The organization of testicular interstitial tissue and changes in the fine structure of the Leydig cells of European moles (Talpa europaea) throughout the year.

Seasonal changes of the testicular interstitial tissue were studied by electron microscopy. During the breeding season in spring, clusters of Leydig cells are surrounded by wide lymphatic sinusoids. In sexually quiescent moles, these sinusoids collapse, and the abundant Leydig cells become closely packed and occupy most of the testis. During sexual activity, the Leydig cells contain abundant smooth endoplasmic reticulum (SER), mitochondria with tubular cristae, and lipid droplets. Some areas of the cytoplasm are occupied exclusively by tubular SER, arranged in parallel. During regression the SER appears tortuous, and large lipid droplets are found in the cytoplasm, although these gradually become smaller. During the long period of sexual quiescence, the size and abundance of Leydig cells and the appearance of SER, lipid droplets and mitochondria were similar to those observed during sexual activity.