Seasonal Change in Fine Structure and Function of Leydig Cells in the Blue Fox (Alopex lagopus)

The correlation between ultrastructural alterations and presumptive change in endocrine activity was studied in the Leydig cells of 17 blue foxes castrated at different times of the year.
In the reproductive season (March and April), with high concentrations of plasma testosterone and very active spermatogenesis the Leydig cells had large and light nuclei, few lipid droplets, ovoid mitochondria with tubular cristae, and a well developed agranular endoplasmic reticulum (AER). During early regression the mitochondria became large and pleomorphic, and the AER was arranged in concentric whorls. Later, when the activity seemed to have reached basal levels, the nuclei were small and dark, the number of lipid droplets increased, the mitochondria were rod-shaped with lamellar cristae, and the whorls of AER decreased. During the period of increasing activity the nuclei enlarged, the endoplasmic reticulum displayed both granular and agranular profiles, the mitochondria were sometimes dark and cup-shaped, and the number of lipid droplets decreased gradually.