GILBERT, S.F.*; LOREDO, G.A.; BRUKMAN, A.; TUAN, R.: Studies in Turtle Shell Development: Osteogenesis of an Evolutionarily Novel Structure

The turtle shell is an evolutionary novelty, a development of the osseous skeleton unique among vertebrates. The turtle shell is composed of a dorsal carapace, a ventral plastron, and a lateral bridge. The carapace contains fifty unique bones, while the plastron contains nine bones. In the carapace, the most dorsal bones are formed by the neural portion of the spine, while the costal bones are formed from the dermis between the ribs. The marginal bones form along the edges of the shell mesoderm. The ribs become trapped in the carapacial ridge (CR), a structure resembling the limb bud. We show that the mesoderm of the CR expresses fgf10. FGF10 is found in the limb bud where it acts as an agent of cell proliferation and chemotaxis. In the carapace, the neural bones and the nuchal bone are the first new bones to be seen. Meanwhile, the anterior ribs show an expanded region of bone around them, and the ribs appear to be inducing new bone from the dermis around them.By 120 days, the ossifying dermal regions between the ribs have coalesced. The future plastron can be identified by nine ossification centers in the ventral dermis. No alcian blue staining is seen presaging these sites. The three ossification centers corresponding to the three anterior plastron bones will fuse first, and the six marginal ossification centers of the plastron grow toward each other and fuse. Condensed mesenchyme presages the advance of these spicules. The bony spicules cross the midline, but avoid fusing until later in development. We are presently attempting to identify the paracrine and transcription factors regulating the development of these evolutionary novel structures.