Abstract
No metazoan cell survives on its own, absent the signals and support of its milieu. For multicellular life with specialized tissues to persist, organization is everything and so defining the association of position with cell state is critical to understanding how tissues function, maintain, and repair. This review focuses specifically on place for progenitor and stem cells. Especially emphasized are hematopoietic cells that balance free movement and stable position and where concepts of regulatory interrelationships have been shown with some precision. It reviews classical and emerging concepts of the niche, particularly considering how niche functions may participate in neoplastic disease.