Localization of125I-labeled FSH in the Testes of Hypophysectomized Rats by Autoradiography at the Light and Electron Microscope Levels12

Abstract

Previous evidence has indicated that the testicular receptors for FSH [fillicle stimulating hormone] reside on plasma membranes in seminiferous tubules, and that the Sertoli cell is a primary target of the hormone. Since the Sertoli cell is large and elaborately structured, it is important to know whether there is a regional distribution of receptors over its extensive surface. In the present study, the specific binding of 125I-labeled h[human]FSH to seminiferous tubules of hypophysectomized rats was examined, utilizing autoradiography at both the light microscope and EM levels. Purified hFSH (LER-1575C) was iodinated to an average specific activity of 29 .mu.Ci/.mu.g, and approximately 1 .mu.g was administered intratesticularly in 40 days old rats which had been hypophysectomized 10 days earlier. In some cases, testosterone was administered after hypophysectomy in order to maintain spermatogenesis. Controls were of 2 types: 40 day old intact littermates in which endogenous FSH presumably occupied most of the receptors, and hypophysectomized rats which had received a 500-fold excess of unlabeled FSH along with iodinated hormone (uptake inhibition control). All material was perfuse-fixed, after being allowed to clear 5-60 min following the intratesticular injection of the hormone. The tissue was then embedded, sectioned and prepared for light microscope and EM autoradiography, with quantitation by grain counts in both cases. Light microscope autoradiographs of tubules from experimentals (with or without testosterone treatment) show a concentration of grains over cytoplasm in the peripheral parts of the seminiferous tubules, outside the approximate level of the Sertoli-Sertoli junctions. The specificity of this localization is verified in both types of controls, since very few grains appear over these tubules. At the EM level, this activity is further localized to Sertoli plasma membranes in this peripheral region of the seminiferous tubule, at about the level of the Sertoli nuclei. Grains also appear over apposed Sertoli and spermatogonial membranes, but with this technique it is not possible to distinguish which the 2 membranes gives rise to the activity. Some FSH receptors may be on spermatogonia. Grains are conspicuously absent over membranes of either Sertoli or germ cells in the more central parts of the tublues. These areas may lack FSH receptors, assuming that the hormone can penetrate the Sertoli-Sertoli junctions. It appears that the regulatory action of FSH is restricted to the basal part of the Sertoli cell, in the general vicinity of the nucleus.