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The comparative ultrastructural analysis of steroid-secretory cells (SC) in gonads of youngs lamprey Lampetra fluviatilis (Linnaeus 1758), Siberian sturgeon Acipenser baerii Brandt 1869, sterlet A. ruthenus Linnaeus, 1758, rainbow trout Oncorhynchus mykiss (Walbaum 1792) and pink salmon Oncorhynchus gorbuscha (Walbaum 1792) at similar stages of gametogenesis was carried out. The SC localization and size, the diameters of mitochondria, tubules of agranular reticulum, lipid inclusions and also the relative volume density of these structures were determined and calculated. It is concluded that localization of SC in gonads and their activity changes are closely related to the processes of sex differentiation and the growth of oocytes. Thus, in females of all studied species there was a change in SC localization in the direction of stroma→theca→granulosa of previtellogenic oocytes with a simultaneous increase of their secretory activity (increase in SC size, size and volume density of mitochondria and of agranular endoplasmic reticulum tubules). In males, during gametogenesis, the SC localization also changed in the direction of epithelium→stroma testicular with an increase of their secretory activity. In this case, the largest and most active SC were found in the stroma of pink salmon testicular during the natural sex inversion. At the same time, however, were revealed differences in SC ultrastructural organization, appeared to be of species-specific nature. The highest average diameter of the agranular endoplasmic reticulum tubules was observed in sturgeon SC and they are characterized by a large number of lipid inclusions in early stages of gametogenesis. This should be taken into account when comparatively analysing of quantitative data in fish of different systematic groups.

Special features of ultrastructural organization steroid-secretory cells in the gonads of juveniles of cyclostomes and fishes