The chromaffin system of the beluga sturgeon Huso huso (Chondrostei): histological, immunohistochemical and ultrastructural study

This histology and fine structure of the chromaffin cells were examined in the beluga sturgeon Huso buso Linnaeus, 1758. Chromaffin cells, identified by the chromaffin reaction and immunohistochemical reactions for tyrosine hydroxylase, dopamine-beta-hydroxylase and phenylethanolamine-N-methyltransferase, were spread over the entire length of the kidney, mainly localized in the walls of the cardinal and caudal veins and their main branches. Ultrastructural analysis revealed two different types of catecholamine containing cells distinguished by their secretory granules, as in other vertebrate species. According to the different electron-densities of the granules, the cells could be distinguished as adrenaline and noradrenaline producing cells. The presence of nerve fibres and a few endings led us to hypothesize the neural control of catecholamine secretion. Moreover, the positivity of the immunohistochemical reaction for neuronal nitric oxide synthase, an enzyme that catalyzes the synthesis of nitric oxide (NO), in nerve elements close to the chromaffin cells suggested the involvement of NO in the regulation of cell function. The distribution, cytological aspects and regulation of the chromaffin system of the beluga sturgeon are compared with the same system in other bony fishes.