either side of the sternum. The cavities for rib 1 are between the "stem" and "heart" of the manubrium. Each of ribs 2–4 and 6 articulates with two cavities, on the sternal components rostral and caudal to it. The fossa for rib 5 is on the articular ridges of the third sternebra. The cavities for rib 7 are on the ventral aspect of the xiphisternum, near its rostral end (Fig. 16). Rostrally, the first rib articulates with the demi (hemi) facet for the head of the rib and articular facets for the tubercle of the rib of T1 (Part A, Section II, Chapter 2 and Figs. 9–12). The heads of ribs 2–10 articulate with the demi-facets for the head of the ribs of T2–T10 (Figs. 4 and 17). The tubercles of these ribs connect with T3–T11 (Figs. 3, 4, and 18). Ribs 11–13 have no tubercles; their heads articulate with T11–T13 (Figs. 4, 19, and 20).

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Figure 5. Ventral view of mouse sternum. Inset: 2D mid-sagittal slice demonstrating ventral sternebral foramen.

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Figure 6. Lateral (right) view of mouse sternum.

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Figure 7. Dorsal view of mouse sternum.

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Figure 8. Internal view of the mouse sternum from an approximately mid-sagittal plane. Right image: 2D slice through the same plane demonstrating trabecular and cortical details.

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Figure 9. Internal view of mouse ribs. (A) 2D frontal slice through sternebrae and ventral segment of ribs. (B–D) 2D internal rostro-caudal view through different levels of joint between rib 1 and T1.

Figure 10. Rostral view of complex of mouse rib 1 and T1.

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Figure 11. Caudal view of complex of mouse rib 1 and T1.

Figure 12. Lateral view of complex of mouse manubrium, rib 1, and T1.

STERNUM, STERNAL-RIB JOINTS, RIBS AND RIB-VERTEBRAL JOINTS

Figure 13. Rostro-ventral view of complex of mouse manubrium and rib 1.

Figure 14. Dorso-rostral view of complex of mouse manubrium and rib 1.

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Figure 15. Ventro-caudal view of complex of mouse T1–T6 and dorsal segments of corresponding ribs.

Figure 16. Ventral view of complex of mouse sternum and ventral segments of ribs 1–7.

STERNUM, STERNAL-RIB JOINTS, RIBS AND RIB-VERTEBRAL JOINTS

Figure 17. Ventral view of complex of mouse T8–T10 and dorsal segments of corresponding ribs.

Figure 18. Dorsal view of complex of mouse T8–T10 and dorsal segments of corresponding ribs.

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Figure 19. Ventral view of complex of mouse T11–T13 and dorsal segments of corresponding ribs.

Figure 20. Dorsal view of complex of mouse T11–T13 and dorsal segments of corresponding ribs.

PA RT B

Appendicular Skeleton

SECTION I

Rostral Appendage

1 CLAVICLE

The clavicle is a bow-shaped bone connecting the shoulder joint to the sternum (Figs. 1–3). It was scanned at 10-µm voxel resolution. The bow perimeter is ~6 mm long (Fig. 1). In its center the bow measures ~0.5 and ~0.3 mm in the dorso-ventral and rostro-caudal dimensions, respectively (Figs. 1 and 2). The lateral end is somewhat flattened and forms part of the rostro-ventral aspect of the shoulder joint, articulating with the humerus and acromion process of the scapula to which it is connected by a thin cartilaginous disc (Figs. 1–3). The clavicular medial ends connect to the "heart leaves" of the sternal manubrium via the cartilaginous omosternum, which is uncalcified and therefore appears as a gap on microtomographs (Figs. 2 and 3). The medial part of the bow, which connects to the omosternum, is thicker, measuring ~0.65 mm in both the rostro-caudal and dorso-ventral dimensions (Figs. 1–3). All parts of the clavicle are devoid of trabecular bone; it consists almost entirely of ~80-µm thick cortex (Fig. 4).

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Figure 1. Rostro-ventral view of mouse shoulder girdle.

Figure 2. Ventral view of mouse shoulder girdle.

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Figure 3. Dorsal view of mouse shoulder girdle.

Figure 4. Internal view of mouse clavicle.

2 SCAPULA

The scapula was scanned at 10-µm voxel resolution. It is a flat triangular bone having its vertex medially at the shoulder joint and its base laterally (Figs. 1 and 2). Its triangular height extends from the lateral mid-base to the vertex at the glenoid cavity (~8.5 mm, Fig. 1). The greatest rostro (fronto)-caudal dimension is between the rostral and caudal angles at the ends of the base (~6.5 mm, Fig. 2). The thinnest parts of the scapula consist of uncalcified membranes, more or less at the middle of the supraspinous fossa and scattered along the base and caudal angle, that appear cribriform on microtomographs (Figs. 1–4). The coracoid process (~1.25 × 0.5 mm) extends ventro-medially from the rostral border of the glenoid cavity (Figs. 1 and 4), forming the ventro-rostral aspect of the shoulder joint (Figs. 1 and 4). The glenoid cavity comprises the main scapular component of the shoulder joint. Its perimeter is elliptical, with its long diameter (~1.4 mm) in a rostro-caudal direction and the short diameter (~1.0 mm) in a dorso-ventral orientation (Figs. 3 and 4). The subscapular fossa extends from near the base approximately halfway to the vertex, thus dividing the ventral aspect into a rostral third and caudal two thirds (Fig. 1). The scapular spine divides the dorsal aspect into the supra- and infraspinous fossae. It has a ridge-like shape (~0.5 mm from the dorsal scapular surface) with a relatively wide base (0.5–1.0 mm) that runs approximately three quarters of the way latero-medially between the base and the vertex (~6.4 mm) and ends dorsally in a knife edge (~0.05 mm) (Figs. 2 and 4). The scapular spine has two processes: (1) the small supraspinous process (~1 mm long, ~0.5 mm wide, ~0.6 mm thick) that extends from the lateral quarter of spine in a dorso-rostral direction; (2) the markedly larger acromion process that extends from the medial end of the spine base medially beyond the glenoid cavity, reaching the same sagittal plane as the medial end of the coracoid process, thus forming the dorsal aspect of the shoulder joint (Figs. 1–4). It is relatively long (~5 mm), with a narrow (~0.5 mm) attachment to the spine and a flat, broad (~1.1 mm) medial end. The broadest part of the acromion process consists of the metacromion process, which bulges from its caudal border (Figs. 2 and 4). Figure 5 is an anterior view of the inner scapular content from a frontal plane, at the level of the spine, demonstrates that the medial half of the scapula consists of dorsal and ventral cortical plates (~0.1–0.2 mm thick) connected by a trabecular network. The overall scapular thickness at the medial half is nonuniform (~0.3–2.3 mm). In most of the lateral half, the two cortical plates are fused to form a single layer of compact bone (~0.18 mm thick). 117

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Figure 1. Ventral view of mouse scapula.

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Figure 2. Dorsal view of mouse scapula.

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Figure 3. Ventro-rostral view of mouse scapula.

Figure 4. Dorso-rostral view of mouse scapula. Orange line indicates the frontal plane shown in Figure 5.

SCAPULA

Figure 5. Top: internal view of the mouse scapula from a frontal plane at the level of the scapular spine. Bottom: 2D frontal slice through the same plane demonstrating trabecular and cortical details.

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3 HUMERUS AND SHOULDER JOINT

The humerus was scanned at 10-µm voxel resolution. It extends from the convex-most end of the humeral head distally to the medial tip of the trochlea (~10 mm, Figs. 1–4). Its maximal dimension is the rostro-caudal diameter of the head (~2.7 mm, Fig. 5). The maximal dimension of the shaft is at the level of the deltoid tuberosity (~2 mm, Figs. 1–4). The maximal diameter at the distal part of the humerus is between the lateral and medial epicondyles (~2.5 mm, Figs. 1 and 4). The narrowest part of the humerus is at the distal third of the shaft (0.75 mm, Figs. 2 and 3). The key anatomical landmarks at the humeral head are the greater and lesser tubercles, separated by the intertubercular sulcus (Figs. 1–5). The tubercular nutrient foramen, ~80 µm in diameter, opens on the rostral of the greater tubercle (Figs. 1 and 5). The head is separated from the shaft by the proximal growth plate, 55–250 µm wide, wriggling cribriform collar (Figs. 1–4). The deltoid tuberosity extends laterally from the proximal half of the shaft (Figs. 1–4). The proximal nutrient foramen, ~100 µm in diameter, opens in a rostro-medial direction in the proximal quarter of the shaft (Figs. 1, 2, and 4). The distal nutrient foramen opens to the rostro-medial surface, at the distal third of the shaft (~80 µm, Figs. 1 and 2). The supinator crest extends along the caudo-lateral aspect of the shaft in a caudo-lateral direction. Its distal end comprises the lateral epicondyle and articular surface for a sesamoid bone (Figs. 1, 3, and 4). The shaft and trochlea are separated by the coronoid and olecranon fossae on the rostral and caudal aspect, respectively (Figs. 1 and 4). The two fossae are connected by a ~200-µm diameter opening, the coronoid-olecranon notch (Figs. 1 and 4). The trochlea is a ~0.6-mm diameter shaft extending from the medial epicondyle laterally to the capitulum (~1.15 mm in length, Figs. 1, 4, and 6).

