Autoradiographic Localization of Specifically Bound 125I-Labeled Follicle-Stimulating Hormone on Spermatogonia of the Rat Testis*

Abstract

In a previous study, autoradiography was used to demonstrate specific FSH[Follicle-Stimulating Hormone]binding sites on Sertoli plasma membranes in the basal compartment of rat seminiferous tubules. The present study was undertaken to determine whether spermatogonia in the rat testis also possess FSH-binding sites. Approximately 1 .mu.g [125I]h[human]FSH was administered as an intratesticular injection to 40 day old rats that had been hypophysectomized 10 days earlier; a 2nd group of rats received a 500-fold excess of cold hormone along with the [125I]-hFSH, and served as a control for specificity of binding. All testes were perfused with a hyperosmotic fixative, which produces preferential shrinkage of spermatogonia and Sertoli cells in the basal compartment. The resulting separation of the cells made it possible to determine whether spermatogonia were the source of any autoradiographic grains. In the absence of excess cold hormone, some grains were clearly localized to spermatogonial surfaces, although the majority of specific binding was to Sertoli membranes. The ratio of Sertoli-bound [125I]hFSH to that on spermatogonia was 6.58 .+-. 0.31 (SEM) [standard error of the mean]. A morphometric analysis, carried out on routinely fixed tissue, established that Sertoli cells in the basal compartment have 4.02 .+-. 0.61 times as much surface area as spermatogonia. A comparison of the density of FSH receptors (grains per .mu.m2 cell surface) on Sertoli cells with that on spermatogonia yielded a ratio (Sertoli to spermatogonia) of 1.68. FSH-binding sites occur over the surface of spermatogonia in concentrations similar to those of Sertoli cells in the basal compartment of the seminiferous tubule.