Variations of concentric hair cells in a Cnidarian sensory epithelium (Coryne tubulosa)

In capitate hydropolyps, the spherical end-knobs of the short tentacles present an exceptional concentration of sensory functions in one of the evolutionarily oldest nervous systems. The tentacular spheres are the basis of sensation and discrimination of objects and of capturing of prey-objects by the discharge of nematocytes (stinging cells). Recent electrophysiological studies of the spheres revealed combined chemo/mechanosensory functioning of the nematocytes and mechanosensitivity of further types of cells. The present electron microscopical study made use of the small size of the spheres of Coryne tubulosa to characterize all cells of some spheres. Five types of ectodermal cells were found to have sensory structural features and to be separated by or enclosed in supporting cells: 1) nematocytes of the stenotele type; 2) short and 3) long ciliated concentric hair cells, which carry a cilium-stereovilli bundle, similar to the cnidocil apparatus of nematocytes; 4) cells having a recessed cilium-microvilli complex equipped with a thick cell-traversing rootlet (rootlet cells); and 5) cells having a recessed short cilium with no microvilli and only a short rootlet and containing, apically as well as basally, aggregations of dense-core vesicles (vesicle-rich cells). Types 1-4 vary the configuration of a concentric cilium-microvilli complex (variations of a concentric hair bundle) and were demonstrated or inferred to be mechanosensitive. Apical exocytotic activity, which is well known for the nematocytes (discharge of their cnidocyst), is indicated by ultrastructure for the nematocyte-resembling concentric hair cells and for the vesicle-rich cells. The tentacular spheres are considered an early paradigm of a sensory epithelium. Its synaptic structures and extensive connectivity are the subject of a subsequent paper.