Tri-iodothyronine directly affects rat Sertoli cell proliferation and differentiation.

The addition of physiological concentrations (1 nM) of tri-iodothyronine (T3) to the culture medium of Sertoli cells from prepubertal (8-day-old) rats stimulated both protein synthesis (+55%) and lactate (+50%) production, while it inhibited DNA synthesis (-30/35%) and aromatase activity (-45/50%); insignificant T3-dependent effects were observed in cultured Sertoli cells from midpubertal (28-day-old) rats. These data suggest an age-dependent role for thyroid hormone in promoting and maintaining Sertoli cell differentiation at puberty; more-over, the hormone is involved in the regulation of Sertoli cell proliferation. The present study validates the role of Sertoli cells as a specific target for T3 action at the testis level; it also demonstrates the existence of an early and critical direct influence of thyroid hormone on Sertoli cell proliferation and functional maturation.