Morphological studies on the origin of adult-type Leydig cells in rat testis.
The differentiation of adult-type Leydig cells (ATLC) in rat testis was studied with the help of electron microscopy and a combined technique of autoradiography and enzymehistochemistry. Male Wistar rats on each alternative day from postnatal day (pnd) 9 to 25 were perfused in situ, and the testicular sections were observed by electron microscopy. The cytoplasm of the ATLC contained smooth endoplasmic reticulum, mitochondria, a few small lipid droplets and a large well developed Golgi apparatus. In addition to typical fibroblasts/fibrocytes, a group of fibroblast-like cells was observed in the peritubular and perivascular spaces of the interstitial tissue. These cells, containing smooth endoplasmic reticulum and a large number of mitochondria with tubular-type cristae, were named as intermediate-type cells. In the autoradiographic experiments the 3-H-thymidine and 14-C-thymidine uptake behaviour of fetal-type Leydig cells, ATLC, intermediate-type cells and myoid cells was studied from 1 till 25 pnd. The intermediate-type cell, observed predominantly between pnd 9 to 21, showed the highest labelling index. The absolute number of fibroblasts/fibrocytes showed a peak on pnd 13 and decreased gradually with age. The duration of DNA synthesis phase ranged for interstitial fibroblasts: 7.9 h, for intermediate-type cell 10.4 h, for ATLC 17.4 h. The results lead to the conclusion that ATLC most probably originate to a large extent from the peritubular and to a lesser extent from perivascular fibroblast-like cells of the interstitial tissue.