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Figure 1. Rostral view of mouse humerus.

An anterior view of the inner humeral content from a frontal plane, at a level caudal of the deltoid tuberosity, demonstrates the trabecular bone content of the epiphyses and metaphyses. In the proximal end of the femur, this trabecular bone encompasses most of the humeral head with the proximal growth plate separating the epiphysis from the metaphysis. At the distal end it fills most of the distal quarter of the humerus including the medial and lateral epicondyles. The diaphyseal shaft is devoid of trabecular bone and consists only of ~150-µm thick compact bone cortex (Fig. 7). The shoulder joint is shown in Figure 8. It has only a partial socket that consists of the glenoid cavity, acromion process, and coracoid process of the scapula and the lateral end of the clavicle. At maximal extension the humeral shaft is more or less in line with the neck of the scapula and approximately at a right angle to the clavicle. The dorsal (middle) third of the head of the humerus is positioned against the glenoid cavity. The acromion process and lateral end of clavicle are opposite the dorso-lateral and lateral aspects of the head, respectively. At a standing position the humeral shaft is at a right angle to the neck of the scapula. The dorso-medial third of the head is opposite the glenoid cavity. The acromion process is at a dorsorostral position to the head, and the lateral end of the clavicle embraces a small portion of its rostro-lateral aspect. At maximal flexion only a relatively small portion of the ventro-medial aspect of the head is against an articular surface. The greater tubercle of the humerus is brought to close proximity with the coracoid process.

HUMERUS AND SHOULDER JOINT

Figure 2. Rostro-medial view of mouse humerus. 2D image panel is a cross-sectional slice demonstrating path of the proximal nutrient foramen.

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Figure 3. Caudo-lateral view of mouse humerus. 2D image panel is a cross-sectional slice demonstrating the path of the distal nutrient foramen.

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Figure 4. Caudal view of mouse humerus.

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Figure 5. Dorsal view of mouse humerus. 2D image panel is a cross-sectional slice demonstrating the path of the tubercular nutrient foramen.

Figure 6. Ventral view of mouse humerus.

HUMERUS AND SHOULDER JOINT

Figure 7. Left: internal view of the mouse humerus from a frontal plane at a level posterior of the deltoid tuberosity. Right: 2D frontal slice through the same plane demonstrating trabecular, cortical, and growth plate details.

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Figure 8. Shoulder joint from extension to flexion. Left: maximal extension. Middle: standing position. Right: maximal flexion. Upper row: ventral view. 2nd row: dorsal view. 3rd row: rostral view. Bottom: caudal view. Hs, humeral shaft; H, head of humerus; Gt, greater tubercle; Lt, lesser tubercle; Ns, neck of scapula; Cp, coracoid process; Ap, acromion process; C, clavicle.

4 FOREARM (ULNA, RADIUS, AND ELBOW JOINT)

Unlike the tibia and fibula, the ulna and radius are not fused, and therefore are described separately. They were scanned at 10-µm voxel resolution. The ulno-radial system is bow shaped with its convex aspect facing caudo-medially (Fig. 1). The ulna extends from the olecranon, which comprises the ulnar head, proximal of the elbow joint, to its distal end at the styloid process, which forms part of the ulnar component of the ulno-lunate joint (~11.5 mm in length, Figs. 1–4). The horizontal plane through the edge of the olecranon process constitutes the largest dimension of the ulna in both rostro-caudally (~1.4 mm) and medio-laterally (~1.1 mm) (Figs. 2–6). The narrowest dimension of the ulnar shaft is the rostro-caudal diameter at a level approximately 1.7 mm proximal of the tip of the styloid process (~0.3 mm, Fig. 4). The key anatomical landmarks at the proximal zone of the ulna are a proximal growth engine, which separates the tip of the olecranon from its shaft, and the semilunar notch facing anteriorly and comprised proximally by the olecranon process and distally by the coronoid process (Figs. 2, 3, and 5). The semilunar notch articulates with the humeral trochlea. The knife-like ulnar ridge extends on the caudal aspect of the ulnar shaft from the coronoid process distally towards the distal ulnar quartile (Figs. 3 and 5). An articular facet for the radial tuberosity is present on the ridge edge ~0.8 mm distal of the coronoid process (Fig. 5). The proximal nutrient foramen opens on the medial aspect of the ulnar ridge, ~0.5 mm distal of the coronoid process (Fig. 3). The ulnar tuberosity, an elongated ridge, extends ~0.6 mm distally from the coronoid process on the caudo-lateral aspect of the ulnar shaft (Fig. 2). Like the proximal nutrient foramen, the distal nutrient foramen opens on the lateral aspect of the shaft, approximately at the center of the distal quartile (Fig. 3). At the distal end of the ulna, the styloid process is separated from the shaft by the distal growth engine (Figs. 2–6). The proximal articular facet for the radius is located on the antero-medial aspect of the coronoid process; the distal facet is on the caudo-medial aspect of the distal epiphysis (Fig. 5).

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Figure 1. Caudo-medial view of mouse forearm.

An anterior view of the inner ulnar content from a mid-sagittal plane demonstrates the trabecular bone content of the epiphyses and metaphyses. In the proximal quartile of the ulna, this trabecular bone encompasses most of the olecranon and coronoid processes, with the proximal growth plate separating the epiphysis from the metaphysis. At the distal end it fills the very small metaphysis and the styloid process. The diaphyseal shaft is devoid of trabecular bone and consists only of compact bone cortex, ~130 µm thick at the distal third of the shaft, thinning to ~60 µm at the proximal third (Fig. 6). The radius extends from the articular surface for the humeral capitulum at the radial head ~9.4 mm distally to the tip of the radial styloid process (Figs. 7–11), which articulates with the navicular and lunate. Unlike the ulna, which has its broadest and narrowest dimensions at the proximal and distal ends, respectively, the radius is narrowest at its neck, immediately distal to the radial head (~0.45 mm rostro-caudally and ~0.7 mm latero-medially), and broadest at the distal part of the shaft, immediately proximal to the distal growth engine (~0.9 mm rostro-caudally and ~1 mm latero-medially) (Figs. 7–10). Proximally, the radial head and neck have a cup-like shape. The head, ~0.67 and ~0.9 mm in the rostro-caudal and latero-medial dimensions, respectively, has a collar-like shape, ~0.35 mm in the proxo-distal dimension; its proximal surface is concave, to fit the humeral capitulum (Figs. 7–9). The distal aspect of the collar shows a facet for articulation with the coronoid process of the ulna (Figs. 8 and 10). Facing rostrally on the neck is the radial tuberosity (Figs. 7, 8, 10, and 11). The radial knife edge ridge protrudes laterally from the level of the distal border of the tuberosity to the distal end of the proximal third of the radius (Fig. 7). The proximal nutrient foramen opens rostrally, ~0.5 mm distal to the tuberosity

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Figure 2. Caudo-medial view of mouse ulna.

(Fig. 10). The distal foramen is located ~0.9 mm proximal of the distal growth engine (Figs. 7 and 10). The articular facet for the distal ulnar epiphysis is located on the rostro-lateral aspect of the distal radial epiphysis (Fig. 10). The radius is made almost entirely of cortical bone, ~130 µm thick, with trabecular bone present only in the epiphyses (Fig. 11). The elbow joint at different positions is illustrated in Figure 12. The humeral trochlea articulates with the ulnar semilunar notch. At maximal extension the rostral aspect of the humerus opposes the ulnar olecranon process, and the ventral aspect of the trochlea is against the flat dorsal aspect of the radial head. As the joint changes from extension to flexion, the trochlea and semilunar notch glide against each other. At maximal flexion, the ulnar coronoid process opposes the caudal aspect of the humerus. A dense trabecular network is present in the immediate vicinity of the humeral and ulnar articular cortexes (Fig. 13).

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Figure 3. Rostro-lateral view of mouse ulna. 2D image panels are longitudinal slices demonstrating the path of the ulnar nutrient foramina.

FOREARM (ULNA, RADIUS, AND ELBOW JOINT)

Figure 4. Lateral view of mouse ulna.

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Figure 5. Medial view of mouse ulna.

