Fine structure of the Leydig cell during postnatal differentiation of the mouse testis

Abstract It is possible to divide the ultrastructural events which take place in the mouse Leydig cell during postnatal differentiation into two periods. The first period includes the changes taking place during the first three weeks after birth, whereas the second period comprises the events occurring from the fourth week on. During the first period the cytoplasm contains a poorly developed smooth endoplasmic reticulum. After the tenth day a slight increase in the number of vesicles and tubules of the smooth endoplasmic reticulum is observed. Clusters of glycogen particles (beta type) and lipid droplets are very numerous in this period. At the beginning of the second period the smooth endoplasmic reticulum hypertrophies considerably occupying a large area of the cytoplasm. This period is also characterized by the appearance of double‐walled tubules, of numerous interdigitations between neighboring Leydig cells and primary lysosomes in close relation to lipid droplets. A marked decrease in the number of glycogen particles and lipid droplets are also found. In the adult mouse (after day 50) numerous cisternae of the smooth endoplasmic reticulum are concentrically arranged (whorls). The formation of new membranes in the Leydig cell undergoing differentiation and the control of such differentiation are discussed.