Relation of Serum T3 and T4 to Sertoli Cell Proliferation and Differentiation in Euthyroid Ram Lambs.

Immature Sertoli cells cease to proliferate, and differentiate into their adult form during the first wave of spermatogenesis. Since Sertoli cell number determines the number of germ cells that can be supported, the rate and duration of Sertoli cell proliferation during postnatal development determines adult sperm production capacity. In the past, the testis was considered to be unresponsive to thyroid hormones, but more recent studies in rats and humans have shown that hyperthyroidism causes Sertoli cells to cease proliferation and hypothyroidism prolongs proliferation. This implies that T3 or T4 may affect the duration of proliferation of Sertoli cells in euthryoid animals during postnatal development, but this question has not been directly tested. The objectives of this study are to determine serum levels of free T3 and T4 in normal Rideau Arcott ram lambs, and to compare this to postnatal Sertoli cell proliferation and differentiation. We designed our experiments to accommodate the age discrepancy in onset of testicular maturation that we see in our flock. Serum was collected twice weekly from the jugular vein of 12 Rideau Arcott ram lambs between 10 and 19 weeks of age, and testicular biopsies were taken once a week from the left testis of these same animals over the same time span. Testicular biopsies were processed for histology and immunohistochemistry, and free T3 and T4 were determined by radioimmunoassay using Coat-A-Count kits (Siemens Med. Solu. Diag. Los Angeles, CA). Serum free T3 levels increased between 12 and 13 wk of age and then decreased, but when free T3 was compared to lumen formation (indication of tight junction formation by Sertoli cells), T3 remained at about 4.5 pmol/l until 1 wk after initiation of lumen formation and then decreased during the first wave of spermatogenesis and leveled off at about 2 pmol/l by the end of first wave of spermatogenesis. Free serum T4 levels decreased at 14 wk of age, but when compared to lumen formation, the decrease was observed 1 wk before initiation of lumen formation. Decrease in neither free T3 nor free T4 correlates with the onset of lumen formation in ram lamb seminiferous tubules. We have developed an immunohistochemistry technique for the sheep biopsies that identifies cells that are both proliferating (anti-PCNA positive) and are Sertoli cells (anti UCHL1 negative) for similar comparison to serum TH levels. (poster)