Structural relationship between epithelial cells in Hydra
The plasma membranes of epithelial cells in Pelmatohydra oligactis are characterized by two dense lines about 30 Å apart, corresponding to the unit membrane structure described by Robertson. At certain localized areas of apposing cell surfaces, continuities between the outer dense components of the respective plasma membranes form an oriented series of parallel platelike lamellae perpendicular to the cell surfaces. Each lamella consists of two dense lines about 25 Å in thickness separated by about 25 Å. The lamellae occur at intervals of 100–150 A. Fixation in buffered OsO4 followed by postdehydration exposure to a solution of 0.1–0.5% phosphotungstic acid in absolute ethanol effectively demonstrates these relationships, as this treatment leads to sharp contrast between the dense membrane system and the pale interlamellar regions. Plate-like intercellular connections are seen after permanganate fixation but the interlamellar regions appear much more dense. Attempts have been made to obtain further information on the molecular structure of these lamellae by treating the tissues with crystalline enzyme preparations. Evidence at this time favors the view that these oriented lamellae function for intercellular attachment. An additional possibility is that they act as a permeability. barrier or seal, detering ambient water from passing between the cells into the intercellular fluid.