Direct binding of radioiodinated human chorionic gonadotrophin to frozen sections of rat testis.

Most published work on the binding of gonadotrophins to testicular tissue has been based on a biochemical approach and whole organs, homogenates or subcellular fractions have been used. Some studies have been concluded on interstitial tissue and/or seminiferous tubules separated from each other by microdissection after the method of Christensen & Mason (1965). Binding of radioiodinated HCG to specific receptors of the interstitial cells as a first step in the chain of events leading to steroidogenesis has been demonstrated (Catt, Tsuruhara & Dufau, 1972; Catt, Watanabe & Dufau, 1972). Few morphological studies aiming at the histological or cytological localization of the binding of the gonadotrophins in the testis have been published to date. Labelled or unlabelled gonadotrophins have been injected in vivo and traced thereafter by various methods and techniques: autoradiography (de Kretser, Catt, Burger & Smith, 1969), immunofluorescence and histochemistry (Mancini, Castro & Seiguer, 1966; Castro, Alonso & Mancini, 1972) and electron microscopy (Castro, Seiguer & Mancini, 1970). Unfortunately, knowledge of the metabolic events concerning injected hormones, especially labelled ones, is such that there is still doubt whether the localized substance is the original hormone or a metabolite and whether the localized uptake of the hormone by some tissue component is really the consequence of a specific hor¬ mone-receptor binding or of some other factors. The following report presents the results that we have obtained using a method in which radioiodinated HCG was directly bound to frozen sections of rat testis in an attempt to avoid the intricacies of the experiments in vivo and approximate to the relative simplicity and precision of biochemical studies in vitro. A similar approach has been followed for the ovary (Midgley, 1973). Frozen sections of testes from adult Wistar rats were obtained by direct im¬ mersion of small pieces (3 to 4 mm) of tissue in liquid nitrogen ( —195°C) and sectioning at 7 μ in a cryostat at — 20°C. Cut sections were attached to glass slides prewarmed at 37°C, left to dry at this temperature for 1 hr on a thermo¬ statically controlled warm stage and were then kept in a desiccator at 0 to 4°C until a sufficient number had been collected for further processing. For binding study, a highly purified preparation of HCG, with a biological activity of 12,000 to 18,000 i.u. and mol. wt ~47,000 (Serono Immunochemi-