Embryonic and early postnatal cranial bone volume and tissue mineral density values for C57BL/6J laboratory mice

Abstract

Background: Laboratory mice are routinely used in craniofacial research based on the relatively close genetic relationship and conservation of developmental pathways between humans and mice. Since genetic perturbations and disease states may have localized effects, data from individual cranial bones are valuable for the interpretation of experimental assays. We employ high-resolution microcomputed tomography to characterize cranial bones of C57BL/6J mice at embryonic day (E) 15.5 and E17.5, day of birth (P0), and postnatal day 7 (P7) and provide estimates of individual bone volume and tissue mineral density (TMD).

Results: Average volume and TMD values are reported for individual bones. Significant differences in volume and TMD during embryonic ages likely reflect early mineralization of cranial neural crest-derived and intramembranously forming bones. Although bones of the face and vault had higher TMD values during embryonic ages, bones of the braincase floor had significantly higher TMD values by P7.

Conclusions: These ontogenetic data on cranial bone volume and TMD serve as a reference standard for future studies using mice bred on a C57BL/6J genetic background. Our findings also highlight the importance of differentiating "control" data from mice that are presented as "unaffected" littermates, particularly when carrying a single copy of a cre-recombinase gene.

Keywords: bone mineralization; bone structure; cranial ossification centers; development.

© 2022 The Authors. Developmental Dynamics published by Wiley Periodicals LLC on behalf of American Association for Anatomy.

Figures

FIGURE 1

Presence/absence of each cranial bone by age. Points are randomly jittered (falsely separated) along the x‐axis at the represented age group to enable visualization of the value for each specimen, with six individuals represented in each age group

FIGURE 2

Average volume by age for each examined cranial bone. When an ossification center was not present, volume was recorded as 0. Y‐axis values are separately specified for each bone due to the wide variance in upper values (ranging from 0.12 mm3 for the zygomatic bone to over 3.5 mm3 for the mandible)

FIGURE 3

Average TMD by age for each examined cranial bone. When an ossification center was not present, TMD was recorded as 0. Y‐axis values are separately specified for each bone due to the wide variance in upper values (ranging from less than 250 mg/cm3 for the squamous occipital and nasal bones to over 400 mg/cm3 for the tympanic ring and zygomatic bones)

FIGURE 4

Heatmaps of average bone volume across chronological age, with bones grouped by tissue origin (mesoderm v. CNCC), ossification process (endochondral v. intramembranous), and cranial module (face, braincase floor, and vault). *P < .05; **P < .01

FIGURE 5

Heatmaps of average TMD across chronological age, with bones grouped by tissue origin (mesoderm v. CNCC), ossification process (endochondral v. intramembranous), and cranial module (face, braincase floor, and vault). *P < .05; **P < .01

FIGURE 6

Illustration of cranial bones analyzed for this study at each age group. Top row: left lateral view, with the rostrum facing left. Bottom row: superior endocranial view, with the rostrum facing left and the superior portion of the cranial vault removed. Bones that cross the midline were fully segmented. For bilaterally occurring bones, only bones of the left side of the skull were segmented. Bones appearing in gray were not segmented