EARLY THYROID-HORMONE TREATMENT IN RATS INCREASES TESTIS SIZE AND GERM-CELL NUMBER

By means of in vivo and in vitro approaches, we studied the effect of thyroid hormone on postnatal development of rat testis. T3 treatment i n neonatal rats is associated with an increase of testis size of about 60%, compared to coeval controls. Increased number of both Sertoli an d germ cells and enlarged diameter of seminiferous cords were found in hyperplastic testes. In the T3-treated group, TSH serum levels were l ow and a slight increase of FSH was found. In vitro treatment of neona tal testis fragments by 10(-7) M T3 for 3 days increased the number of gonocytes (P<0.001 vs control) and decreased the percentage of degene rating germ cells (P<0.001 vs control). In the adult testis, both in v ivo and in vitro treatments with thyroid hormone did not induce morpho logical modifications, thus demostrating that the critical window of t hyroid hormone effectiviness coincides with the prepuberal period. Sin ce thyroid hormone stimulates Sertoli cells to secrete growth factors and nutrients for germ cell development, we suggest that the increased testicular size and germ cell number following the T3 treatment is me diated by a direct hormonal effect on the somatic cell of the seminife rous epithelium.