FOREARM (ULNA, RADIUS, AND ELBOW JOINT)

Figure 6. Left: internal view of mouse ulna from a mid-frontal plane. Right: 2D frontal slice through the same plane demonstrating trabecular, cortical, and growth plate details.

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Figure 7. Left: caudo-medial view of mouse radius. Right: 2D cross-sectional slice demonstrating the path of the radial distal nutrient foramen.

FOREARM (ULNA, RADIUS, AND ELBOW JOINT)

Figure 8. Rostro-lateral view of mouse radius.

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Figure 9. Lateral view of mouse radius.

FOREARM (ULNA, RADIUS, AND ELBOW JOINT)

Figure 10. Right: medial view of mouse radius. Left: 2D frontal slice demonstrating the path of the radial proximal nutrient foramen.

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Figure 11. Left: internal view of mouse radius from a mid-frontal plane. Right: 2D frontal slice through the same plane demonstrating internal details.

FOREARM (ULNA, RADIUS, AND ELBOW JOINT)

Figure 12. Elbow joint from extension to flexion. Left: maximal extension. Middle: midway flexion. Right: maximal flexion. Upper row: rostral view. 2nd row: caudal view. 3rd row: medial view. Bottom: lateral view. H, humerus; R, radius; U, ulna; T, trochlea; C, capitulum; Me, medial epicondyle; Le, lateral epicondyle; Rf, radial fossa; Cf, coronoid fossa; Co, coronoid-olecranon notch; Sc, supinator crest; S, sesamoid bone; O, olecranon; Op, olecranon process; Sn, semilunar notch; Cp, coronoid process; Ut, ulnar tuberosity.

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Figure 13. Two- (left) and three- (right) dimensional views of frontal plane through elbow joint. H, humerus; T, trochlea; Op, olecranon process; Sn, semilunar notch; Cp, coronoid process.

5 MANUS

The manus was scanned at 10-µm voxel resolution. It is composed of three major sections: the carpal, metacarpal, and phalangeal. The carpal, the most proximal of the three, is comprised of ten smaller bony structures arranged in two parallel rows (Figs. 1 and 2). The proximal row includes the fused navicular and lunate as well as the smaller triangular. The navicularlunate and triangular articulate with the radius and ulna, respectively, to form the wrist joint (Figs. 1–4). The distal row consists of the greater and lesser multangulars, falciformis, capitate, centrale, hamate, and pisiform (Figs. 1–4). Distally, the carpus articulates with the five metacarpal bones at the carpometacarpal joint. The 1st metacarpal bone articulates with the greater multangular, the 2nd metacarpus with the lesser multangular and centrale, the 3rd metacarpus with the capitate, and the 4th and 5th metacarpal bones with the hamate (Figs. 1 and 2). The carpal and metacarpal bones are cortical, hollow bones with a minute trabecular component. In sexually mature mice they do not show cartilaginous growth engines (Fig. 5). Further distal, each metacarpal bone articulates with its corresponding phalanx at a metacarpo-phalangeal joint. Each of these joints has a couple of sesamoid bones at their palmar aspect (Figs. 1–4). Each of phalanges 2–5 are divided into the proximal, intermediate, and distal phalangeal bones, which like the other manus bones are mainly hollow with no growth engines. Phalanx 1 has only two parts: the proximal and distal (Figs. 1–4).

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Figure 1. Dorsal view of mouse manus. Orange lines highlight the carpometacarpal and metacarpophalangeal joints.

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Figure 2. Ventral view of mouse manus. Orange lines highlight the metacarpophalangeal joints.

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Figure 3. Medial view of mouse manus.

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Figure 4. Lateral view of mouse manus.

Figure 5. Mid-longitudinal internal view through third mouse metacarpal bones, demonstrating trabecular, cortical, and joint details.

SECTION II

Caudal Appendage

1 PELVIC GIRDLE

The pelvic girdle consists of the ilium and pubis ventrally and the sacrum and ischium dorsally (Figs. 1, 2, and 4). The sacrum is described elsewhere (Part A, Section II, Chapter 4). In the adult mouse the ilium, pubis, and ischium are fused to form one bony unit: the coxae (Figs. 1–4). It was scanned at 10-µm voxel resolution. Together with the sacrum, the ilium comprises the rostral part of the girdle. Its rostral aspect consists of the iliac crest and the rostro-ventral (anterior superior) iliac spine (Figs 1–3 and 5). Caudally, it forms most of the acetabulum, the pelvic component of the hip joint (Fig. 3). Its rostro-caudal dimension, between the acetabular notch and the iliac crest, is ~9.7 mm long (Figs. 1, 3–5). The rostral third of the ilium is shovel-shaped, with its concave aspect facing medially where it articulates with the ala of sacrum. Its convex aspect faces laterally (Figs. 1–5). The respective dorso-ventral and latero-medial dimensions of this part of the ilium are ~2.2 and ~0.5 mm (Figs. 2, 3, and 5). Dorsally, it is bordered by the sciatic notch (Figs. 3–5). Caudal to the notch, the ilium resembles a shaft, ~2.5 mm long and ~1.2 and ~0.8 mm in the dorso-ventral and latero-medial dimensions, respectively (Figs. 3 and 4). Caudal to the shaft-like structure, the ilium increases dorso-ventrally and latero-medially to form the iliac spine, acetabulum and iliopectineal eminence (Figs. 1–5). The acetabulum is a crater-like socket facing laterally that articulates with the femoral head. Its inner diameter is ~1.3 mm. The outer diameter at the crater margin is ~1.7 mm (Fig. 3). The crater-like structure protrudes 0.6–0.75 mm from the lateral iliac surface (Figs. 1, 2, and 4). The crater wall opens caudo-ventrally to form the ~0.33 mm wide acetabular notch (Fig. 3). Two ridges project from the crater margin: the iliopectineal eminence, which bulges laterally and ventrally, and a ridge projecting rostrally and ventrally of which the rostral end is the iliac spine (Figs. 1–3 and 5). The pubis ventrally and the ischium dorsally form the caudal part of the pelvic girdle and the perimeter of the obturator foramen, a large skeletal void in the coxae, ~4.3 and ~1.5 mm in the rostro-caudal and dorso-ventral dimensions, respectively (Fig. 5). The pubis consists of the ascending and descending rami, with a ~30º angle between them. The convex aspect of the angle is at the rostral end of the symphysis pubis that connects the right and left coxae (Figs. 1 and 3). The ~3 mm 153

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Figure 1. Ventral view of mouse pelvic girdle.

long ascending ramus forms the caudal part of the iliopectineal eminence and a small sector of the caudal border of the acetabulum, including the acetabular notch (Figs. 1, 3, and 5). In a mid ventro-lateral-dorso-medial plane the ascending ramus is 0.25 mm thick (Fig. 2). Its widest dimension is ~1.85 mm along a line connecting the iliopectineal eminence with the dorsal border of the acetabular notch (Fig. 3). Its latero-medial dimension at the level of the angle of the pubic angle is ~1 mm (Fig. 1). The ischium is a flattened bone with an average thickness (latero-medially) of 0.35 mm. It consists of a body, ramus, and tuberosity (Figs. 3–5). The rostral end of the body includes the caudal section of the acetabular margin, where the dorsoventral dimension measures ~1.25 mm. It then increases in size to reach ~2.5 mm near its caudal end (Fig. 3). The ischial ramus extends from this region ~2.5 mm ventrally to fuse with the descending ramus of pubis. Its maximal (rostro-caudal) dimension in this region is ~0.8 mm (Figs. 1, 3, and 5). The ischial tuberosity is a ~1.25-mm long ridge that protrudes ~0.15 mm from the caudal border of the ischium (Figs. 1, 3, 4, and 5). The coxal structure includes a major trabecular component that exhibits a marked symmetry between the right and left sides (Fig. 6). The trabecular network is particularly dense at the acetabular region (Figs. 6 and 7), where the skeleton is subjected to higher mechanical loads. The trabeculae are enveloped by a 90–170-µm thick cortex (Figs. 6 and 7).

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Figure 2. Rostro-ventral view of mouse pelvic girdle.

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Figure 3. Lateral (left) view of mouse pelvic girdle.

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Figure 4. Dorsal view of mouse pelvic girdle.

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Figure 5. Medial view of mouse coxae.

PELVIC GIRDLE

Figure 6. Slice through a frontal plane showing internal view of the mouse coxae at the level of the dorsal border of the acetabular notch.

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Figure 7. Left: internal view of the mouse coxae from a sagittal plane at the level of the third trochanter. Right: 2D slice through the same plane demonstrating trabecular and cortical details.

