DEVELOPMENT OF THE ADULT-TYPE LEYDIG-CELL POPULATION IN THE RAT IS AFFECTED BY NEONATAL THYROID-HORMONE LEVELS

We have investigated the effects of neonatal-prepubertal changes in th yroid hormone levels on the early phases of adult-type Leydig cell dev elopment in the rat testis. Hypothyroidism was induced by adding 6-pro pyl-2-thiouracil (PTU) to the drinking water, while hyperthyroidism wa s induced by daily injections with triiodothyronine (T-3). The prolife rative activity of the Leydig cells in PTU-treated animals was not dif ferent from that in age-matched controls through the age of 16 days. N evertheless, the percentage of Leydig cells (i.e., the proportion of L eydig cells among the total interstitial cell population) was approxim ately 83% and 67% lower at the ages of 12 and 16 days, respectively. T he proliferative activity of the Leydig cells in the T-3-treated anima ls compared to the controls was increased approximately 3-fold at the ages of 12 and 16 days. The percentage of Leydig cells in T-3- treated animals was also considerably increased at these two ages (400% and 7 25%, respectively). Concomitantly, the percentage of peritubular cells was decreased, suggesting that the increase in the percentage of Leyd ig cells may at least partially be the result of differentiation of pe ritubularly located precursor cells. Plasma testosterone levels fluctu ated considerably at these ages. Hence, injection of T-3 during the ne onatal-prepubertal period not only affects Sertoli cell proliferation and differentiation but also directly or indirectly affects the onset of the formation of the adult-type Leydig cell population and its func tion.