Spermatogonial stem cells and in vitro spermatogenesis.

Spermatogonial stem cells (SSCs) provide the basis for the life-long production of enormous numbers of sperm. The nature of these mysterious cells is being clarified. Although they were regarded to be mostly dormant, dividing rarely and remaining static in a niche, their rather dynamic behavior in the seminiferous tubules has been disclosed. The territories of each colony of SSCs can also quickly change in size. The development of a culture method for SSCs also shed light on their stable, but at the same time, fragile characteristics. In addition, an in vitro system for spermatogenesis was developed which can produce functional sperm from SSCs. These new developments will contribute to reproductive medicine.