The Use of Modern Microscopical Techniques for the Study of Fish Gill

Abstract

The branchial epithelium of the fish gill consists of at least four different cell types: pavement cells, chloride cells, mucus cells and neuroepithelial cells (Laurent, 1984). Of these, the pavement ceils (or respiratory cells when located on the lamellae) and the chloride cells, also called ionocytes or mitochondria-rich (MR-) cells, are thought to be directly involved in gas exchange, ionoregulatory and acid/base-regulatory processes. Although gill cells have been extensively studied, in particular chloride cells (reviewed by Zadunaisky, 1984; Pisam and Rambourg, 1991; Marshall, 1995; McCormick et al., 1995), important questions about these cells are as yet unanswered. For example, on the basis of ultrastructural and biochemical studies, the chloride cells are generally thought to be responsible for ion transport across the branchial epithelium of fish gills. However, recent evidence suggests the presence of proton pumps and Na+/H+- exchanger in the respiratory cells, indicating a possible role in Na+-uptake in freshwater fish (Lin and Randall, 1995; Sullivan et al., 1995). To define the ion transporting function of chloride and respiratory cells direct in situ measurements of ion transport, membrane potentials or intracellular ion concentrations are needed. Up to now intracellular electrodes could not be used for these measurements because of the complex structure and vascularization of the gills, which hinder good access to individual cells.