Previous evidence has indicated that the testicular receptors for FSH 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 25I-labeled hFSH to seminiferous tubules of hypophysectomized rats was examined, utilizing autoradiography at both the light and electron microscope levels. Purified hFSH (LER-1575C) was iodinated to an average specific activity of 29 μCi/μg, and approximately 1 μg was administered intratesticularly in 40 day 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 two 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 unlabelled FSH along with the 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 and electron microscope 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 ofthe 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 electron microscope level, this activity is further localized to Sertoli plasma membranes in this peripheral region ofthe 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 ofthe two membranes gives rise to the activity. We therefore cannot rule out the possibility that some FSH receptors are on spermatogonia. Grains are conspicuously absent over membranes of either Sertoli or germ cells in the more central parts of the tubules. This suggests that these areas lack FSH receptors, assuming that the hormone can penetrate the Sertoli-Sertoli junctions. In any event, it appears that the regulatory action of FSH is restricted to the basal part of the Sertoli cell, in the general vicinity ofthe nucleus.
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Orth et al. (1977) studied this question.