2 FEMUR AND HIP JOINT

The femur was scanned at 10-µm voxel resolution. It extends from the proximal tip of the greater trochanter, distally to the medial and lateral condyles that comprise the femoral aspect of the knee joint (~11 mm in length, Figs. 1–4). Its maximal latero-medial dimension is at the level of the proximal metaphysis between the lateral aspect of the greater trochanter and the medial aspect of the femoral head (~3 mm, Figs. 1 and 4). The greatest rostro-caudal (antero-posterior) dimension at this level is between the distal tip of the lesser trochanter and the rostral (anterior) aspect of the proximal metaphysis (~1.65 mm, Fig. 3). The maximal width of the femoral shaft is between the lateral-most edge of the third trochanteric ridge and the medial aspect of the shaft proper (~2 mm, Figs. 1 and 4). The narrowest part of the femur is at the mid-shaft (~1.5 mm lateromedial diameter; ~1 mm rostro-caudal diameter, Figs. 1–4). The maximal dimensions of the distal part of the femur are at the level of the distal femoral metaphysis (~2.3 mm in either rostro-distal or latero-medial dimension, not including the fabellae, Figs. 1–4). The key anatomical landmarks at the proximal third of the femur are the trochanters, the intertrochanteric crest, which connects the greater and lesser trochanters, the femoral head (half a sphere, ~1.4 mm in diameter), and the femoral neck (~1 mm long, 0.8 mm diameter, Figs. 1–5). The head and neck are separated by the proximal growth plate, which appears as a cribriform, ~130-µm wide collar surrounding the proximal end of the neck (Figs. 1 and 4). The fovea, the entry into the femur of the ligament of head that contains the acetabular branch of the obturator artery, is a cribriform concavity, ~0.4 mm in diameter, on the rostro-medial aspect of the femoral head (Figs. 1, 2, and 5). A couple of nutrient foramina (each ~70 µm in diameter), which connects the periosteal surface with the medullary space, open at the medial aspect at the level of the lesser trochanter (Fig. 2). Another nutrient foramen opens at the caudo (postero)-medial surface approximately from the distal femoral end (Figs. 2–4). The distal growth plate forms a 140–300-µm wriggling collar, separating the shaft from the condyles, intercondylar fossa and the articular surface for the patella (Figs. 1–4, and 6). Its postero-lateral and medial aspects form part of the articular surface for the fabellae, approximately 0.4-mm diameter sesamoid bones that modulate the path of the tendons at the origin of the heads of the triceps surae. The condyles are sep161

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Figure 1. Rostral view of mouse femur.

arated rostro-ventrally (antero-inferiorly) by the articular surface for the patella and caudo-ventrally (postero-inferiorly) by the intercondylar fossa (Figs. 1 and 4). An anterior view of the inner femoral content from a frontal plane, at the level of the third trochanter, demonstrates the trabecular bone content of the epiphyses and metaphyses. In the proximal end of the femur, this trabecular bone encompasses most of the femoral head and neck, greater and lesser trochanters, and intertrochanteric crest. It also fills most of the distal third of the femur with the distal growth plate separating the epiphysis from the metaphysis. The diaphyseal shaft is devoid of trabecular bone and consists only of ~170-µm thick compact bone cortex (Fig. 7). Figure 8 comprises 2D images to highlight the distal femoral cartilaginous growth engine (growth plate) and adjacent bone structures. The 3D geometry of the growth engine is that of a central proximal bulge that corresponds roughly to the intercondylar fossa. The peripheral portions of the engine flank distally. The non-calcified layer of the cartilage, ~25 µm thick, is bordered distally by the epiphyseal trabeculae. On its proximal side is the zone of provisional calcification, an approximately 50-µm thick, partially mineralized layer bordered proximally by the primary spongiosa, another 15–35-µm thick layer comprised of a highly mineralized, irregular trabecular structure. These structures, as well as the proximal tibial growth engine (see Part B, Section 2, Chapter 3), are shown in great detail because they are often used as hallmarks for determining a reference window for morphometric measurement of the distal femoral metaphysis, perhaps the most common site for bone morphometry in the mouse.

FEMUR AND HIP JOINT

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Figure 2. Medial view of mouse femur. 2D image panels are cross-sectional slices demonstrating the path of the proximal nutrient foramina.

The hip joint at different positions is illustrated in Figure 9. At maximal extension the caudal two thirds of the femoral head are completely enclosed within the acetabulum. As the joint flexes, the femur passes through a position at 90º-angle to the ilium. At this position the acetabulum embraces the ventral two thirds of the femoral head. At maximal flexion the femoral shaft is almost parallel to the ilium, with the rostro-ventral half of the femoral head within the acetabulum.

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Figure 3. Lateral view of mouse femur.

FEMUR AND HIP JOINT

Figure 4. Caudal view of mouse femur. 2D image panel is a cross-sectional slice demonstrating the path of the distal nutrient foramen.

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Figure 5. Dorsal view of mouse femur.

Figure 6. Ventral view of mouse femur.

FEMUR AND HIP JOINT

Figure 7. Left: internal view of the mouse femur from a frontal plane at the level of the third trochanter. Right: 2D frontal slice through the same plane demonstrating trabecular, cortical, and growth plate details.

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Figure 8. Sections through the mouse distal femoral cartilaginous growth engine. Left: mid-frontal slice demonstrating the engine and adjacent structures. Right: cross-sectional slice through the distally flanking, peripheral ends of the engine, at the level of the zone of provisional calcification.

FEMUR AND HIP JOINT

Figure 9. Hip joint from extension to flexion. Left: maximal extension. Middle: standing position. Right: maximal flexion. Upper row: lateral view. 2nd row: medial view. 3rd row: rostral view. Bottom: caudal view. Fs, femoral shaft; Fn, femoral neck; H, head of femur; Gt, greater trochanter; Lt, lesser trochanter; Tt, third trochanter; Il, ilium; P, pubis; Is, ischium.

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3 TIBIO-FIBULAR COMPLEX AND KNEE JOINT

In the mouse, the tibia and fibula are fused along most of their distal half (Figs. 1–4, 7, and 8). Therefore, they are referred to as a single bony component, the tibio-fibular complex (TFC). The TFC scan was carried out at 10-µm voxel resolution. The TFC extends from the medial and lateral tibial condyles and head of the fibula, which comprise the TFC aspect of the knee joint, to the lateral and medial malleoli, which articulate with the talus and calcaneum (~15.5 mm in length, Figs. 1–4). The maximal TFC horizontal dimension is at the level of maximal convexity of the tibial ridge where the distance between medial border of the tibia and the lateral border of the fibula is ~3.7 mm (Figs. 1 and 4). The maximal tibial width is the latero-medial diameter at the plane through the distal-most ends of the growth plate (~2.35 mm, Figs. 1 and 4). In spite of the conventional terminology referring to this growth plate as the "proximal tibial growth plate," the present µCT scans in sexually mature mice failed to identify a "distal growth plate." Thus, the proximal growth plate is referred to as just the "growth plate." The same also applies to the fibula. The greatest latero-medial dimension of the fibula is at the distal border of the head (~0.75 mm, Figs. 1 and 4). The maximal rostro-caudal dimension of the TFC is at the plane of the distal end of the fibular growth plate (~2.6 mm, Figs. 2 and 3). The maximal fibular rostro-caudal dimension is also at this level (~1.35 mm, Fig. 3), while the maximal tibial rostro-caudal diameter is distal to its growth plate (~1.5 mm, Fig. 2). The narrowest TFC dimension is the medio-lateral diameter at the level of the fused shaft (~0.75 mm, Figs. 2 and 7). The maximal dimension of the distal part of the TFC is at the level of the malleoli (~1.8 mm in the rostro-caudal dimension, Fig. 1), with more or less equal contributions of the tibia and fibula (Fig. 7). The key anatomical landmarks at the proximal zone of the TFC are the tibial condyles and the fibular head (Fig. 5). Posteriorly, the condyles are separated by the intercondylar fossa, with a central nutrient foramen near its rostral end. A couple of additional foramina is present approximately in the middle of horseshoe-shaped articular surface. Based on 2D images, these foramina may serve for either passage of vessels or as ligament attachments (Fig. 5). In both the tibia and fibula, the growth plate forms a ~170-µm wide cribriform collar separating the tibial condyles and fibular head from their shafts (Figs. 1–4). Further distally are the tibial and fibular proximal nutrient 171

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Figure 1. Rostro-lateral view of mouse TFC.

