Ordered arrays of intramembrane particles on the surface of fish gills

Freeze fracture of euryhaline fish gill epithelium reveals large smooth surfaces with intramembrane particles or corresponding pits often aggregated into hexagonal arrays. They belong to the outer plasma membranes of respiratory cells, the main surface of contact between the internal and external milieu of the fish. Possible functions for the arrays are discussed in relation to the fine structure of the respiratory cell.