The blood-testes barrier is formed by tight intracellular junctions between adjacent Sertoli cells in the seminiferous tubule. The barrier separates the basal and adluminal compartments of the seminiferous tubule. All substances passing from the basal to the adluminal compartment therefore must pass through the cytoplasm of the Sertoli cell. Thus, the germ cells also cross the barrier as they pass from the basal compartment to the adluminal compartment. The tight junctions between Sertoli cells loosen up to enable passage of the maturing germ cells through the junction, only to tighten up after they have passed.
The blood-testis barrier protects the spermatocytes, spermatids and spermatozoa from blood-borne toxic substance and circulating antibodies. It also prevents byproducts of gametogenesis from entering circulation lest they should stimulate an autoimmune reaction. Not unexpectedly, the breakdown of the barrier sometimes leads to autoimmune response against the germ cells. Steroids penetrate the barrier with ease.
The blood-testis barrier enables the seminiferous tubule to maintain a somewhat different composition of fluid inside its lumen. The fluid in the lumen of the seminiferous tubules contains very little protein and glucose but is rich in androgens, estrogens and K+ ions.