Abstract
Facial shape is the basis for facial recognition and categorization. Facial features reflect the underlying geometry of the skeletal structures. Here, we reveal that cartilaginous nasal capsule (corresponding to upper jaw and face) is shaped by signals generated by neural structures: brain and olfactory epithelium. Brain-derived Sonic Hedgehog (SHH) enables the induction of nasal septum and posterior nasal capsule, whereas the formation of a capsule roof is controlled by signals from the olfactory epithelium. Unexpectedly, the cartilage of the nasal capsule turned out to be important for shaping membranous facial bones during development. This suggests that conserved neurosensory structures could benefit from protection and have evolved signals inducing cranial cartilages encasing them. Experiments with mutant mice revealed that the genomic regulatory regions controlling production of SHH in the nervous system contribute to facial cartilage morphogenesis, which might be a mechanism responsible for the adaptive evolution of animal faces and snouts.
Highlights
The shape of a face strongly depends on the geometry of skeletal elements directly under the skin, adipose tissue and muscles
Taking into account known achondroplasia facial phenotypes, we hypothesized that even minor changes in the facial cartilaginous template may lead to significant or even major changes in the overlaying membranous bone geometry and the overall facial shape
We used a dose of 2.5 mg of tamoxifen administered at E12.5 and, in an alternative experiment, double injection at E13.5-E14.5 to avoid a strong phenotype with dramatic face shortening and brain shape distortion, and we analyzed the mutant embryos at E17.5 and E15.5 correspondingly (Figure 1)
Summary
The shape of a face strongly depends on the geometry of skeletal elements directly under the skin, adipose tissue and muscles. Models involving Fgfr do not allow for precise discrimination of cartilage or bone-dependent parts of the phenotype in affected subjects Still, these effects strongly suggest that chondrocranium shape might be truly important for producing initial facial geometry and for influencing the formation of membranous bone on top of the cartilaginous template. The frontonasal prominence and other facial regions are enriched in signaling systems Activity in these systems leads to progressive induction and shaping of craniofacial structures, including chondrogenic mesenchymal condensations that turn into cartilage (Minoux and Rijli, 2010). Expression of Shh in the forebrain turned out to be important for the correct formation of FEZ and early steps of facial shaping in general (Hu et al, 2015a; Chong et al, 2012) Still, how these and other signaling centers synchronize in order to build the 3D shape of facial cartilaginous elements is not understood. Various genomic regulatory regions that direct the expression of Shh to the developing nervous system participate in the fine-tuning of the shape of the facial cartilaginous skeleton
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