A second sensory--motor--interneuron with neurosecretory granules in Hydra.

Using serial-sectioning techniques for conventional transmission and high-voltage electron microscopy, we characterized the ultrastructural features and synaptic contacts of the sensory cell in tentacles of Hydra. The sensory cell has an apical specialization characterized by a recessed cilium surrounded by three rodlike stereocilia. This ciliary--stereociliary complex constitutes the receptive or dendritic pole of the sensory cell. The dense filamentous cores of the stereocilia project proximally into a narrow circumciliary cytoplasmic region connected by septate junctions to marginal processes of an enveloping epitheliomuscular cell. The central cilium has a characteristic marginal flare midway along its length and a dense filamentous substructure at its base. Pairs of branched, striated rootlets extend from the axial centriole into a mitochondria-rich region of the cell. Pigment-like granules are present in the cytoplasm around the circumciliary space. The perikaryon is characterized by an elongate nucleus surrounded by a narrow rim of cytoplasm containing prominent Golgi complexes, assorted vacuoles and dense-cored vesicles, free ribosomes, short segments of rough endoplasmic reticulum, microtubules, glycogen particles, and lipid droplets. Generally, one or two thin, naked axons extend laterally from the perikaryon into the nerve net region above the myonemes of the large epitheliomuscular cells. Within the axons are found occasional aggregates of dense-cored vesicles and en passant synapses characterized by the presence of clear or dense-cored vesicles in contact with paramembranous densities and associated intracleft cross filaments. Using these ultrastructural criteria, we demonstrated for the first time that the granule-containing sensory cells have synaptic contacts with other neurons, nematocytes, and epitheliomuscular cells hence, we considered these cells to be sensory--motor--interneurons with neurosecretory granules. We hypothesize that this unique, apparently multifunctional neuron may be a modern representative of a primitive stem cell that give rise evolutionarily to the sensory cells, motor neurons, interneurons, and neurosecretory cells of higher animals.