The fine structure of fungiform papillae and epithelium of the tongue of a South American toad, Calyptocephalella gayi

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Abstract
The epithelium covering the surface of the tongue of the South American toad Calyptocephalella gayi has been analyzed using large electron microscope photo‐montages and has been compared with that covering the tongue of the frog, Rana pipiens. In both species four kinds of epithelia are similarly distributed over the surface, and relatively minor differences in the cytoplasm and intercellular relations have been identified. Stratified ciliated epithelium covers the ventral, nonpapillary surface. A simple columnar epithelium of mucous cells covers most of the irregular dorsal surface. A poorly stratified nonkeratinizing epithelium covers the apex of filiform papillae while stratified, columnar epithelium forms the thick epithelial disc of fungiform papillae. Emphasis is given to fungiform papillae as the basis for functional studies since they are the only structures receiving large numbers of nerve fibers. Sensory terminals do not appear to be acted on directly by chemicals since they are located near the base of the epithelium and since tight junctions unite cells at the outer surface. However, axons selectively contact the base of certain elongate cells which extend to the surface and are presumed to be receptor cells. It seems probable that recent intracellular potential changes, recorded from the toad tongue in response to chemical stimulation, originate from mucous cells. It is suggested that these potentials could affect the receptor cells if low resistance pathways are provided by tight junctions.