Comparative study of ultrastructures of the lateral-line organs and the palatal taste organs in the African clawed toad, Xenopus laevis.

The fine structure of the lateral-line organ and the palatal taste organ in the African clawed toad, Xenopus laevis, was examined by means of electron microscopy. The lateral-line organ consisted of hair and accessory cells. The apical surface of a hair cell was studded with one kinocilium and 20 to 40 stereocilia. Synaptic bodies and subsynaptic cisternae were found in the cytoplasm of a hair cell adjacent to the synaptic contacts with the afferent and the efferent nerve endings, respectively. Crystalline bodies were observed in both the nucleoplasm and the cytoplasm of almost all hair cells. The palatal taste organ consisted of three types of cells: the taste, sustentacular, and the Merkel cells. The taste cells contained numerous dense-cored vesicles which accumulated in close association with both the afferent synapses and the basal plasma membrane. The possibility was raised from the ultrastructural results that these vesicles had dual functions as both neurotransmitter and hormone. The existence of Merkel cells in the palatal taste organ suggested that this organ might function not only as a chemoreceptor but also as a mechanoreceptor. In spite of possible chemosensory function of the lateral-line organ in Xenopus, its ultrastructure was significantly different from that of the typical gustatory organ, the palatal taste organ, in this animal.