The Adult Stem Cell Niche: Multiple Cellular Players in Tissue Homeostasis and Regeneration

Adult somatic stem cells, also called progenitors, are essential for tissue regeneration and homeostasis. Stem cells have been identified in almost all tissues including those with little regenerative capacity. It is a central interest in the field of regenerative medicine to understand the signals that govern stem cell function, particularly in tissues that typically do not regenerate. Progenitors are present in small numbers in most tissues and reside in a specific cellular context that is referred to as the stem cell niche. In this article, we survey the stem cell niche in several tissues and examine its role in stem cell function. While the anatomical appearance of the stem cell niche differs among tissues, common features exist that serve similar roles. We review here the stem cell niche in two tissues capable of robust regeneration (skeletal muscle and skin) and two tissues that display poor regenerative capacity (CNS and heart). We propose that an increased understanding of the stem cell niche, particularly in tissues that are ‘poor regenerators,’ will have a significant impact upon regenerative medicine. An understanding of the molecular signals that are transmitted between the progenitors and neighboring niche cells will help identify new therapeutic targets to augment regenerative capacity as well as increase our fundamental understanding of these important cellular players.