Ultrastructure and ion permeability of the two types of epithelial cell arranged alternately in the gill of the fresh water branchiopod Caenestheriella gifuensis (Crustacea)

Abstract

 The adult freshwater branchiopod, Caenestheriella gifuensis, has, as respiratory organs, fifteen pairs of slender cone-shaped gills composed of a thick epithelium. The silver nitrate/nitric acid technique revealed that the gill epithelium consisted of two kinds of cell, types I and II, which were alternately arranged with irregular interdigitations to form a unique, daisy pattern. Only type I cells were darkly stained by this technique, indicating high permeability of these cells to chloride ions and appearing to be responsible for the ion transport and osmoregulation. Further, electron microscopy disclosed fine structural characteristics of the two distinct types of epithelial cell covered by an extremely thin and soft cuticle layer, suggesting high permeability to gases and ions. The type I epithelial cell was characterized by an abundance of mitochondria, well-developed infoldings of the basal cell membrane exceeding two-thirds of the epithelial thickness, (which produce a magnification of the basolateral surface area of the cell), sparse microvillous projections of the apical border, and complicated interdigitations with the other type of epithelial cell. In the type II epithelial cell, on the other hand, these characteristics were less developed. These results suggest that in addition to their respiratory function, type I epithelial cells are of the ion-transporting type and play an important role in the active absorption of electrolytes to maintain a constant osmotic pressure of the hemolymph in extremely salt-deficient, freshwater environments. The type II epithelial cells may function mostly as respiratory epithelial cells.