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 in neonatal rats is associated with an increase of testis size of about 60%, compared to coeval controls. Increased number of both Sertoli and germ cells and enlarged diameter of seminiferous cords were found in hyperplastic testes. In the T3-treated group, TSH serum levels were low and a slight increase of FSH was found. In vitro treatment of neonatal 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 degenerating germ cells (P < 0.001 vs control). In the adult testis, both in vivo and in vitro treatments with thyroid hormone did not induce morphological modifications, thus demonstrating that the critical window of thyroid hormone effectiveness coincides with the prepuberal period. Since 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 mediated by a direct hormonal effect on the somatic cell of the seminiferous epithelium.