The surface epithelium of teleostean fish gills. Cellular and junctional adaptations of the chloride cell in relation to salt adaptation.

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Abstract
Various species of teleostean fish were adapted to fresh or salt water and their gill surface epithelium was examined using several techniques of EM. In both fresh and salt water the branchial epithelium is mostly covered by flat respiratory cells. They are characterized by unusual outer membrane fracture faces containing intramembranous particles and pits in various stages of ordered aggregation. Freeze fracture studies showed that the tight junctions between respiratory cells are made of several interconnecting strands, probably representing high resistance junctions. The organization of intramembranous elements and the morphological characteristics of the junctions do not vary in relation to the external salinity. Towards the base of the secondary gill lamellae, the layer of respiratory cells is interrupted by mitochondria-rich cells (Cl- cells), also linked to respiratory cells by multistranded junctions. There is fundamental reorganization of the Cl- cells associated with salt water adaptation. In salt water young adjacent Cl- cells send interdigitations into preexisting Cl- cells. The apex of the seawater Cl- cell is part of a mosaic of sister cells, linked to surrounding respiratory cells by multistranded junctions. The Cl- cells are linked to each other by shallow junctions made of only 1 strand and permeable to La. Salt water adaptation may trigger a cellular reorganization of the epithelium in such a way that leaky junctions (a low resistance pathway) appear at the apex of the chloride cells. Cl- cells are characterized by an extensive tubular reticulum which is an extension of the basolateral plasma membrane. It is made of repeating units and is the site of numerous ion pumps. The presence of shallow junctions in sea water-adapted fish makes it possible for the reticulum to contact the external milieu. In the freshwater-adapted fish the Cl- cell''s tubular reticulum is separated by deep apical junctions from the external environment.