Merkel cells are responsible for the initiation of taste organ morphogenesis in the frog

Taste organs in the frog have a distinctive cell type located exclusively i n the basal portion. In the same fashion as type III cells in mammalian tas te buds, these basal cells show immunoreactivity for serotonin antibody. Fu rther, these cells are morphologically similar to epidermal Merkel cells. T o determine the significance of these serotonergic basal cells, we examined the early development of taste organs during metamorphosis of the frog by focusing on the origin and possible roles of serotonergic basal cells. For convenience of description, five stages of development (metamorphic stage t o climax stages A-D) are defined. In the metamorphic stage, a few noninnerv ated Merkel cells appear at the upper layer of the lingual epithelium. No n euronal elements are seen in the epithelium at this stage. At climax stages A-B, immature fungiform papillae become discernible in the dorsal surface of the tongue, where the Merkel cells are located. Merkel cells then move d ownward and extend their cytoplasmic processes toward the basal lamina. The se cells are identified by their intense immunoreactivity for serotonin. Du ring the later stages, many nerve fibers in the subepithelial connective ti ssue approach the epithelium containing Merkel cells. At climax stages C-D, Merkel cells extend cytoplasmic processes along the basal lamina toward th e center of the newly forming fungiform papillae. The morphology of these M erkel cells exactly coincides with that of serotonergic basal cells in adul t taste organs. Profuse exocytotic release of dense-cored granules of Merke l cells toward the nerve fibers through the basal lamina is frequently seen in these stages. The present study indicates that serotonergic basal cells are derived from intraepithelial Merkel cells, which act as target sites f or growing nerves and may be responsible for the initiation of taste organ morphogenesis. J. Comp. Neurol. 406:129-140, 1999. (C) 1999 Wiley-Liss, Inc .