Seasonal changes in subcellular structures of Leydig and Sertoli cells in the Japanese black bear, Ursus thibetanus japonicus.

The fine structure of Leydig and Sertoli cells in Japanese black bears, Ursus thibetanus japonicus, was studied at different stages of reproductive activity in different seasons. Structures of Leydig cells in the testes that were spermatogenically active (May and June), inactive (November and January), and resumptive (March and April) were found to be different. The nucleus and smooth endoplasmic reticulum in Leydig cells had regressed when observed in November and January, and began to develop in March and April. Lipid droplets were numerous in Leydig cells in November and January, but few in March and April. These seasonal variations in the subcellular structures of Leydig cells corresponded to those in the steroidogenic activity of the cells. In Sertoli cells, changes in mitochondria and lipid inclusions with the seasons were detected, but the nuclei and other organelles, including the rough endoplasmic reticulum, did not change throughout the year. Lipid inclusions of both electron-dense and electron-lucent types in Sertoli cells were numerous in November and few in January. This change was reversely proportional to the number of degenerating cell masses in the luminal space, indicating that most of these lipid inclusions might be the end products of germ cells phagocytosed by the Sertoli cells. The results suggest that the seasonal changes in the morphology of Leydig cells are closely associated with their spermatogenic activity