ABSTRACT
It is an essential prerequisite for most vertebrate species to maintain various chemical and physical conditions of the body fluids (blood and extracellular fluids) within certain physiological ranges in order to ensure normal activities of the cells that constitute the body. Teleost species that inhabit aquatic environments face osmotic problems, which differ from those in terrestrial vertebrates. The aquatic animals possess the gill, which is permeable to both ions and water to a large extent, as their respiratory organ. Chloride cells are scattered all over the body surface in this early stage, in which the yolk has been absorbed but the gills are still underdeveloped. It could be thus speculated that extrabranchial chloride cells play a central role in osmoregulation during early developmental stages in place of gill chloride cells. Chloride cells in the yolk-sac membrane are flattened compared to the elongated or spherical shape of chloride cells in the gills.
