The cell junctions of hydra as viewed by freeze-fracture replication

Abstract

The cell junctions of hydra have been examined in freeze-fracture replication. Septate junctions appear as linear arrays of particles and complementary furrows containing fewer particles. In the epidermis, the particles are most numerous on the cytoplasmic fracture face (A-face), and the furrows are most obvious on the external fracture face (B-face). The polarity of particle distribution is reversed in the gastrodermis. Gap junctions are of the “B” type, having an aggregation of particles arranged in a plaque on the B fracture face. The particles are embedded in a matrix material. A complementary image appears on the A fracture face. Focal aggregates of particles appear on the A fracture face of epidermal myoid processes. No complementary B-face profile of pits has been identified. These observations support the view that the septate junction of hydra corresponds to a tight and intermediate junction of higher organisms and that the B-type gap junction is strongly adhesive. The significance of the focal particle aggregates is not known, but they may represent a punctate gap junction, a special receptor site, or an anchorage site for intracellular components.