Apical surface of hydrozoan nematocytes: Structural adaptations to mechanosensory and exocytotic functions

At the apical cell pole of the nematocytes of a hydrozoan (Hydra vulgaris), cytoskeletal elements of different molecular families (microfilaments, microtubules, cross-striated rootlets) are combined in a complex framework. As determined by transmission and scanning electron microscopy, the stimulus-transducing cnidocil apparatus of these mechanosensitive cells is forced into a lateral position, facilitating the close apposition of the nematocyst to the apical cell membrane. The nematocyst, a single, extraordinarily large, exocytotic organelle, is held in position by a microtubular basket. The cnidocil apparatus and microtubular basket are linked to an ellipsoid arrangement of pseudovilli, i.e., small surface protrusions containing cross-striated rods. These nematocyte-specific, cytoskeletal elements mediate the anchorage of the entire cytoskeletal framework to the apical cell membrane. The apical membranes of the nematocyte and nematocyst are separated by a distance of only ∼ 50 nm. Structural modifications on the external side of the cyst membrane resemble those of synaptic membranes. © 1994 Wiley-Liss, Inc.