Whereas a woman "produces oocytes only between the age of puberty and that of menopause, a man produces sperm in permanence from puberty." This production of sperm, which diminishes with age, "is made possible by the existence in the testicle of spermatogonial stem cells" which "are able to renew themselves independently and differentiate into sperm indefinitely."
Several causes of male sterility are observed. It may be due to an "insufficient production of sperm owing to the existence of too few spermatogonial stem cells" in the testicle of the patient, or to a "blockage of the process of forming sperm."
It may also be caused by the existence "in the testicle of support cells called Sertoli cells."
Lastly, it may be a case of "secondary male sterility […] resulting from a toxic treatment given for a cancerous disease, for example."
For this last type, secondary male sterility, finding therapeutic solutions remains a difficult challenge. Dr Virginie Barraud-Lange, from the Histology-Embryology and Reproduction Biology department of Cochin hospital, explains that it is not possible today to "preserve the stem cells before an anti-cancer treatment or even to try to restore spermatogenesis of a man not producing enough sperm." Tests have been made only with mice. In 1994, research succeeded in restoring "complete spermatogenesis in a mouse rendered sterile, after transplanting spermatogonial stem cells taken from another mouse." This transplant enabled "the host mouse to produce offspring possessing the characteristics of the donor mouse," and "no disorder in their development or genetic modification was observed in two generations of mice descended from the transplanted male." The same tests have been carried out on other species of mammals such as goats, pigs and cattle, and "recently on the macaque rhesus monkey." The journal Cell Stem Cell has also reported on the recent research of the Hermann group, in Pittsburgh, which shows that "spermatogonial stem cells transplanted into the testicles of sterilised animals are also capable of producing functional sperm capable of fertilizing oocytes in vitro by micro-injection (ICSI)."
In man, researchers have not yet managed to "individualise correctly" the spermatogonial stem cells. Once this technique has been developed, Dr Virginie Barraud-Lange points out that "it could then be possible to preserve the stem cells of per-pubertal boys before subjecting them to treatments carrying a risk of sterility" and "to enrich the testicle of a sterile man by multiplying in the laboratory his own spermatogonial stem cells before transplanting them to resupply his testicle."