Effects of thyroid and luteinizing hormones on the onset of precursor celldifferentiation into Leydig progenitor cells in the prepubertal rat testis

Leydig cells in the adult rat testis differentiate during the neonatal-prep ubertal period. However, the stimulus for the initiation of their different iation is still not clear. In the present study our objectives were to test the effects of thyroid hormone and LH on the initiation of precursor cell differentiation into Leydig cells in the prepubertal rat testis. Four group s of Sprague-Dawley rats were used. All treatments began at postnatal Day 1 . Rats in groups I, II, and III received daily s.c. injections of saline (2 00 mu l, controls), triiodothyronine (T-3, 50 mu g/kg body weight, hyperthy roid), and LH (ovine LH 10 mu g/rat/day), respectively. Rats in group IV we re made hypothyroid from postnatal Day 1 by adding 0.1% propylthiouracil (P TU) to their mother's drinking water. Testes of rats were collected at 7, 8 , 9, 10, 11, 12, 16, and 21 days of age, fixed in Bouin's solution, and emb edded in paraffin for immunocytochemical studies. Immunoexpression of 3 bet a-hydroxysteroid dehydrogenase (3 beta-HSD) and LH receptors (LHR) in testi cular interstitial cells (other than the fetal Leydig cells) was observed u sing the avidin-biotin method. In control rats, out of all spindle-shaped c ell types in the testis interstitium, only the peritubular mesenchymal cell s showed positive immunolabeling for 3 beta-HSD, beginning from the postnat al Day 11. However, positive immunolabeling for LHR was first detected in t hese cells at Day 12, i.e., after acquiring the steroidogenic enzyme activi ty. In T-3-treated rats 3 beta-HSD positive spindle-shaped cells were first observed at Day 9 (i.e., 2 days earlier than controls), and LHR-positive c ells were first observed on Day 11 (2 days later than obtaining 3 beta-HSD immunoactivity); they were exclusively the peritubular mesenchyma( cells. T he 3 beta-HSD- and LHR-positive spindle-shaped cells were absent in the tes tis interstitium of LH-injected rats from Days 7 through 12 but were presen t at postnatal Day 16. In addition, more fetal Leydig cell clusters and fet al Leydig cells in mitosis were present in LH-treated rats compared to rats in all other treatment groups. Following their first detection, the number of positive cells for each protein continued to increase at each subsequen t age in controls, T-3-, and LH-injected groups. In PTU rats, 3 beta-HSD an d LHR-positive spindle-shaped cells were absent throughout the experimental period. From these observations, it is possible to suggest the following r egarding the developing rat testis interstitium. 1)The precursor cells for the adult generation of Leydig cells in the postnatal rat testis are the pe ritubular mesenchymal cells. 2) Luteinizing hormone does not initiate the o nset of mesenchymal cell differentiation into Leydig cells, instead it dela ys this process. However, daily LH treatment causes mitosis in fetal Leydig cells and increase in fetal Leydig cell clusters. 3) Thyroid hormone is cr itical to initiate the onset of mesenchymal cell differentiation into adult Leydig cells.