Making sense of hematopoietic stem cell niches.

The hematopoietic stem cell (HSC) niche commonly refers to the pairing of hematopoietic and mesenchymal cell populations that regulate HSC self-renewal, differentiation, and proliferation. Anatomic localization of the niche is a dynamic unit from the developmental stage that allows proliferating HSCs to expand before they reach the bone marrow where they adopt a quiescent phenotype that protects their integrity and functions. Recent studies have sought to clarify the complexity behind the HSC niche by assessing the contributions of specific cell populations to HSC maintenance. In particular, perivascular microenvironments in the bone marrow confer distinct vascular niches that regulate HSC quiescence and the supply of lineage-committed progenitors. Here, we review recent data on the cellular constituents and molecular mechanisms involved in the communication between HSCs and putative niches.