Date of Award

2002

Document Type

Restricted Thesis

Terms of Use

© 2002 Günes Bender. 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

Department

Biology Department

First Advisor

Scott F. Gilbert

Abstract

Most of the dermal bones of the face and cranium arise from the cranial neural crest cells. Most of the intramembranous bones of the vertebrate exoskeleton have been assumed to be derived from neural crest cells, although it has not been experimentally shown to be true (Smith and Hall, 1993). Recent evidence shows that some of the posterior bones in the human cranium are actually not neural crest derivatives (Jiang et al., 2002). It was hypothesized in this study that the new bones of the turtle shell, which are also formed by exoskeletal intramembranous ossification come from the neural crest cells. Neural crest involvement in the formation of turtle shells has also never been shown. We studied whether the nine bones of the plastron as well as the nuchal bone of the carapace synthesized molecules characteristic of neural crest cells. Turtles are the only extant organisms with an extensive exoskeleton in the trunk region, which is partially made up of intramembranous bones. The intramembranous bones of the turtle shell consist of the nuchal and peripheral bones in the carapace and all of the plastron bones (Figure. 1). Both transverse and sagittal sections were made from the shells of 50 and 55 day old Trachemys scripta embryos. First, these sections were stained with Mallory's trichrome stain in order to locate the intramembranous ossification centers, and then they were immunohistostained against HNK-1 and PDGFRα. HNK-1 is a marker for neural crest cells and PDGFRα is a marker specifically for bone-forming neural crest cells. In both experiments, the intramembranous bones of the turtle plastron and the carapace were stained positively, which endorsed the hypothesis that the intramembranous ossification of dermal bones of the turtle shell involves neural crest cells.

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