LEWIS, K.K.*; CEBRA-THOMAS, J.A.; GILBERT, S.F.; Swarthmore College; Swarthmore College; Swarthmore College: The expression of signaling molecules in the developing scute of the turtle Trachemys scripta
The reptilian scale is one of the major adaptations that allowed early reptiles to colonize the land. Yet reptile scales, especially the chelonian scute, are all but unknown in terms of the molecular signals that drive their formation. While the formation of cutaneous appendages in the avian and mammalian lineages have been studied in detail, little more than morphological studies of reptilian scales have been done in the past. Given that the avian feather is thought to have evolved from the reptilian scale, it seems likely that these two structures are homologous. In this study our approach has been to look in the developing chelonian scute for signaling molecules that are important in the developing feather placode. Some of the best known signaling molecules involved in early feather bud formation are Sonic hedgehog (SHH), members of the fibroblast growth factor family (FGFs) and the Wnts. Nuclear &beta-catenin (an indicator of Wnt signaling) is one of the earliest markers of feather epithelial placode formation. Consistent with this observation, nuclear &beta-catenin was detectable in the marginal scute epithelium of stage 17 turtle embryos, coincident with the earliest morphological changes. The requirement for Wnt signaling appears to be transient, as nuclear &beta-catenin was not observed in the marginal scutes of either stage 16 or 18 embryos. In addition, SHH is expressed in each of the developing marginal scutes in the stage 17 embryo.