Pericytes, integral components of adult hematopoietic stem cell niches

The interest in perivascular cells as a niche for adult hematopoietic stem cells (HSCs) is significantly growing. In the adult bone marrow (BM), perivascular cells and HSCs cohabit. Among perivascular cells, pericytes are precursors of mesenchymal stem/stromal cells (MSCs) that are capable of differentiating into osteoblasts, adipocytes and chondrocytes. In situ, pericytes are recognised by their localisation to the abluminal side of the blood vessel wall and closely associated with endothelial cells, in combination with the expression of markers such as CD146, neural glial 2 (NG2), platelet derived growth factor receptor β (PDGFRβ), α-smooth muscle actin (α-SMA), nestin (Nes) and/or leptin receptor (LepR). However, not all pericytes share a common phenotype: different immunophenotypes can be associated with distinct mesenchymal features, including hematopoietic support. In adult BM, arteriolar and sinusoidal pericytes control HSC behaviour, maintenance, quiescence and trafficking through paracrine effects. Different groups identified and characterized hematopoietic supportive pericyte subpopulations using various markers and mouse models. In this review, we summarize recent work performed by others to understand the role of the perivascular niche in the biology of HSCs in adults, as well as their importance in the development of therapies.