Abstract
Ruminant Sertoli cells, instead of a nucleolus with nucleolonema, have a vesicular nucleolus, first described as a multivesicular nuclear body (MNB). It consists of membrane vesicles and tubules covered with ribo-some-like granules. We studied the testes of several domestic and wild ruminants using transmission electron microscopy. In domestic ruminants with continuous spermatogenesis, the MNB occurs throughout the year. In roe deer, the seasonal breeder, the most developed MNB is during the rut. During testicular rest, Sertoli cells have a nucleolus without vesicles. The cycle of Sertoli cells nucleolus is related to the onset and cessation of spermatogenesis and cyclical changes in Leydig cell, which include their regression, apoptosis and autophagy. Before and during spermatogenesis, Leydig cells have all the features of steroid-producing cells. At the end of the rut, the amount of smooth endoplasmic reticulum (SER) decreases rapidly and lipid droplets and glycogen appear in their cytoplasm. In January, small inactive Leydig cells contain lipid droplets, remnants of SER and glycogen. Our observations indicate that the MNB performs a major function of the nucleolus – a key role in ribosome biogenesis. Why membrane vesicles are present in the MNB, and what their role is in ruminant Sertoli cells, remains a mystery more than 50 years after its discovery.