BENDER , G; TAN, F; SISTLA, S; ESTES, E; CEBRA-THOMAS, J; GILBERT, S. F. ; Swarthmore College; Swarthmore College; Swarthmore College; Swarthmore College; University of Pennsylvania; Swarthmore College: The turtle shell as co-opting developmental programs of the vertebrate head

The 50 new bones of the turtle shell superficially resemble facial and calvarial bones in their appearance and in the sutures between the bones. Our recent studies have demonstrated several deeper similarities. First, like much of the skull and face, the nine plastron bones, the nuchal bone, and the peripheral bones form by intramembranous ossification in the dermis. Second, we show here that these intramembranous bones appear to arise from both neural crest and mesodermal components. The developing plastron and nuchal bones stain for the neural crest cell markers HNK-1 and PDGFR-alpha, whereas the developing peripheral bones do not. Third, we show that the costal bones that comprise the majority of the carapace form in a manner reminiscent of the mandible. Just as mandibular bone forms around Meckel’s cartilage, so do the costal bones form around each rib. We hypothesize that during their endochondral ossification, the ribs secrete Indian Hedgehog, which then induces bone morphogenetic proteins (BMPs) in the adjacent dermis. These BMPs would then induce bone formation. Thus, the event placing the rib within the dermis would have enabled the normal development of the rib to activate genes in the dermis that otherwise would not have been expressed. We are presently examining the similarities of costal bone and mandible development and present preliminary evidence in support of this model.