Signaling in adult stem cells

Abstract

Adult stem cells are set aside during development in order to provide a source for replenishment of tissue over time in response to damage or simply wear and tear. The literature suggests that stem cells can be found in most major organ systems, and that they possess defining characteristics, namely the ability to both self-renew and differentiate down one or more specific lineages. Many groups have sought to define stem cell specific physiology in a molecular fashion by identifying those genes specifically expressed in stem cells. Although these data suggest that there are genes frequently found to be upregulated in stem cells from various tissues, they do not definitively demonstrate that these cells all function similarly. There is also considerable data showing how various signaling pathways influence stem cell growth and differentiation. A review of this literature suggests that many of the well-described pathways affect adult mammalian stem cells from different tissues similarly, and that these effects are sometimes unique to stem cells as opposed to their progeny. In this review we summarize the effects of well-known signaling pathways on several of the most well defined stem cells and argue that the similarity with which unique stem cells from different tissues respond to external stimuli suggests that they share functional mechanisms.