Adult Stem Cells in Tissue Maintenance and Regeneration

Adult stem cells, also known as somatic stem cells, reside in specific tissues and possess fundamental properties of stem cells, that is, self-renewal capacity and, though limited in magnitude, multipotency. Generally, these primitive cells are stored in a specialized environment called niche, where they are connected to supporting cells, protected from external harmful stimuli, and kept quiescent until the arrival of an appropriate activating signal. In accordance with the demand of the organ, tissue-specific adult stem cells proliferate, migrate to leave the niche, and differentiate to replace senescent or deteriorated cells, maintaining the organ structure and function. Also, they are known to repair mild injuries in various organs including the skin, liver, intestine, kidney, and bone marrow. Such endogenous regenerative mechanisms, however, appear insufficient to cope with severe damage, as in the case of myocardial infarction [1, 2] or cerebral ischemia [3], in which the damage is mostly irreversible despite the presence of local stem cells. Possibly, tissue-specific biological cues that determine the fate of adult stem cells and their committed progenitors in normal and pathological conditions pose limits to cell differentiation and survival in vivo.