Mitotic activity in monkey and rat Leydig cells.

The testes of the monkey, Macaca fascicularis, from birth to adulthood and of rats from puberty to adulthood were examined by both light and electron microscopy to estimate the mitotic activity of the Leydig cells. In monkeys, mitotic activity was measurable only in neonates: 2.3% in Leydig cells and 0.4% in fibroblast-like cells. From measurements of 3H-thymidine incorporation, mature Leydig cells were labeled in neonates (2%) and adult animals (1%). The labeling indices were significantly increased with increasing time after isotope exposure, and some pairs of labeled cells were found. In prepubertal (28 days of age) and pubertal (45 days of age) rats, mitotic indices were 2 and 0.2%, respectively, in the Leydig cells and 0.4 and 0.1% in fibroblast-like and myoid cells, respectively. Mitoses were not detected in postpubertal rats at 63 days of age. Taken together, these results demonstrate the self-renewal ability of mature Leydig cells during normal development.