The blood-testis barrier: a morphologic or physiologic phenomenon?

Blood urea is concentrated in rat seminiferous tubule fluid. Data are presented that provide strong evidence that this urea movement is dependent on active sodium transport. In contrast, in the hamster blood urea is excluded partially from the seminiferous tubule. This may be caused by transport of urea from the lumen to the blood rather than secondary to the morphologic blood, testis barrier at the Sertoli to Sertoli cell tight junctions.