Meeting Abstract

65.5  Wednesday, Jan. 6  Lateral inhibition systems in baleen development HIERONYMUS, TL*; THEWISSEN, JGM; GEORGE, JC; Northeastern Ohio Universities College of Medicine; Northeastern Ohio Universities College of Medicine; Department of Wildlife Management, North Slope Borough thieronymus@neoucom.edu

Baleen is a key adaptation of keratinized palatal tissue that has allowed mysticete cetaceans to specialize as plankton-feeders. The lingual surfaces of major baleen plates are lined by a fringe that grossly resembles hair, but both fringe and the larger plates that contain it are histologically distinct from most other mammalian integumentary appendages. Instead, fringe and baleen plates show the greatest similarity to large keratinized epidermal structures such as horse hoof, bovid horn sheath, and rhinoceros horn. Although the morphological development of baleen has been documented, neither the signaling systems involved nor the tissue-level homologies of baleen are known. Fetal bowhead (Balaena mysticetus) palatal and skin tissues were screened for morphogens that have been implicated in palate and hair follicle development (BMP2/4, SHH, FGF-7&10) by immunohistochemistry. Preliminary results suggest that the initial arrangement of baleen epithelial placodes proceeds by lateral inhibition. An inhibitory signal transmitted by relatively high concentrations of BMP2/4 on the lingual side of the developing baleen ridge may limit the size of the developing baleen placodes in that region. This may lead to the mediolateral morphological gradient of fringe, minor baleen plates, and major baleen plates seen on the posterior palate in adults. Morphological and developmental similarities suggest that baleen may be homologous to typical mammalian palatal structures, with baleen arising as an exaptation of existing palatal morphology in primitive mysticetes.