The testis is composed of two compartments: the seminiferous tubules. in which spennatogenesis takes place, and the interstitium, which is composed of blood vessels, nerve fibres, macrophages, endothelial cells, and also contains Leydig cells, which are the steroid-producing cells of the testis. The seminiferous tubules consist of spennatogenic epithelium formed by Sertoli cells and developing genn cells, surrounded by peri tubular myoid cells. Sertoli cells are important for physical support of the developing germ cells. Furthermore, adjacent Se.10li cells form tight junctions, thus fonning a barrier (the so-called Sertoli cell barrier or blood-testis barrier; Dym and Fawcett, 1970; Setchell et aI., 1988), which provides the developing germ cells behind tIlis barrier WitIl a separate environment. Spermatogonia are found outside the blood-testis barrier, in the basal compartment, whereas leptotene spennatocytes start to migrate through the Sertoli cell barrier; the adluminal compartment contains more advanced spematocytes and spermatids (Dym alld Fawcett, 1970). In order to reach cells in the adluminal compartment, substal,ces must pass through Sertoli cells, which thereby determine the composition of the spermatogenic micro-environment. Moreover, Sertoli cells produce a number of factors, which are important for germinal cells.