Many tissues and organs maintain a process known as homeostasis, in which cells are replenished as they die as a result of apoptosis or injury. The continuously growing mouse incisors are an excellent model for studying the molecular mechanisms of cell homeostasis, renewal, and repair. We elucidated these mechanisms in mouse incisors by detecting adult stem cells and analyzing the stem cell lineage by bromodeoxyuridine (BrdU) labeling analysis. The stem cells divide slowly, giving rise to a daughter cell that remains in the cervical loop and a second daughter cell that enters the zone of rapidly dividing inner enamel epithelial cells (transit-amplifying cell population). During subsequent rounds of cell division, the latter cells move toward the incisal end and differentiate into ameloblasts that form the enamel matrix. Recent evidence from gene knockout mice suggests that fibroblast growth factor (Fgf10) plays an important role in the formation and maintenance of stem cells in the development of mouse incisors. The role of dental stem cells in odontogenic tumors is discussed.