DAVIS, M.C.*; VOGELI, K.M.; Univ. of California, San Francisco; Univ. of California, San Francisco: Apical ectodermal ridge development in the paired fins of chondrichthyans and basal actinopterygians

In tetrapods, limb outgrowth is maintained by a thickened epithelium along the distal margin of the limb bud called the apical ectodermal ridge (AER). The paired fin buds of teleosts are similarly rimmed by a thickened ectoderm. In contrast to the tetrapod AER, the thickened ectoderm of teleosts elongates distally and folds on itself to form the apical ectodermal fold (AEF) characteristic of fish fins. Fin bud mesenchyme then invades the AEF and contributes to formation of the dermal fin skeleton (actinotrichia and lepidotrichia). Despite the similarities in the molecular mechanisms of limb/fin development there remain questions regarding the functional homology of the AER in tetrapods and teleosts. Complicating this issue, AER/AEF development has not been adequately described for more basal vertebrates. Here we present new data on AER/AEF development in the paired fins of the shark Cephaloscyllium ventriosum, and the basal actinopterygians Polyodon spathula and Lepisosteus osseus. Confocal microscopy reveals that formation of the AER/AEF involves changes in both cell shape and cell polarity. Correlated with these changes are patterns of intracellular reorganization. These include elaboration of the Golgi apparatus and changes in nuclear-cytoplasmic ratio. Reorganization within the cell may reflect initiation of specific developmental programs such as endoskeletal condensation within the bud mesenchyme or secretion of collagens during dermal ray formation within the AEF. When placed in a phylogenetic context, these results offer insight into the evolution of AER/AEF development in derived taxa. Likewise, these observations provide the framework for ongoing functional studies that seek to address how the molecular mechanisms of appendage development affect cell morphology.