Anatomical relationships of the ionocytes (chloride cells) with the branchial venous compartment: definition of two types of epithelium in fish gills.

The fish gill, a gas exchanger and osmoregulatory structure, is separated from the external milieu by two well defined types of epithelium. One, the primary lamellar epithelium, faces the venous compartment of the primary lamellae. It contains chloride cells whose extracellular spaces drain into the venous system. The other, the secondary lamellar epithelium, faces the arterio-arterial capillaries of the secondary lamellae and is made-up of an …