The adrenocortical homolog of the beluga sturgeon Huso huso (Chondrostei)

In the beluga sturgeon Huso huso we studied the cell structure and the presence of glycogen in the corticosteroidogenic tissue. In chondrosteans this tissue is the main component of the yellow bodies spread throughout the length of the kidney region and constitutes the adrenocortical homolog of the adrenal gland of higher vertebrates. This study showed that these cells are present according to four main cyclic phases in relation to different moments of their secretory activity. Moreover, differently from teleosts, they contain abundant lipidic material stored in droplets. At the electron microscopic observation the lipid content of the droplets may appear highly osmiophilic or lightly osmiophilic, or electron lucent if extracted by the fixation. According to the cell characteristics, three main zones may be considered: a peripheral zone in which the lipid droplets appear very electron dense, an intermediate zone with medium dense droplets, and a central zone with electron lucent droplets. In this latter, degenerative processes are frequent, while in the other zones the images suggest dedifferention processes. The three zones have not precise outlines, but there is a gradual and centripetal cell transformation from peripheral zone cells to intermediate zone cells and from these latter to central zone cells. While new cells differentiate in the most external band of the peripheral zone, in the central zone the cells degenerate. We suggested that the high osmiophily in the peripheral lipid droplets could be due to a high unsaturation of triglycerides and cholesterol esters. A progressive hydrolysis of fatty acids can modify the stored lipid content and its electron density. The β-oxidation of the fatty acids could provide an energy source and the steroidogenic cells could so also accomplish an adipocyte-like role. Even if rarely, contacts of the peripheral cells with chromaffin cells are present, indicating a possible paracrine action of the catecholamines on the cortisol secretion, as proved in other vertebrates.