Autoradiographic study of binding and internalization of follicle-stimulating hormone in the mouse testis minces in vitro

The internalization of FSH-receptor complexes was demonstrated in mouse testis by means of light and electron microscopic autoradiography. Chopped testicular pieces were incubated with radioiodinated FSH (¹³¹I-NIADDK-rat FSH-I-4) for 10, 20, 60 and 180 min. After incubation the pieces were fixed with glutaraldehyde containing tannic acid, and embedded in Spurr's resin. Semithin and ultrathin sections were cut for light and electron microscopic autoradiography, respectively. In light microscopic autoradiographs, silver grains were preferentially localized over Sertoli cells, regardless of incubation time. Sixty to 70% of the total number of grains were located over Sertoli cells which account for only about 4% of the total cell population of the seminiferous tubules. The majority of these grains correspond to the specific FSH binding sites, because few grains remained after incubation with an excess amount of unlabeled FSH. In electron microscopic autoradiographs, the half-distance (HD) value for the ¹³¹I-labeled line source was about 216 nm in the present study. After 10 min of incubation, 56.6% of the total number of silver grains were located over the plasma membrane of Sertoli cells. In testicular pieces incubated for longer periods (20, 60 and 180 min), both the percentage and relative concentration of grains increased in the Golgi apparatus and lysosomes and decreased in the plasma membrane. These results suggest that [¹³¹I]iodo-FSH first binds to FSH receptors on the plasma membrane of Sertoli cells, then FSH-receptor complexes are internalized. The increase in the number of grains over the lysosomes following longer incubation, indicates that internalized [¹³¹I]iodo-FSH or FSH-receptor complexes are subjected to degradation.