Carapacial Ridge and Costal Bone Formation in the Carapace of the Red-eared Slider Turtle, Trachemys Scripta

The turtle shell represents a unique and extreme adaptation of the tetrapod body plan. Previous\ud research characterized the major developmental steps in the formation of the dorsal carapace: a novel structure termed the carapacial ridge develops along the dorsolateral flank of approximately stage 15 (Greenbaum, 2002) turtle embryos (Gilbert et. al., 2001, Loredo et. al., 2001). The ribs extend dorsolaterally rather than proceeding ventrally as they do in all other tetrapods and enter this carapacial ridge. Together, these structures form the carapacial disc that will become the shell. Morphologically and histologically, the carapacial ridge resembles the limb bud (Burke, 1989) leading to the hypothesis that the turtle has co-opted the programs of limb development, namely the positive feedback loop between FgflO and Fgf8, for the formation of the carapacial ridge. Loredo et. al. (2001) localized turtle FgflO expression to the mesenchyme of the carapacial ridge of Greenbaum stage 15 Trachemys scripta\ud embryos. Here, we continue the exploration of the limb hypothesis by testing the role of Fgfs in the maintenance of the carapacial ridge though explant culture with an Fgf receptor inhibitor, SU5402. Treatment appears to block formation of the carapacial ridge and alter the migration pattern of the scleretome-derived rib precursor cells. We have also localized Fgf8 expression to the distal tips of the ribs as they enter the carapacial ridge, around Greenbaum stage 17, leading to the hypothesis that the positive feedback loop between Fgf8 in the rib and FgflO in the carapacial ridge mesenchyme maintains outgrowth of the carapacial disc. Ossification of the dermally derived novel bones of the shell proceeds post-hatching. We hypothesize that the normal endochondral ossification of the ribs induces bone formation in the competent dermal cells surrounding them. Antibody staining specific to activated Smadl, a Bmp activated transcription factor, points towards a key role in dermal ossification for Bmp4. These data taken together suggest an overall mechanism for the formation of the carapace involving the redirection of the ribs by the carapacial ridge into a dorsolateral position followed by consequent dermal ossification