RUI: Roles of Paracrine Factors in Formation of the Turtle Shell

Swarthmore College, Swarthmore PA

Investigators

Abstract

0316025 Gilbert The turtle shell is a remarkable evolutionary novelty. No other extant vertebrate has a casing made almost exclusively of bone, and no other vertebrate has its limbs forming within its ribs. The turtle shell is composed of two main parts, the dorsal carapace and the ventral plastron. Between them, on the lateral sides, is a bridge. The carapace contains fifty bones and is supported by the ribs, which grow dorsolaterally rather than ventrally. The major effort of this project will expand earlier studies of carapace development. These earlier studies found (1) that the costal bones that form the bony plate of the carapace grow out from the ribs in both directions, (2) that the costal bones begin to form after the endochondral ossification front of the rib has passed that region of the dermis, and (3) that the bones around the anterior ribs form earlier than those of the posterior ribs. The current project seeks to test whether the ribs induce the ossification of the carapacial dermis and the formation of the bony plate of the carapace. Specifically, it tests the hypothesis that the ossification of the turtle dermis is a by-product of paracrine factors secreted by the ribs as they undergo endochondral ossification. The first part of this developmental scenario contends that as the ribs undergo their normal endochondral ossification, the Indian hedgehog that they secrete during this process would induce the dermal cells around them to express and secrete BMP2/4/7. The second part of this scheme is that the Bone Morphogenetic Proteins would act in both paracrine and autocrine fashions to ossify the dermis, causing a progressive front of intramembranous ossification in the dorsal dermis of the developing turtle. To test this, the turtle homologues for the msx2, ihh, bmp2, bmp4, bmp7, fgf8, and noggin genes would be cloned and their expression analyzed by in situ hybridization. Second, beads or cells secreting the BMP-inhibitory protein, Noggin, would be implanted into hatchling carapaces to see if this inhibition would prevent turtle shell formation. Conversely, BMP-containing beads would be implanted into the dorsal dermis to see if they could induce precocious bone development in other parts of the carapacial dermis. If this hypothesis is validated, this research has wide-ranging implications. First, it would go very far to explaining the "age-old" question of "how the turtle got its shell." Second, this research would show that the developmental program used to form the turtle carapace is a consequence of putting rib cartilage into new positions in the body. Third, the results of this research will aid in the reconstruction of the phylogenetic path leading to the "sudden" emergence of turtles in the late Triassic. The data would provide a mechanism for the rapid development of the carapace (as inferred by the fossil record). Pedagogically, if this hypothesis were correct, this research would provide an excellent and readily understood example of developmental co-option in vertebrates. This work is especially relevant for the training of biology students and the integration of teaching and research. During the past two years, the project has benefited from the talents of ten undergraduates, one graduate student, and three postdoctoral fellows. These have included several women, as well as persons from disabled and under-represented minority groups. The first year of this program will be undertaken at the Thomas Jefferson University School of Medicine, where the Orthopedics Research group is studying the role of hedgehog and BMPs in jaw development. During the second two years this research will be performed at Swarthmore College (during the academic year) and Thomas Jefferson University (during the summers). .