Examining the Correlation Between Cellular and Gross Morphological Changes In the Developing Mouse Skull
Authors: Jarvis Sommer G.E. , Lee Jae Eun , Kelly Gregory M. , Willmore Katherine E. 
Published: June 2021

INTRODUCTIONThe mammalian skull develops from intramembranous and endochondral ossification. Thus, differentiation and proliferation of osteoblasts and chondrocytes play an integral role in skull development and the resulting morphology. While this connection between cellular behaviour and morphology is intuitive, the relationship between changes at the cellular level and gross phenotypic changes in the skull are still poorly understood. In this study we ask: How do changes in osteoblast and chondrocyte differentiation and proliferation within facial sutures and cranial base synchondroses affect the gross morphology of the mouse skull?We use a mutant mouse model (Cx43I130T/+) wherein the GJA1 gene encoding for the gap junctional protein Connexin43 has been mutated, reducing gap junctional conductance by 50%. Cx43 is critical in bone development, and is known to stimulate osteoblast differentiation. Presently, the literature is vague surrounding the effects of Cx43 on chondrocytes. Cx43I130T/+ mice display craniofacial anomalies, such as smaller facial bones, elongated nasal cartilages and microcephaly. Preliminary data from our lab shows that the cranial base is also altered in Cx43I130T/+ adult mice. We predict that these morphological changes in the face and cranial base will be mirrored by altered cellular behaviour in the facial sutures and cranial base synchondroses.METHODSSkulls of P0 Cx43I130T/+ mice and their wildtype (WT) littermates were imaged using μ‐CT. Three‐dimensional landmarks were collected from scans and differences in shape were analyzed using geometric morphometrics. Paraffin sections were taken from skulls of each genotype and stained for histomorphometry and markers of cell proliferation and osteoblast and chondrocyte differentiation. Measures of cell density were collected for facial sutures and cranial base synchondroses and compared between groups.RESULTSWhile we found no significant differences in skull size between genotypes, we did find significant shape differences. Mutant mice have proportionately smaller facial bones and orbits and relatively larger basicrania compared to WT mice. Mutant skulls are also severely under‐mineralized. There is no difference in osterix‐positive cell density in facial sutures between groups, but we did find an altered pattern of osteocalcin expression in mutant facial sutures. Total area of each synchondrosis is similar for both genotypes, but there are significant differences in resting zone area of each synchondrosis between groups. Cell density of Sox9‐positive cells within the synchondroses are the same for mutant and WT mice.CONCLUSIONSOur shape analyses indicate that the morphological changes observed in adult Cx43I130T/+ mice are present at birth. While we did find some differences in osteoblast and chondrocyte profiles between groups, the link between cellular changes at the tissue level and gross phenotypic changes is not straightforward.Support or Funding InformationThis work is supported by NSERC.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.