Date of Award

Spring 2005

Document Type

Restricted Thesis

Terms of Use

© 2005 Paul N. Riccio. 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

Judith Cebra-Thomas

Second Advisor

Scott F. Gilbert


This thesis presents the latest evidence in a string of observations supporting a hypothesis of turtle shell development that alludes to the modularity of paracrine signaling pathways and their evolutionary cooption. Epithelial-mesenchymal interactions give rise to a diverse set of cutaneous appendages in vertebrates. It seems the early ontogeny of the Chelonian carapacial ridge-the epithelial-mesenchymal interaction responsible for the dorsoventral position of the ribs in turtles-is similar to that of scutate placodes and feather buds. Here, I report the expression of Shh and β-catenin in the ecotdermal tissue of the carapacial ridge of T. scripta embryos. Analysis of turtle explants with disrupted Fibroblast Growth Factor signaling, reveals that members of this family of paracrine factors are necessary for maintenance of proper carapacial ridge morphology. Also discussed is the possibility of visualizing the patterning of somite derivatives in these FGF-disrupted embryos, using riboprobes to Pax9 and antibodies to Pax7 in histological analyses. Through comparison with better characterized epithelial-mesenchymal interactions, a possible ontogeny of signaling events for the induction of the carapacial ridge is hypothesized. Several evolutionary implications are drawn, including a hypothesis of the role of various paracrine signals in the divergent patterns of dermal ossification observed in hard and soft-shelled turtles.