foramina opening onto the caudal and rostral surfaces, respectively (60–140 µm in diameter, Figs. 2 and 3). Landmarks at the distal zone are the distal tibial crest (Figs. 1 and 2), the distal tibial nutrient foramen (~25 µm in diameter, Fig. 3), and the malleoli. The fusion between the tibia and fibula extends approximately 6 mm from the distal end of the interosseous space (~9 mm from the proximal TFC end, Figs. 1, 4, and 7). In the midst of the fusion zone the fibula and tibia are almost indistinguishable from each other. The tibial medullary space is obliterated along most of the fusion zone. The tibia and fibula become separated again only ~0.5 mm from the distal TFC end (Fig. 7). The tibial longitudinal growth apparatus is reminiscent of the distal femoral growth zone. However, unlike the femur, where the metaphyseal trabeculae do not demonstrate a particular orientation, here they are generally oriented more or less parallel to the long axis of the tibia (Fig. 8). Other than a minimal amount of trabecular bone at the proximal fibular metaphysis and distal ends of tibia and fibula, the remaining medullary space is devoid of trabecular bone (Fig. 8). Figure 9 presents 2D images to highlight the proximal tibial cartilaginous growth engine (growth plate) and adjacent bone structures. The 3D geometry of the growth engine is wavy, corresponding roughly to the tibial articular surface of the knee joint. The non-calcified layer of the cartilage, ~25 µm thick, is bordered proximally by the epiphyseal trabeculae. On its distal side is the zone of provisional calcification, an approximately 50-µm thick, partially mineralized layer bordered distally by the primary spongiosa, another 15–35-µm thick layer comprised of a highly mineralized, irregular trabecular

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Figure 2. Rostral view of mouse TFC. 2D image panel is a cross-sectional slice demonstrating the path of the fibular nutrient foramen.

structure. These structures, as well as the distal femoral growth engine (see Part B, Section II, Chapter 2), are shown in great detail because they are often used as hallmarks for determining a reference window for morphometric measurement of the proximal tibial metaphysis, a site for bone morphometry in the mouse. The knee joint at different positions is illustrated in Figure 10. In all mice scanned, both menisci are calcified at their outer aspect. The menisci articulate with the femoral and tibial condyles. At maximal extension they are located opposite the rostral portion of all four condyles and glide caudally as the joint flexes. The caudal gliding is relative to both the femoral and tibial condyles. As the joint changes from extended to flexed positions, the patella and its femoral articular surface glide against each other, with a relative patellar traveling path from the most superior aspect of the surface to the femoral intercondylar fossa. As the knee joint flexes, the medial fabella changes position relative to its femoral articular surface from caudal to rostral; the lateral fabella "rolls" in an opposite direction. At the most flexed position, the lateral fabella travels away from the femur to make contact with the lateral tibial condyle and head of the fibula. Opposite the menisci, the femoral and tibial epiphyseal trabeculae are oriented at right angles to the articular surface, along the line of load; at the intercondylar fossae the trabeculae do show a patterned alignment (Fig. 11).

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Figure 3. Caudal view of mouse TFC. 2D image panels are cross-sectional slices demonstrating the path of the tibial nutrient foramina.

TIBIO-FIBULAR COMPLEX AND KNEE JOINT

Figure 4. Caudo-lateral view of mouse TFC.

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Figure 5. Dorsal view of mouse TFC. 2D image panels are cross-sectional slices demonstrating the path of the nutrient foramina.

Figure 6. Ventral view of mouse TFC.

TIBIO-FIBULAR COMPLEX AND KNEE JOINT

Figure 7. Left: horizontal slices through fusion zone of mouse TFC. Numbers in slice headings indicate distances in mm from reference horizontal plane through rostral edge of fusion. Red asterisks mark the fibula. Right: slice position (transparent yellow); black line indicates position of reference line.

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Figure 8. Left: internal view of mouse TFC from a frontal plane at the level of the proximal tibial nutrient foramen. Right: 2D frontal slice through the same plane demonstrating trabecular, cortical, and growth plate details.

TIBIO-FIBULAR COMPLEX AND KNEE JOINT

Figure 9. Sections through the mouse proximal tibial cartilaginous growth engine. Left: mid-frontal slice demonstrating the engine and adjacent structures. Right: cross-sectional slice through the distally flanking, peripheral ends of the engine, at the level of zone of provisional calcification.

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Figure 10. Knee joint from extension to flexion. Left: maximal extension. Middle: midway flexion. Right: maximal flexion. Upper row: rostral view. 2nd row: caudal view. 3rd row: medial view. Bottom: lateral view. Mm, medial meniscus; Lm, lateral meniscus; Mfc, medial femoral condyle; Lfc, lateral femoral condyle; Mtc, medial tibial condyle; Ltc, lateral tibial condyle; P, patella; Ps, patellar articular surface of femur; Fif, femoral intercondylar fossa; Mf, medial fabella; Lf, lateral fabella; M, medial fabellar articular surface of femur; L, lateral fabellar articular surface of femur; F, head of fibula.

TIBIO-FIBULAR COMPLEX AND KNEE JOINT

Figure 11. Two- (left) and three- (right) dimensional views of frontal plane through knee joint. Mm, medial meniscus; Lm, lateral meniscus; Mfc, medial femoral condyle; Lfc, lateral femoral condyle; Mtc, medial tibial condyle; Ltc, lateral tibial condyle; Fif, femoral intercondylar fossa; Dfe, distal femoral epiphysis; Pte, proximal tibial epiphysis.

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4 HINDFOOT

The hindfoot (pes) was scanned at 10-µm voxel resolution. It is composed of three major sections: the tarsal, metatarsal, and phalangeal. The tarsus, the proximal of the three, is comprised of eight smaller bony structures arranged in two parallel rows (Figs. 1 and 2). The proximal row includes the larger calcaneum and talus, the latter forming the ankle joint with the distal end of the tibia (Figs. 3 and 4), and the smaller tibiale (Figs. 1, 2, and 4). The distal row comprises the three cuneiform bones (Figs. 1, 2, and 4), the navicular, and the cuboid (Figs. 1–4). Distally, the tarsus articulates with the five metatarsal bones at the tarsometatarsal joint. The 1st, 2nd, and 3rd metatarsal bones articulate with the medial, intermediate, and lateral cuneiforms, respectively. The 4th metatarsus is articulated mainly with the cuboid and the 5th forms its main articulation with the 4th (Figs. 1 and 2). The tarsal and metatarsal bones are cortical, hollow bones with a minute trabecular component. In sexually mature mice they do not show cartilaginous growth engines (Fig. 5). Further distal, each metatarsal bone articulates with its corresponding phalanx at a metatarsophalangeal joint. Each of these joints has a couple of sesamoid bones at its plantar aspect (Figs. 1–4). Each of phalanges 2–5 is divided into the proximal, intermediate, and distal phalangeal bones, which like the other pes bones is mainly hollow with no growth engines. Phalanx 1 has only two parts: proximal and distal (Figs. 1–4). Figure 6 shows the spatial relationship of the tarsal and metatarsal bones. Proximally, at the proximal part of the tarsus, the tibiale, cuboid, and calcaneum form a row. The talus lies dorsal to the cuboid and dorso-medial, and dorso-lateral to the calcaneum and tibiale, respectively. Further distally the tarsal and then metatarsal bones are organized in one medio-lateral row.

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Figure 1. Dorsal view of mouse hindfoot. Orange lines highlight the tarsometatarsal and metatarsophalangeal joints.

HINDFOOT

Figure 2. Ventral view of mouse hindfoot. Orange lines highlight the tarsometatarsal and metatarsophalangeal joints.

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Figure 3. Medial view of mouse hindfoot. Orange line highlights the tarsometatarsal joint.

Figure 4. Lateral view of mouse hindfoot. Orange line highlights the tarsometatarsal joint.

HINDFOOT

Figure 5. Mid-longitudinal internal view through third mouse metatarsal bone, demonstrating trabecular, cortical, and joint details.

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Figure 6. Frontal slices through mouse heel. Numbers in slice headings indicate distances in mm from reference frontal plane (indicated by black line) through anterior edge of calcaneum. I–V, metatarsal bones; M-c, medial cuneiform; I-c, intermediate cuneiform; L-c, lateral cuneiform; C-d, cuboid; N, navicular; C, calcaneum; Ta, talus; Ti, tibia.

