Date of Award

Spring 2000

Document Type

Restricted Thesis

Terms of Use

© 2000 Alla K. Brukman. All rights reserved. Access to this work is restricted to users within the Swarthmore College network and may only be used for non-commercial, educational, and research purposes. Sharing with users outside of the Swarthmore College network is expressly prohibited. For all other uses, including reproduction and distribution, please contact the copyright holder.

Degree Name

Bachelor of Arts



First Advisor

Scott F. Gilbert


The turtle shell, an evolutionarily novel structure, is the only vertebrate casing made exclusively of bone. The mechanisms of shell development in turtle embryos have not been studied very extensively. Results presented in this paper suggest that the shell forms by co-opting existing developmental pathways. In situ hybridizations with antisense riboprobes derived from Trionyx spiniferous fibroblast growth factor-l0 (fgflO) and Trachemys scripta fgf8 were performed on sections of early T. scripta embryos. fgflO expression was localized in the mesenchyme ofthe carapacial ridge (CR), which forms the dorsal part of the shell. In the chick limb bud, Fgf10 is known to specify the limb-forming mesenchyme. Fgf10 expression induces Fgf8 expression in the ectoderm of the limb bud and causes limb outgrowth. The localization of fgf10 in the CR supports the hypothesis that the early shell utilizes the same developmental pathway as the early limb. fgf8 expression, however, was only found in the growing turtle limb, indicating that the pathway is not co-opted directly from the limb. Bone/cartilage staining with alizarin red/alcian blue indicated that ossification of the shell begins soon after hatching. In order to determine whether bone morphogenetic proteins (BMPs) play a role in shell ossification, whole-mount in situ hybridizations with recombinant human BMP4 and BMP2 antisense riboprobes were performed on T. scripta hatchlings. The expression of BMP4 in the ventral part of the shell and in ribs embedded in the dorsal part of the shell suggests that BMP4 acts in shell dermis ossification.