Expression of Neural Crest Specifiers During the Two Waves of Trunk NCC Migration in Turtle Embryos.

The bones of the plastron, the ventral portion of the turtle shell, develop through intramembranous ossification, in the same manner as facial bones, suggesting they are also produced by neural crest cells (NCCs). Previous work has demonstrated the existence of a unique second migration of NCCs away from the neural tube of turtle (Trachemys scripta) embryos; these migrate ventrally, appear to be skeletogenic and may contribute to the bones of the plastron. The goal of this project is to examine the expression of neural crest specifiers in the premigratory NCCs located in the neural tube and in migrating NCCs using immunofluorescence and whole mount in situ hybridization. During the switch-over period, the neural crest specifiers Sox9 and Sox10 are both expressed in premigratory NCCs. However, their expression diverges in migrating NCCs; the cells that retain Sox10 are located within the major migratory stream in the rostral somite leading to the production of the dorsal root ganglion and other peripheral nerves, consistent with a neurogenic cell fate. By contrast, cells that retain Sox9 appear to take a novel migratory pathway through the caudal portion of the somite, and are excluded from the dorsal root ganglia, consistent with a non-neurogenic and perhaps skeletogenic cell fate. We are currently examining the expression of transcription factors that have been found to be preferentially expressed in either trunk NCCs or cranial NCCs in the well-characterized chicken (G. gallus) system. Elucidating the molecular control of NCC specification in the unique population of trunk NCCs would further our understanding of its role in the development of the turtle shell.