STRUCTURE AND FUNCTION OF CHLORIDE CELLS IN THE GILLS OF ANGUILLA ANGUILLA

Abstract

Gills of yellow eels in fresh water contain relatively few chloride cells distributed on both primary and secondary filaments. Seawater gills contain many chloride cells located solely on primary filaments. The incidence of chloride cell - chloride cell contacts is, therefore, much higher in seawater than in freshwater gills. The internal structures of chloride cells from freshwater and seawater gills are generally very similar, both cells being especially characterised by an extensive smooth tubular system (STS), that is derived from basal and lateral infoldings of plasma membranes, and numerous mitochondria. No internal differences were found between chloride cells that could account for the inwards and outwards pumping of NaCl across freshwater and seawater gills respectively. Intercellular spaces were particularly prominent between adjacent chloride cells, i.e. in seawater gills, and specialisation appears to occur at the short junctional region near the apical membranes of the cells, where a swollen area is commonly found between two short, ostensibly tight junctions. We consider that these areas are junctions through which NaCl passes outwards across the gills into seawater. To account for current ultrastructural, biochemical and physiological data we propose the following mechanism for NaCl pumping in seawater gills. The high density of NaCl pumps on the STS of adjacent chloride cells generates a particularly high concentration of NaCl in the intercellular spaces. NaCl moves outwards from these spaces into seawater through ion permeable junctions by diffusion aided by hydrostatic pressure stemming from arterial blood flow.