The terminal buds from three species of fresh water fishes (carp, cat-fish and loach) were examined electron microscopically. The buds consist of three types of cells and of nerve elements innervating the buds.
1. The receptor cells are characterized first by the presence of specialized electron-lucid apical processes which show wide variety in dimension and form. The supranuclear cytoplasm contains numerous electron-dense tubules of 400-600Å in diameter, sometimes running straight along the axis of cells, or sometimes presenting tortuous or spiral appearances. The infranuclear portion has abundant vesicles, 300-1000Å in diameter. Some of these vesicles assemble to the particular site of contact of the cell with nerve element and resemble those in the synaptic regions in the central nervous system.
2. The supporting cells contain abundant filaments through the whole extent of their cytoplasm, from just below the free surface with microvilli, to the fine sheets which are formed by branching of the infranuclear part of the cell and intervene the various elements in the basal portion of the bud.
3. The basal cell is placed at the bottom of the bud directly on the basement membrane and contains numerous small and large vesicles. Some of the larger ones contain dense cores in them, while not a few of the smaller ones are accumulatad to the specialized site of contact with neural element, presenting seemingly the same situation as was found between the receptor cells and the neural element.
4. The myelinated fibers in the core of the barbels branch into finer unmyelinated fibers which proceed into the raised papilla of the dermis underneath the bud to reach the base of the latter. After having entered the bud, the fibers seem to divide into finer branches, which, forming the intragemmal plexus, have a specified contact with either the receptor cells or the basal cells. On the bases of these findings, possible functional implications of the morphological constituents of the terminal buds were discussed.