APICAL SURFACE OF HYDROZOAN NEMATOCYTES - STRUCTURAL ADAPTATIONS TO MECHANOSENSORY AND EXOCYTOTIC FUNCTIONS

At the apical cell pole of the nematocytes of a hydrozoan (Hydra vulga ris), cytoskeletal elements of different molecular families (microfila ments, microtubules, cross-striated rootlets) are combined in a comple x framework. As determined by transmission and scanning electron micro scopy, the stimulus-transducing cnidocil apparatus of these mechanosen sitive cells is forced into a lateral position, facilitating the close apposition of the nematocyst to the apical cell membrane. The nematoc yst, a single, extraordinarily large, exocytotic organelle, is held in position by a microtubular basket. The cnidocil apparatus and microtu bular basket are linked to an ellipsoid arrangement of pseudovilli, i. e., small surface protrusions containing cross-striated rods. These ne matocyte-specific cytoskeletal elements mediate the anchorage of the e ntire cytoskeletal framework to the apical cell membrane. The apical m embranes of the nematocyte and nematocyst are separated by a distance of only similar to 50 nm. Structural modifications on the external sid e of the cyst membrane resemble those of synaptic membranes. (C) 1994 Wiley-Liss, Inc.