Previous work has indicated that the contractile “myoid cells” surrounding the seminiferous tubules serve as a barrier to the penetration of substances into the germinal epithelium. In the present study, the nature of the junctions separating myoid cells was investigated with en bloc uranyl acetate staining and with lanthanum nitrate, a small electron-opaque tracer useful for testing the patency of junctions. While some of the junctional complexes exhibited a continuous interspace of 200 Å, the majority displayed occluding junctions which effectively prevented passage of perfused lanthanum into the germinal epithelium. However, in 10–15% of the cross sections of tubules examined, lanthanum did penetrate the myoid cell layer through open junctions. At these sites the tracer also entered the interspaces between the Sertoli cells and the spermatogonia clearly outlining the latter. The interspaces between the spermatogonia and the Sertoli cells are continuous with those separating adjacent Sertoli cells, but the tracer was only able to penetrate for short distances further toward the tubular lumen. High magnification electron microscopy revealed that tight junctions connected adjacent Sertoli cells a short distance from the spermatogonia. Thus, while the myoid cells constitute a significant permeability barrier, the tight junctions between Sertoli cells form a second, and apparently a more effective barrier to the penetration of substances through the germinal epithelium.

These results indicate that in the seminiferous tubules two compartments are formed by the Sertoli cells and their junctional complexes: (1) a basal compartment between the Sertoli junctions and the basal lamina, containing the spermatogonial population and early preleptotene spermatocytes, and (2) an adluminal compartment above the Sertoli junctions, containing the other germ cells. The possible functional significance of the barrier at the level of the peritubular contractile cells and of the compartmentalization of the seminiferous epithelium is discussed.