The formation, distribution and fate of lipid inclusions within the seminiferous tubules of the rat has been studied throughout the spermatogenic cycle. The occurrence of lipid inclusions within the Sertoli cell exhibited cyclic variation with the stages of the rat seminiferous cycle. At stage 9 of the cycle, residual bodies of maturing spermatids were phagocytosed by the Sertoli cell and released numerous lipid droplets which appeared to coalesce into large inclusions at the base of the Sertoli cell at stage 10. The Sertoli cell lipid inclusions persisted throughout the completion of meiosis (stages 11 to 14) and the formation of young spermatids (stages 1 to 2) and their numbers appeared to reach a peak at stages 12 to 13 of the cycle. The inclusions decreased markedly within the Sertoli cell cytoplasm during stage 2 and remained low until stage 9 when lipid from the residual bodies again became available to the cell. This cyclic variation of lipid inclusions within the Sertoli cell does not support previously held views that there is a gradual decline in Sertoli cell lipid during stages 10 to 14 of the spermatogenic cycle. A hitherto unnoticed finding was the presence of large lipid inclusions in the cytoplasm of late pachytene to diakinetic spermatocytes, and some observations suggest a transfer of these lipid inclusions from the Sertoli cells to primary spermatocytes.
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