Ontogenesis of the nuclear 3,5,3′-triiodothyronine receptor in the rat testis

The aim of this study was to investigate the ontogenesis of the nuclear T₃ receptor among the different cell types in the rat testis from fetuses at the 19th day of gestation and animals 1, 5, 15, 20, and 60 days after birth. Whole testis, tubular fraction, nontubular fractions, and Sertoli cells cultured in vitro or enriched in vivo by irradiation were used. The results demonstrate that high affinity, low capacity T₃-binding sites are localized only in Sertoli cells; the binding specificity and affinity (K_d ranges from 0.8 ± 0.2 to 2.6 ± 0.4 nM) do not change significantly with the age of the animals and are comparable to those observed for T₃ receptors in other mammalian tissues. In the whole testis, the concentration of receptors changes during gonadal development, being maximally expressed in the fetus (154.3 ± 8.1 fg T₃ bound/10⁶ nuclei) and from 1 (203.4 ± 10.9 fg T₃ bound/10⁶ nuclei) to 5 (185.3 ± 15.1 fg T₃ bound/10⁶ nuclei) days of postnatal life, decreasing significantly at 15 and 20 days (65.4 ± 2.0 and 57.9 ± 1.9 fg T₃ bound/10⁶ nuclei, respectively) and being virtually absent in the adult. The same change in receptor concentration was found in Sertoli cells obtained by different techniques. This ontogenetic profile coincides with the pattern of Sertoli cell proliferation and differentiation, thus suggesting a role of thyroid hormones in the regulation of growth and maturation of the somatic cells of the seminiferous epithelium.