PA RT C

Murine Comparative Microanatomy

1 STRAIN DIFFERENCES

Differences between strains are presented for 8-week-old females of the C57Bl/6J, SJL, and C3H inbred mouse strains. These strains were selected because they are in widespread use in skeletal research. Female mice of this age are at, or toward, the end of their steep skeletal growth phase/peak bone mass (for details, see Part C, Chapter 2). The morphometric parameters were assessed in femora, the most commonly used skeletal site for µCT analyses. The overall femoral parameters are used here to represent general skeletal growth and development. The overall femoral size was greatest in C57Bl/6J mice, reflecting their longest disto-proximal and radial dimensions (Figs. 1 and 2). The overall femoral size of SJL and C3H mice is similar (Fig. 1). Although SJL bones are slightly shorter than C3H bones (Fig. 1), they are wider, as demonstrated by their greater diaphyseal diameter (Fig. 2). By contrast, the overall bone content, measured as either the overall bone volume or overall bone density, is highest in C3H mice (Fig. 1), resulting from both thickest cortex (Fig. 2) and highest trabecular bone volume (Fig. 3). C57Bl/6J and SJL animals have similar overall bone volumes, although C57Bl/6J overall bone density is lower, because the overall volume in these mice is greater (Fig. 1). Strain-related differences in the femoral cortex are demonstrated in a mid-diaphyseal representative segment. Both the diaphyseal and medullary cavity diameters are greatest in C57Bl/6J and smallest in C3H mice (Fig. 2), indicating a similar strain-related variation in the expansion of the cortical envelope. The cortex is thickest in C3H mice, with similar values presented by C57Bl/6J and SJL animals (Fig. 2). In the femoral metaphysis, all trabecular bone parameters are highest in C3H mice (Fig. 3). Other than the trabecular number, which is similar in C57Bl/6J and SJL mice, C57Bl/6J animals present the lowest trabecular bone parameters (Fig. 3). The gender-related variations in trabecular thickness and number are reflected in trabecular bone volume density and connectivity. 191

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Figure 1. Gender-related changes in overall femoral dimensions of 8-week-old C57Bl/6J, SJL, and C3H female mice. Boxes represent mean – 1SD to mean + 1SD interval.

STRAIN DIFFERENCES

Figure 2. Gender-related changes in mid-diaphyseal femoral dimensions of 8-week-old C57Bl/6J, SJL, and C3H female mice. Boxes represent mean – 1SD to mean + 1SD interval.

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Figure 3. Gender-related changes in trabecular parameters in distal femoral metaphysis of 8-week-old C57Bl/6J, SJL, and C3H female mice. Boxes represent mean – 1SD to mean + 1SD interval.

2 GENDER AND AGE DIFFERENCES

Dynamic changes in overall bone mass and in cortical and cancellous structure consist of initial rapid alterations, followed by a prolonged phase of substantially slower changes or sustained steady state. Most parameters show higher or similar values in males as compared to females (Figs. 1–4). Cortical parameters are presented for the femoral mid-diaphyseal segment; trabecular parameters were assessed in both the distal femoral metaphysis and third lumbar vertebra. These sites are commonly used for the respective measurements. The overall femoral parameters are used here to represent general skeletal growth and development. In the females, the steep growth phase in overall femoral size, defined by the external femoral envelope, lasts for 8 weeks. During this time the femoral size is similar in females and males. Thereafter, growth continues at a slower linear rate (Fig. 1). In males, the rapid growth phase continues for 2 additional weeks, resulting in increased overall size compared to the females. The male growth curve past the peak size parallels that of the females (Fig. 1). One component of femoral size, femoral length, is similar in males and females. However, the rapid phase lasts for 8 weeks in females and 12 weeks in males. The similarity therefore results from continued slow growth in females and maintenance of the peak length in males (Fig. 1). The difference in overall size between males and females is therefore the consequence of higher femoral diameter in males. In both genders, the steep growth in femoral diaphyseal diameter continues for 13 weeks. Subsequently, males present further linear growth at a lower pace and the female diaphyseal diameter remains at the same size (Fig. 2). The steep increase in absolute overall bone volume, which includes all mineralized femoral components, ends during weeks 8 and 13 in females and males, respectively. In both genders it is then followed by mild linear growth, resulting in a higher male bone volume after week 10 (Fig. 1). The trend of changes in femoral bone volume is similar to that of overall femoral volume, except that in bone volume the rapid growth phase is somewhat steeper and the slow phase is milder (Fig. 1). Consequently, the overall femoral bone volume density, which presents overlapping dynamic changes in males and females, slightly declines after reaching a peak at week 12 (Fig. 1). 195

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Figure 1. Curves describing age-related changes in overall femoral dimensions of female and male C57Bl/6 mice based on measurements at 4, 8, 13, 17, and 40 weeks of age. Curves represent mean – 1SD to mean + 1SD interval. Light gray: males; dark gray: females.

Age-related alterations in the femoral cortex are demonstrated in a mid-diaphyseal representative segment. The 13week rapid growth phase is similar for all mid-diaphyseal parameters. In both males and females the cortical thickness doubles from weeks 4 to 13, consequent to a small increase in diaphyseal diameter, which results from periosteal bone formation, and a substantial decrease in the medullary cavity diameter that results from a net gain in endosteal bone (Fig. 2). During the initial phase, the growth in cortical thickness is the same in females and males, in spite of differences in the rate of increase in diaphyseal and medullary cavity diameters. This is because the diaphyseal diameter grows faster in males while the reduction in medullary cavity diameter is faster in females (Fig. 2). Changes in cortical thickness during the steady-state period are minimal as a consequence of balanced, slow radial growth, consisting of net gains in diaphyseal and medullary cavity diameters resulting from respective net increases in bone formation and resorption (Fig. 2). In the femoral metaphysis, all trabecular bone parameters are higher in males than in females. In females, the peak in the trabecular density and number is at 6–7 weeks of age. In males, the peak in trabecular density and number is at weeks 12 and 10, respectively (Fig. 3). Past the peak, these parameters decline linearly. In particular, female trabecular density reaches a very low level at week 40 (Fig. 3). Trabecular thickness and connectivity density exhibit different overall trends. Trabecular thickness increases rapidly until age 12 weeks in males and 15 weeks in females. Thereafter, the males and females show a very slight increase and decrease, respectively (Fig. 3). In males, the peak connectivity density is at week 13, followed by a sharp decrease toward week 20 and a milder decline thereafter. In females, connectivity density demonstrates a sharp decline from weeks 4 to 13. Although the further decline that follows is milder, connectivity density becomes almost negligible by week 40 (Fig. 3).

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Figure 2. Curves describing age-related changes in femoral diaphyseal dimensions of female and male C57Bl/6 mice based on measurements at 4, 8, 13, 17, and 40 weeks of age. Curves represent mean – 1SD to mean + 1SD interval. Light gray: males; dark gray: females.

Except for trabecular number, the differences between male and female trabecular bone are minimal in the lumbar vertebral body. Following a steep increase from weeks 4 to 12, trabecular bone volume density and trabecular thickness exhibit a steep decrease toward week 15, followed by a milder decrease thereafter (Fig. 4). In males, the vertebral trabecular number declines linearly from weeks 4 to 40. Female trabecular number, as well as male and female connectivity density, show a steep decrease from weeks 4 to 13, with a milder decrease thereafter (Fig. 4).

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Figure 3. Curves describing age-related changes in trabecular parameters in distal femoral metaphysis of female and male C57Bl/6 mice based on measurements at 4, 8, 13, 17, and 40 weeks of age. Curves represent mean – 1SD to mean + 1SD interval. Light gray: males; dark gray: females.

GENDER AND AGE DIFFERENCES

Figure 4. Curves describing age-related changes in trabecular parameters in lumbar vertebrae (L3) of female and male C57Bl/6 mice based on measurements at 4, 13, 17, and 40 weeks of age. Curves represent mean – 1SD to mean + 1SD interval. Light gray: males; dark gray: females.

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Index

A Acetabular notch, 153–154, 156, 159 Acetabulum, 153–154, 160, 163 Acromion process, 113–115, 117–118, 120, 124, 130 Ala of sacrum, 79, 80–82, 153–154, 157, 159 Alveolar foramina, 34 Alveolar ridge, 11, 31–33 Ampullae, 26 Anapophysis, 73–76, 78 Angular process, 27, 31–32, 34 Ankle joint, 183 Anterior ethmoidal foramen, 6 Anterior inferior iliac spine, 154–155, 159 Anterior lambdoid suture, 8, 14, 17 Anterior sciatic spine, 154, 159 Anterior tubercle, 41–43, 45–46, 51, 53 Apex of sacrum, 80, 82, 154 Arcuate eminence, 20–21, 15, 19 Atlas, 14–15, 18, 41–46, 63 Auditory ossicles, 21–24 Axis, 18, 41–44, 47–54, 63

B Basisphenoid 8–10, 13, 15, 18–21 Body of vertebra of axis, 42–44, 48–49, 53–54 of caudal vertebrae, 87–92 of cervical vertebrae, 44, 54–57, 59–60, 62–63, 65 of lumbar vertebrae, 73–76, 78, 197 of sacral vertebrae, 79–80, 83–84, 156 of thoracic vertebrae, 67–71

C Calcaneum, 171, 176, 184–188 Calvaria, see skull Canal carotid, 15 hypoglossal, 15 incisive, 7, 10 infraorbital, 8 mandibular, 31–32, 35, 37 neural, see neural canal optic, 13

