Spermatogonial stem cells.

The mammalian seminiferous epithelium consists of a highly complex yet well-organized cell population, with germ cells in mitosis and meiosis and postmeiotic cells undergoing transformation to become spermatozoa. To study the factors which control renewal and differentiation of spermatogonial stem cells, animal models are now available which allow for arrest and restart of spermatogonial differentiation. In addition, marked progress has been made in understanding the control of apoptosis and its role in spermatogonia. For the future, spermatogonial stem cell transplantation may have important practical applications.