Stem Cell Heterogeneity Actual and Potential Stem Cell Compartments in Mouse Spermatogenesis SHOSEI YOSHIDA, YO-ICHI NABESHIMA, AND TOSHINORI NAKAGAWA Department of Pathology and Tumor Biology, Graduate School of Medicine, Kyoto University, Yoshida-Konoe, Sakyo, Kyoto, Japan ABSTRACT: In order to assess the behavior of the spermatogonia that contribute to the stem cell system in steady-state mouse spermatogen- esis, a series of pulse-labeling experiments of a subset of “undifferenti- ated spermatogonial” was performed using transgenic approaches. The results suggest that, in addition to the spermatogonial population that actually acts as the stem cells (actual stem cell compartment), a second set of undifferentiated spermatogonia also exists that possesses the potential to self-renew, but does not normally do so (potential stem cell compart- ment). The potential stem cells turn over rather rapidly, suggesting that they act as the transit amplifying cells in the normal situation. The po- tential stem cells seem to change their mode from transientamplification to self-renewal upon transplantation, regeneration, or loss of actual stem cells that occurs during the long reproduction period. Such a hetero- geneous composition of the stem cell system may be important for the robustness of overall spermatogenesis. KEYWORDS: spermatogenic stem cells; mouse; steady-state spermato- genesis; undifferentiated spermatogonia; pulse-chase; transplantation; actual stem cells; potential stem cells; transit amplifying cells; Ngn3 (neurogenin3) BACKGROUND It has been established that the spermatogenic stem-cell functions reside in the “undifferentiated spermatogonia,” the most primitive set of spermato- gonia, including A single (isolated spermatogonia, also known as A s ), A paired (interconnected pairs of spermatogonia; A pr ), and A aligned (chains of 4, 8, 16, Address for correspondence: Shosei Yoshida, M.D., Ph.D., Department of Pathology and Tumor Biology, Graduate School of Medicine, Kyoto University, Yoshida-Konoe, Sakyo, Kyoto, Kyoto 606- 8501, Japan. Voice: +81-75-753-4426; fax: +81-75-753-4478. shosei@lmls.med.kyoto-u.ac.jp Ann. N.Y. Acad. Sci. 1120: 47–58 (2007). C  2007 New York Academy of Sciences. doi: 10.1196/annals.1411.003 47