A dysmorphic mouse model reveals developmental interactions of chondrocranium and dermatocranium.

The cranial endo and dermal skeletons, which comprise the vertebrate skull, evolved independently over 470 million years ago and form separately during embryogenesis. In mammals, much of the cartilaginous chondrocranium is transient, undergoing endochondral ossification or disappearing, so its role in skull morphogenesis is not well studied and it remains an enigmatic structure. We provide complete 3D reconstructions of the laboratory mouse chondrocranium from embryonic day (E) 13.5 through E17.5 using a novel methodology of uncertainty-guided segmentation of phosphotungstic enhanced 3D micro-computed tomography images with sparse annotation. We evaluate the embryonic mouse chondrocranium and dermatocranium in 3D, and delineate the effects of a variant on embryonic chondrocranial cartilages and on their association with forming dermal bones using the Crouzon syndrome mouse. We show that the dermatocranium develops outside of and in shapes that conform to the chondrocranium. Results reveal direct effects of the variant on embryonic cartilage, on chondrocranium morphology, and on the association between chondrocranium and dermatocranium development. Histologically, we observe a trend of relatively more chondrocytes, larger chondrocytes, and/or more matrix in the embryos at all timepoints before the chondrocranium begins to disintegrate at E16.5. The chondrocrania and forming dermatocrania of embryos are relatively large, but a contrasting trend begins at E16.5 and continues into early postnatal (P0 and P2) timepoints, with the skulls of older mice reduced in most dimensions compared to littermates. Our findings have implications for the study and treatment of human craniofacial disease, for understanding the impact of chondrocranial morphology on skull growth, and potentially on the evolution of skull morphology.