Capitate, 145–146, 148–149 Capitulum, 123–128, 132, 143 Carotid canal, 15 Carotid tubercle, 44, 59–65 Carpal bones, 145 Carpometacarpal joint, 145–146 Caudal vertebrae, 87–92 Central nutrient foramen, 171 Centrale, 145–146 Chassaignac's tubercle, 44, 59–65 Clavicle, 113–115, 124, 130 Cochlea, 21–26 Common limb, 15, 19, 25–26 Conchae, 7–8 Condylar process, 27, 31–32, 36, 37 Condyle of mandible, 14–15, 18, 32, 38 lateral, of femur, 163–166, 173, 180–181 lateral, of tibia, 171, 173–176, 178, 180–181 medial, of femur 161–166, 173, 180–181 medial, of tibia, 171, 173–176, 178, 180–181 occipital, 7–9, 15, 17–18 Coracoid process, 114, 117–120, 124, 130 Coronal suture, 8, 13–14, 17 Coronoid fossa, 123–124 Coronoid process of mandible, 27, 31–32, 34, 37 of ulna, 131–137, 143–144 Coronoid-olecranon notch, 123–124, 127, 129, 143 Costal dorso-ventral joint, 96–97 Costovertebral joint, 97 Coxae, 153–154 Cranium, see skull Cribriform plate of ethmoid, 6–7, 12, 14–16 Cricoid cartilage, 27–28 Crista galli, 13, 16 Cuboid, 183–188 Cuneiform bones, 183–188

D Deltoid tuberosity, 114, 123–127, 129 Diarthrodial joint, 28, 30 Distal nutrient foramen, 123, 126, 131 of femur, 163, 165 of tibia, 172, 174 Distal tibial crest, 172

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202

E Ear, 21–22, 24–26 Elbow joint, 131, 133, 143–144 Ethmoid bone, 5, 11–13, 18, 20 Eustachian tube, 19, 20–21 External acoustic meatus, 9, 21–22 External auditory meatus, 22 External ear, 21–22, 24 Extravertebral foramen, 43, 50, 58, 62, 64, 70, 75, 77, 79–80, 85, 91, 154

F Fabellae, 161 Falciformis, 145–148 False ribs, 95–96 Femoral condyles, 173 Femoral intercondylar fossa, 173, 180–181 Femur, 124, 161–167, 173 Fibula, 171–176, 178 Fibular nutrient foramen, 173 Fibular proximal nutrient foramen, 171–172 Floating ribs, 95–96 Foramen anterior ethmoidal, 6 extravertebral, 43, 50, 58, 62, 64, 70, 73, 75, 77, 79–80 incisive, 7 infraorbital, 8, 11 internal acoustic, 21 inter-pterygoid, 9, 13, 20–21 intervertebral, 43, 50, 55, 58, 64, 70–71, 74, 77, 79–80, 84–85, 154–155, 157 lateral pterygoid, 20 mandibular, 27, 32 magnum, 7, 9, 14–15 mental, 27, 31, 34, 37 nutrient, see nutrient foramen obturator, 153–157 optic, 6, 13, 15, 20 ovale, 9, 15, 20 posterior lacerated, 9, 15, 21 posterior palatine, 9, 10, 12 postglenoid, 9, 15, 18–19 rostral lacerated, 13, 15, 19–20 rostral palatine, 5, 9, 10 sphenopalatine, 6, 8–9, 11 transeptal, 31, 35–36 transverse, see transverse foramen ventral sternebral, 97, 99, 102 Foramen magnum, 7, 9, 14–15 Foramen ovale, 9, 15, 20 Forearm, 131–144 Fossa coronoid, 123–124, 143 glenoid 9, 13, 18, 32 infraspinous, of scapula, 117, 119–120 intercondylar, of femur, 161–162, 165–166, 173, 180, 181 intercondylar, of tibia, 171, 173–176, 178 olecranon, 123, 127 pterygoid, 9, 19, 20, 27, 32 radial, 124–143 scaphoid, 7, 9, 19–21

INDEX suprascapular, 117–118, 120 supraspinous, of scapula, 117, 119–120 Fovea, 161–163, 166 Frontal bone, 8–9, 11–14, 18–19 Frontal spine, 7, 8, 11–14

G Glenoid cavity, 117–118, 120, 124 Glenoid fossa, 9, 13, 18, 32 Greater horn of hyoid, 27–30 Greater multangular, 145–146, 148 Greater trochanter, 161–167, 169 Greater tubercle, 123–124, 126–128, 130 Growth engine (plate) of femur, 161–162, 167–168, 172–173 of humerus, 123–124, 129 or radius, 132–133, 138–142 of tibio-fibular complex (TFC), 171–172, 178–179 of ulna, 131–137 of vertebra, 73, 78

H Hamate, 145–149 Helicotrema, 19, 22, 26 Hindfoot, 183–188 Hip joint, 153, 163, 169 Humerus, 113–115, 123–130, 133, 143–144 Hyoid, 27–30 Hypoglossal canal, 15

I Iliac crest, 153 Iliac spine, 153, 156–157 Iliopectineal eminence, 153–154, 156, 158 Ilium, 79–81, 84, 153, 155–156, 158, 160, 163, 169 Incisal dental papilla, 31 Incisal socket, 5, 7–8, 31–32 Incisive canal, 7, 10 Incisive foramen, 7 Incisor teeth, 5, 7, 9–10, 12, 16–17, 31–36 Incus, 21–23, 25 Inframandibular foramina, 27, 31–33 Infraorbital canal, 8 Infraorbital fissure, 6, 9, 15 Infraorbital foramen, 8, 11 Infraspinous fossa, 117, 119–120 Intercondylar fossa of femur, 161–162, 165–166 of tibia, 171, 173–176, 178 Intercostal joints, 95–96 Internal acoustic foramen, 21 Internal acoustic meatus, 15, 19, 23–24, 26 Internal ear, 25–26 Interosseous space of forearm 132 of tibio-fibular complex 172–174, 177 Interparietal bones, 8, 13–14, 17, 19 Interpterygoid foramen, 9, 13, 20–21 Intertrochanteric crest, 161–163, 165

MICROCOMPUTED TOMOGRAPHIC ATLAS OF THE MOUSE SKELETON Intertubercular sulcus, 123–124 Intervertebral foramen, 43, 50, 55, 58, 64, 70–71, 74, 77, 79–80, 84–85, 154–155, 157 Ischium, 153–159, 169

J Joint ankle, 183 elbow, 131, 133, 143–144 hip, 153, 163, 169 knee, 161, 173, 180–181 shoulder, 113, 117, 123–130 wrist, 145

Medial fabella, 163–165, 173, 180 Medial malleoli, 171–176, 184, 186 Medial meniscus, 181 Medial pterygoid process, 9, 13, 19–20 Mental foramen, 27, 31, 34, 37 Metacarpal bone, 145–149 Metacarpophalangeal joint, 145–147 Metacromion process, 117, 119, 120 Metatarsal bone, 183–188 Metatarsophalangeal joint, 183–186 Middle ear, 21–22, 24, 26 Molar teeth, 7–9, 11, 13–14, 16, 31–36 Mylohyoid groove, 32

N K Knee joint, 161, 173, 180–181

L Labyrinth, 7, 11–12, 14, 17, 22, 26 Lacrimal bone, 6, 8–9 Larynx, 27 Lateral condyle of femur 163–166 of tibia 174–176, 178 Lateral epicondyle, 123–124, 127, 143 Lateral fabella, 162–167, 173, 180 Lateral malleoli, 171–176, 184, 186 Lateral meniscus, 180–181 Lateral pterygoid foramen, 20 Lateral pterygoid process, 9, 13, 19–20 Lesser horn of hyoid, 27–30 Lesser multangular, 145–146, 148 Lesser trochanter, 161–166, 169 Lesser tubercle, 123–125, 128–130 Lumbar vertebrae, 73–78, 195, 197, 199 Lunate, 132, 146–148

M Malleus, 19, 22–24 Mandible, 27, 31–37 Mandibular canal, 31–32, 35, 37 Mandibular condyle, 18, 32 Mandibular foramen, 27, 32 Manubrium, 96–95, 98–102, 104–105, 113–115 Manus, 145–149 Masseteric ridge, 27, 31 Masseteric tuber, 27, 31 Maxilla, 5–13, 15, 18, 20 Maxillary sinus, 7–9, 11 Maxillary zygomatic process, 6, 8, 18 Maxillary-frontal suture, 12 Maxillo-basisphenoid suture, 10, 13, 16 Maxillo-palatine suture, 10, 12 Medial condyle of femur, 161, 163–166 of tibia, 174–176, 178 Medial epicondyle, 123–125, 127, 143

