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Sertoli cell proliferation in the rat is completed by Days 15-20 postnatally. Thyroid hormones appear to regulate the duration of Sertoli cell proliferation, affecting adult Sertoli cell number and hence the capacity of the testis to produce sperm. In the present study, a combination of immunohistochemistry, immunoblot analysis, and reverse transcription-polymerase chain reaction was used to demonstrate the expression pattern of thyroid hormone receptors (TR) in the juvenile and adult rat testis. The results indicated that TRalpha1 was expressed in proliferating Sertoli cell nuclei, its expression decreasing coincident with the cessation of proliferation. TRalpha2, TRalpha3, and TRbeta1 mRNAs were expressed at low levels during development; however, the corresponding protein was not detected by immunoblot analysis. In addition, TRalpha1 was found to be expressed in germ cells from intermediate spermatogonia to mid-cycle pachytene spermatocytes. Immunohistochemistry also demonstrated TR expression in a subset of interstitial cells. The demonstration of TR expression in germ cells undergoing spermatogenic differentiation suggests a possible role for thyroid hormones in the adult testis.

This study uses a high purity cell culture system to extend previous observations of factors controlling the end of the Sertoli cell proliferative phase. Thyroid hormone, retinoic acid, and testosterone were assessed for their ability to halt the proliferative phase and regulate the expression of markers associated with maturation of the Sertoli cell. We show that these hormones share similar suppressive effects on the rate of Sertoli cell division without any apparent additive effects. We demonstrate that these hormones induce the progressive accumulation of cell cycle inhibitors p27Kip1 and p21Cip1 in Sertoli cells, a likely regulatory mechanism controlling the suppression of proliferation. We used real-time RT-PCR to examine the effects of these factors on the expression of mRNA encoding the Id proteins, demonstrating an increase in Id2 and Id3 expression in Sertoli cells treated with thyroid hormone, retinoic acid, or testosterone. Finally, we examined the expression of a number of genes that have been implicated in the Sertoli cell differentiation process. Our results suggest that these hormones can induce aspects of Sertoli cell differentiation in vitro, providing a valuable in vitro model for studying Sertoli cell function.

This study describes the testicular levels of inhibin/activin subunits by Northern analysis and in situ hybridization and serum and testicular levels of inhibins A and B and activin A by enzyme linked immunosorbent assays (ELISA) during postnatal development in the rat. We show that serum inhibin A levels are less than 4 pg/ml throughout postnatal life. Serum inhibin B levels peak at 572 +/- 119 pg/ml (mean +/- se) at d 40 post partum (pp) before falling to 182 +/- 35 pg/ml in mature males. Serum activin A decreases from 294 +/- 29 pg/ml at d 6 to 132 +/- 27 pg/ml at maturity. Within the testis, inhibin A levels fall from 0.330 +/- 0.108 ng/g at d 15 to less than 0.004 ng/g at maturity. Inhibin B levels peak at 43.9 +/- 4.2 ng/g at d 6 before falling to 1.6 +/- 0.13 ng/g at maturity. Testicular activin A levels fall from 18.6 +/- 2.2 ng/g at d 6 to 0.094 +/- 0.013 ng/g at maturity. Northern profiles of testicular inhibin/activin subunits correlate with immunoreactive levels demonstrated by ELISA. In situ hybridization suggests that beta(A) and beta(B) subunit expression is largely restricted to the seminiferous tubule, particularly Sertoli cells, spermatogonia, and primary spermatocytes. These data support the view that inhibin B is the major inhibin in the male rat and that levels relate to Sertoli cell number and activity. Furthermore, the demonstration of high local concentrations of activin A during the period of Sertoli cell proliferation and the onset of spermatogenesis support its proposed role because a modulator of testicular development and function.