"Niche" concept and the hematopoietic stem cell niches

A stem cell niche is defined as the most suitable tissue microenvironment where somatic cells (parent cells) or in some cases extracellular matrix elements and stem cells are present in an exclusive interaction. In recent years, promising data were published revealing the molecular and functional significance of cell-cell contacts in stem cell proliferation (self-renewal) and differentiation into mature cells. In this brief review, recent progress has been documented with special emphasis on the structural-functional relationship between stem cells and surrounding cells. Hematopoietic stem cells and their niches are among the most studied stem cell types and cell-cell interactions, which have specific as well as some common properties observed in other niche sites in the human body. Some of the signaling pathways, which are illustrated by original drawings herein, obviously play major roles in stem cell self-renewal capacity and differentiation status. Therefore, stem cell niches stand as the most critical biological sites in stem cell physiology and could also be considered as the new targets for the treatment of certain diseases.