Stem cell interactions in a bone marrow niche.

Stem cells differ from other cells of the body in their potential for multilineage differentiation and their continued proliferation without substantial loss of potential (so-called self-renewal). These properties are maintained and regulated by a specific microenvironment referred to as "niche." This term has been used to indicate the specific location of stem cells within tissues, as well as the cellular and molecular components that critically determine stem cell behavior. Whereas other, perhaps less complex, stem cell niches (e.g., Drosophila germarium) have been more clearly dissected in the 30 years that have passed since these observations, the hematopoietic stem cell (HSC) niche has proven challenging due to the difficulty to detect HSCs under normal conditions and the dynamism of HSCs and other cells of the bone marrow that influence HSC behavior. This article reviews the recent development of the HSC niche field with emphasis on prospective integrative mechanisms within bone marrow homeostasis and multisystem physiology. For that purpose, we will first highlight anatomical and histological features of the bone marrow of relevance for HSC behavior; then, we will summarize the principal findings concerning different cell types and potential mechanisms by which they critically regulate HSC function.