Involution of human fetal Leydig cells. An immunohistochemical, ultrastructural and quantitative study.

The testes of stillborn fetuses (from 13 to 28 weeks of gestational age), fetuses born alive (from 29 weeks of gestational age) who died a few days later, and infants dying 1 to 8 months after birth were processed for light and electron microscopy. Paraffin-embedded material was stained with the avidin-biotin peroxidase complex (ABC) method for immunohistochemical detection of testosterone (T) in order to quantify the age-related changes in the number of T-positive interstitial cells. This number decreased progressively from the 24th week of gestation up to birth and remained unchanged up to the second month of postnatal life. During the third month of age, the number of T-positive cells rose markedly but fell again from the fourth month to the end of the study. The ultrastructural study revealed the following types of interstitial cells at all ages studied: fibroblast-like cells, myofibroblast-like cells, developed fetal Leydig cells, degenerating fetal Leydig cells and infantile Leydig cells with a multilobed nucleus and focal cytoplasmic accumulations of smooth endoplasmic reticulum and lipid droplets. Quantitative ultrastructural studies revealed that the changes in the number of fetal Leydig cells with age were similar to those found in the number of T-positive cells although, for each age studied, absolute values were higher in the ultrastructural study. The number of infantile Leydig cells increased with age.