The anchoring of nematocysts and nematocytes in the tentacles of hydra

Abstract

The ultrastructure of nematocyte junctions in hydra tentacles was examined by electron microscopy using thin sections and freeze-fracture replicas. The apical region of a nematocyte is joined to the battery cell by septate junctions. A second basal junctional relationship simultaneously anchors a nematocyte to a surrounding battery cell and the underlying mesoglea. We term this basal junction the NBM complex. Each nematocyte contains 10- to 14-nm cytoplasmic filaments basally; these do not bind NBD-phallacidin, an actin-specific probe. A tongue of battery cell cytoplasm containing microtubules intervenes between a nematocyte and the mesoglea. A condensation of extracellular filaments extends from the battery cell into the underlying mesoglea. We interpret the NBM complex as a tandem arrangement of a fascial desmosome and a fascial hemidesmosome. Nematocyte-free hydra possess epidermal cell-mesogleal junctions resembling the NBM complex, suggesting that sites for nematocyte positioning may be predetermined.