CHEMOSENSORY CELLS IN THE ORAL EPITHELIUM OF RAJA-CLAVATA (CHONDRICHTHYES)

Examination by scanning and transmission electron microscopy (SEM and TEM) has found no actual taste buds in the mouth of Raja clavata. Prominences of the epithelium on the roof and floor of the mouth, and on the oral valves, contain large numbers of innervated bipolar cells, not associated in the form of taste buds, with a cytology intimating that they have a chemosensory function. The apices of these sensory cells, each bearing a group of microvilli, protrude between the superficial epithelial cells. Neurite profiles are associated with the sensory cells; synaptic specializations are marked by a cluster of vesicles with inconspicuous dense cores and some densities on the cell membrane. Shrunken, electron-dense, cell profiles are interpreted as apoptotic. Shrunken sensory cell profiles are commoner than similar epithelial cells, especially in young individuals, indicating a relatively rapid turnover of sensory cells. The epithelium contains a variety of granulocytic leucocytes, some of which contain large phagosomes.