Fine structure of the terminal buds on the barbels of some fishes.

The terminal buds (end buds, beaker organs) of the fish were first observed by LEYDIG (1851) under the name of "becherformiges Organ". Already in a few years after that, morphological details of the structure had been elucidated to the extent that reached almost the limit of the light microscopic observation, even estimated today (SCHULZE 1863, SCHWALBE 1868, NAGEL 1894 etc. See KOLMER 1927 for the review). Similar structures were then found in almost all the vertebrate series above fishes, and the functional meaning of them seemed to have been established as the "taste buds". As for the terminal buds of the fish in particular, the general conclusion of various morphological and physiological studies was that they are homologous with the taste buds of higher vertebrates. However, those buds in fishes, which were found both inside and outside of the oral cavity particularly on the barbels were considered to be sensitive to some additional kinds of stimulus other than chemical, and were thus named "Wechselsinnesorgan" (KOLMER 1927). Though considered to be homologous with, the terminal buds of the fish have been found to show some structural differences from the taste buds of higher vertebrates. In the terminal buds of fishes, two kinds of cells (sensory and supporting cells) can be identified rather easily as compared with the cases of higher vertebrates, and nerve fibers, having entered the buds, first make the basal plexus before branching into fine fibers and ascending between the cellular elements (KOLMER 1927). Few intensive works on these structures, except for some genetic and histochemical investigations, had been further attempted, until the electron microscope was applied to the morphological analysis of them. This new approach has given us a large amount of knowledge on the structures in problem, particularly on the apical differentiation of the taste cells and on the neuro-epithelial relationships, although the results thus obtained so far are confined mostly to the taste buds of mammals (TRUJILLO-CENOZ 1957, DE LORENZO 1858, MURRAY and MURRAY 1960, NEMETSCHEK-GANSLER and FERNER 1964, FARBMAN 1965, GRAY and WATKINS 1965 etc.); the terminal buds of the fish have escaped scrutinizing attacks of electron microscopy except for the short report of TRUJILLO-CENOZ (1961) and the quite recent work on the apical region of sensory cells made by DESGRANGES (1965). The present author could find electron microscopically some peculiar structural characteristics to be reported here that have not been pointed out in mammalian taste