The chloride cell of Cyprinodon variegatus: cytochemical studies on a cell specialized for electrolyte transport

Abstract

Cells specialized for high levels of electrolyte transport characteristically possess amplified plasmalemmas which effectively increase the exposure of the cell's surface to extracellular spaces. Of particular interest is the possibility that the surface coat of such amplified surfaces may possess polyanions which could bind cations of the extracellular space and thereby play a role in electrolyte transport at the membrane. Previous light microscope cytochemical studies have provided evidence for the existence of polyanionic surface coats for several such specialized cells. In the present study ultrastructural cytochemistry was undertaken to more clearly define the nature of the surface coat of a cell type specialized for high levels of electrolyte transport. Morphological and cytochemical studies were completed on the chloride cell of Cyprinodon variegatus branchial epithelium. Gills from salt water—adapted specimens were processed with three different techniques useful for the ultrastructural localization of polyanions: ruthenium red; colloidal thorium dioxide at low pH; and colloidal iron at low pH. Morphological studies revealed that the fine structure of the chloride cell of this species is identical to that of other teleost species adapted to salt water. Of particular interest is the form of the surface amplification in this cell type, i.e., the cell surface takes the form of an extensive tubular system which fills the cytoplasm. Ruthenium red demonstrated a thick surface coat on the plasmalemma forming the tubular system. Colloidal thorium dioxide micelles were found on the outer leaflet of the tubular system plasmalemma. Colloidal iron micelles stained both leaflets of the tubular system plasmalemma, suggesting the presence of acid groups in the membrane itself. All three techniques support the view that the tubular system of the chloride cell possesses surface-associated polyanions. Surface associated polyanions were also demonstrated in other cell types of the branchial epithelium.