Spermatogenesis is a series of complex processes that produce spermatozoa in male testis and it occurs through consecutive cell divisions and differentiation of germ cells (Russell et al., 1990). This process is initiated by a small number of spermatogonial stem cells (SSCs) that are only two or three per 104 testis cells in mouse case, and finally gives rise to many functional spermatozoa. Similar to the characteristics found in other adult stem cells, SSCs have the capability of self‐renewal and differentiation (Meistrich and van Beek, 1993). SSCs that have these two capabilities are the source of maintaining male postnatal fertility for lifetime. SSCs that exist inside the male testis maintain the numerical equilibrium through self‐renew after birth and they are the only germ‐line stem cell that can transfer the genetic information to the next generation through spermatogenesis. Therefore, when genetic modification is performed at the SSC stage, it can produce transgenic offspring in the next generation as well as germ‐line modification so that it can deliver the transformed character stably to the descendants. Past studies regarding the SSC had been dependent on morphological observations due to the absence of a marker system that can distinguish the SSCs. Brinster et al. (1994) published a groundbreaking turning point in identifying characteristics of SSC by developing SSC transplantation technique (Brinster and Avarbock, 1994; Brinster and Zimmermann, 1994). Utilizing the SSC transplantation technique, the self‐renew and production capability of differentiated tissue derived from transplanted SSC within the recipient’s seminiferous tubule can be directly analyzed. The biological activity of SSCs can also be investigated objectively by the SSC transplantation technique. Since the advent of the SSC transplantation technique, there have been a lot of progresses in the biological field of SSC. Recently, the enrichment technique of SSCs using FACS and specific surface marker, in vitro culture, and genetic modification techniques of SSCs have been developed in rodents. These techniques have potential to enhance the practical applications of SSCs. Characterization and development of useful technique for SSCs are now extending to livestock species.

• Buom-Yong Ryu(Department of Animal Science and Technology, Chung-Ang University)