The blood–testis barrier is a physical barrier between the blood vessels and the seminiferous tubules of the testes. The term is a bit misleading as it is not a blood-organ barrier, but rather one that is formed by the tight junctions between Sertoli cells of the seminiferous tubules. For this reason, this barrier is also referred to as the “Sertoli cell barrier”. The barrier isolates further developed germ cells from the blood.
The presence of the blood-testis barrier allows Sertoli cells to control the environment in which germ cells (i.e., spermatocytes, spermatids, and sperm) develop. It also prevents toxins from entering the seminiferous tubules, protecting the germ cells as they develop. The fluid in the open space of the seminiferous tubules (called the lumen) is different from the plasma in blood. It contains very little protein and glucose, and is rather composed of androgens, estrogens, and other substances. The blood-testis barrier helps maintain this unique chemical composition.
The blood-testis barrier can be found in all animals. In humans, it is absent until around the the age of 8, and begins to really assemble during the early stages of puberty (around the age of 11 or 12). This protective barrier is necessary for the creation of sperm (i.e., spermatogenesis), and any sort of disruption to it could lead to infertility in males.