The Adult Stem Cell Niche

Abstract

Tissue-specifi c adult stem cells generally exist in a quiescent state with only a small percentage actively dividing to meet the demand of homeostatic tissue replacement. However, a signifi cant number of stem cells can be recruited into cycle in response to injury. The actively dividing stem cell pool will produce cells that differentiate to replace the mature cells that were damaged, but will also rigorously maintain a critical number of stem cells. Given that stem cells have tremendous potential for use in tissue repair and replacement, understanding of how these stem cell fates are controlled has become an area of intense research. However, our ability to expand stem cells ex vivo for therapeutic purposes is still poorly developed, probably due to a lack of understanding of critical factors that maintain pluripo-tency. Much of our understanding of stem cell regulation comes from studies of the hematopoietic system, including the concept of a stem cell niche, a specialized microenvironment that maintains stem cells in the pluripotent state. In this chapter, we will review the major concepts that have emerged regarding the identity of the cellular/secreted components that infl uence stem cell fate.