Navicular, 132, 146–149, 184–188 Neural arch of atlas, 41, 43 of axis, 43 of caudal vertebrae, 87, 92 of cervical vertebrae, 41–43, 54–55, 59–60 of lumbar vertebrae, 73–74 of sacral vertebrae, 79 of thoracic vertebrae, 67–68, 103–104 Neural canal of atlas, 41–44, 46, 52, 54 of axis, 42–44, 47–48, 50–52, 54 of caudal vertebrae, 87 of cervical vertebrae, 41, 54–55, 58–60, 64 of lumbar vertebrae, 73–74 of sacral vertebrae, 79, 84 of thoracic vertebrae, 67–68, 70, 103–104 Nutrient foramen in femur, 161, 163, 165, 171 in fibula, 171–173 in humerus, 123–128 in radius, 132–133, 138, 141 in tibia, 171–172, 174, 178 in ulna, 131–135 Nasal crest, 10–12 Nose, 5–12 Nasopharynx, 8, 20 Naso-frontal-ethmoidal junction, 11–12, 14 Nasal bone, 5, 6–11, 13–16 Nasal cavity, 5, 7–8, 11–12, 17, 20 Nasal concha, 6–11, 15–16 Nasal septum, 7, 11, 16

O Obturator foramen, 153–157 Occipital bone, 8–9, 14–15, 17–21 Occipital condyle, 7–9, 15, 17–18 Occipital crest, 6, 8, 14, 17–18 Odontoid process, 43–44, 47, 49–53 Olecranon, 131–137, 143 Olecranon fossa, 123, 127 Olecranon process, 132–137, 143–144 Optic canal, 13 Optic foramen, 6, 13, 15, 20

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204 Oropharynx, 20 Otic capsule, 22–24, 28 Oval window, 22–24

P Palate, 5, 10 Palate-basisphenoid suture, 10 Palatine bone, 5, 7, 9–10, 12, 14 Parietal bone, 8, 13–14, 17–19 Parietal suture, 13–14 Paroccipital processes, 6, 14, 21 Patella, 161–162, 166, 173, 180 Pelvic girdle, 153–160 Periodontal ligament space, 10, 31, 34–36 Periotic capsule, 6, 14, 18, 21 Perpendicular plate, of ethmoid, 7, 11–12, 16 Petrotympanic fissure, 15, 20 Phalangeal bones, 145–149, 183–188 Pharyngeal tubercle, 9, 15 Pharyngotympanic tube, 22 Pisiform, 145, 147, 149 Posterior lacerated foramen, 9, 15, 21 Posterior lambdoid suture, 14, 17 Posterior palatine foramen, 9–10, 12 Posterior tubercle, 41–45 Postglenoid foramen, 9, 15, 18–19 Postzygapophysis of axis, 42, 47–48, 51 of cervical vertebrae, 42, 44, 55–57, 59, 61–63, 65 of thoracic vertebrae, 68–71, 104 of lumbar vertebrae, 73–76, 78 of sacral vertebrae, 80–81, 83, 157 of caudal vertebrae, 87–88 Premaxilla, 5–11, 13, 15 Premaxilla-maxilla suture, 10–11, 16 Premaxillary-frontal suture, 11 Presphenoid bone, 7, 9, 12–13, 15, 19–20 Prezygapophyses of cervical vertebrae, 44, 54, 56–63, 65 of thoracic vertebrae, 67–71, 103 of lumbar vertebrae, 73–76, 78 of sacral vertebrae, 80–81, 83–84, 154, 157 of caudal vertebrae, 87–90 Promontory, 23, 26, 80, 154, 159 Proximal nutrient foramen, 123, 125, 131–132, 163, 174, 178 Pterygoid fossa, 9, 19–20, 27, 32 Pterygoid processes, 19–20 Pubic symphysis, 153–155 Pubis, 153–154, 156, 158, 169

R Radial fossa, 124, 143 Radial ridge, 132, 138 Radial tuberosity, 132, 138–139, 141–142 Radius, 131–132, 138–143, 145–149 Ramus of mandible, 153–154 Rib-vertebral joints, 95–98 Rostral lacerated foramen, 13, 15, 19–20 Rostral palatine foramen, 5, 9, 10 Round window, 23, 25

INDEX

S Saccule, 26 Sacral vertebrae, 80–85 Sacrum, 79–85, 153 Sagittal suture, 8, 13, 17 Scala tympani, 19, 23–24, 26 Scala vestibuli, 19, 23–24, 26 Scaphoid fossa, 7, 9, 19–21 Scapula, 113–114, 117–121, 124 Scapular spine, 117, 119–121 Sciatic notch, 153, 158 Sella turcica, 7, 13, 19–20 Semicircular ducts, 19, 23, 26 Semilunar notch, 131–137, 143–144 Shoulder joint, 113, 117, 123–130 Skull, 3–38 Sphenoido-squamosal suture, 13 Sphenopalatine foramen, 6, 8–9, 11 Spinous process of axis, 42, 47, 49–50, 63 of caudal vertebrae, 87, 89–91 of cervical vertebrae, 44, 52, 54–61, 64–65 of lumbar vertebrae, 73–78 of sacral vertebrae, 80–81, 83–85, 156–157 of thoracic vertebrae, 67–71, 103–104 Spiral lamina, 23–24, 26 Squamosal bone, 5–6, 8, 14, 18, 21 Squamosal zygomatic process, 18, 38 Stapes, 21–26 Sternal-rib joints, 95–98 Sternebra, 95–97, 99–102 Sternum, 95–102, 113 Styloid process of radius, 138, 140–141 of ulna, 131–137 Subscapular fossa, 117–118, 120 Supinator crest, 123–124, 126–127, 143 Supraspinous fossa, 117, 119–120 Supraspinous process, 117, 119–120 Suture anterior lambdoid, 8, 14, 17 coronal, 8, 13–14, 17 maxillary-frontal, 12 maxillo-basisphenoid, 10, 13, 16 maxillo-palatine, 10, 12 palate-basisphenoid, 10 parietal, 13–14 posterior lambdoid, 14, 17 premaxilla-maxilla, 10–11, 16 premaxillary-frontal, 11 sagittal, 8, 13, 17 sphenoido-squamosal, 13 temporal, 6, 8, 13–14, 18 Symphysis menti, 27, 32 Symphysis pubis, 153–155

T Talus, 171, 176, 183–188 Tarsal bone, 183 Tarsometatarsal joint, 183–186

MICROCOMPUTED TOMOGRAPHIC ATLAS OF THE MOUSE SKELETON Temporal suture, 6, 8, 13–14, 18 Temporomandibular joint, 32, 38 Third trochanter, 162, 164–167, 169 Thoracic vertebrae, 67–71, 95 Thorax, 93–108 Thyroid cartilage, 27–28 Tibia, 171–179, 183, 186–188 Tibiale, 183–186 Tibio-fibular complex (TFC), 171 Transeptal foramen, 31, 35–36 Transverse foramen of atlas, 41–46 of axis, 47–48, 50–51, 53 of caudal vertebrae, 90 of cervical vertebrae, 54–55, 59–60, 64 of lumbar vertebrae, 75–76 of thoracic vertebrae, 69 Transverse ligament, 42–43, 45–46 Transverse line, 79–80, 83 Transverse processes of atlas, 41–44, 46 of axis, 50 of cervical vertebrae, 58, 64 of thoracic vertebrae, 67–71, 103–104 of lumbar vertebrae, 73–76, 78 of sacral vertebrae, 79–82, 85, 154–155, 157 of caudal vertebrae, 87–91 Triangular, 145–149 Trochanteric fossa, 162–163, 165 Trochlea, 123–125, 127–128, 131, 133, 143–144 True ribs, 95–96 Tympanic bulla, 5–7, 9, 14, 18, 20–23 Tympanic ridge, 19, 21–22

U Ulna, 131–137, 143–145 Ulnar coronoid process, 133 Ulnar nutrient foramina, 134 Ulnar olecranon process, 133 Ulnar ridge, 131, 134, 136 Ulnar shaft, 131–132 Ulnar tuberosity, 131–133, 143 Ulno-lunate joint, 131 Uncinate process, 44, 54, 56–57, 60–62 Utricle, 25–26

V Ventral sternebral foramen, 97, 99, 102 Vertebrae, 41–65 Vertebral column, 39–92, 96–98 Vestibule, 22–25 Vomer, 7, 8, 11

W Wrist joint, 145

X Xiphisternum, 95–102 Xiphoid cartilage, 95–97, 99–102

Z Zygomatic arch, 5, 9, 17–18, 38 Zygomatic bone, 8–9, 14, 18 Zygomatic process, 6, 18

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