Introduction: A functional chloride cell is structurally characterized by the presence of many mitochondria and abundant tubular system. These cells involved in ion transport in the fish gill, also they play key roles in acid-base regulation by altering chloride and carbonate exchange. It is clear that ionic composition of an aquatic environment is effected by so many factors, and gill epithelia should have the ability to excrete and or absorb salts. So, morphological researches are the main route for providing the basic data which can be indicative for physiological investigations. From these viewpoints, the purpose of this study is to identify the fine structure of the chloride cells in Alosa caspio caspio. This species owing to its tolerance to a wide range of salinities is from euryhaline species. The specimens of M.aurata in this research lived in south of Caspian Sea (salt osmolity 12.64-12.48 gr/lit pH=7.5-9). Methods: The gills were dissected out and processed for TEM. The gills were fixed by immersion in 2% Phosphate-buffered glutaraldehyde (pH=7.4), postfixation in 2% osmium tetroxide for 2 hours, dehydrated in graded concentrations of alchol, from 50% to absolute, followed by Propylene oxide, Epoxy resin & Propylene oxide and embedded in Epoxy resin. After trimming the blocks, ultratin sections were cut, harvested on grids and stained with Uranyl acetate and lead Citrate before viewing on a LEO electron microscope. Results: Chloride cells were located in epithelium of filament and lamellae, especially at the base of areas where lamellar platelets emerged. They were ovoid to elongated cells with basal nucleus. They had no tight junctions with adjucent chloride cells. In some cells, the mucosal surface was below the pavement cells and produced small invaginations, but in other cells the surface was free and without invagination. In this species, the chloride cells were ultrastructurally as light (fewer electrons dense) cells with free ribosomes, large mitochondria and developed tubulo-vesicular system, the basolateral tubular system was less developed and they had a close contact with mitochondria. There were also chloride cells with clear cytoplasm without mitochondria and tubular system that can be initial stage of their life spent period. Conclusions: the morphological characteristics of chloride cells of the Alosa caspio Caspio in the present study indicate to one type of these cells that are localized on the filament and lamellae. The occurance of lamellar chloride cells is thought to satisfy the physiological demand of ion uptake. One type of chloride cells can refer to \"alpha- chloride cell\" that are pale cells with extended membranous tubules. In this euryhaline species the chloride cell have more similar to freshwater species.

Keywords