Arterioles and sinusoids of the bone marrow (BM) are accompanied by stromal cells that express nerve/glial antigen 2 (NG2) and leptin receptor (LepR), and constitute specialised niches that regulate quiescence and proliferation of haematopoietic stem cells (HSCs). However, how niche cells differentially regulate HSC functions remains unknown. Here, we show that the effects of cytokines regulating HSC functions are dependent on the producing cell sources. Deletion of chemokine C-X-C motif ligand 12 (Cxcl12) or stem cell factor (Scf) from all perivascular cells marked by Nestin-GFP dramatically depleted BM HSCs. Selective Cxcl12 deletion from arteriolar NG2+ cells, but not from sinusoidal LepR+ cells, caused HSC reductions and altered HSC localisation in BM. By contrast, deletion of Scf in LepR+ cells, but not NG2+ cells, led to reductions in BM HSC numbers. These results uncover distinct contributions of cytokines derived from perivascular cells in separate vascular niches to HSC maintenance.

Stem cells ensure tissue regeneration, while overgrowth of adipogenic cells may compromise organ recovery and impair function. In myopathies and muscle atrophy associated with aging, fat accumulation increases dysfunction, and after chronic injury, the process of fatty degeneration, in which muscle is replaced by white adipocytes, further compromises tissue function and environment. Some studies suggest that pericytes may contribute to muscle regeneration as well as fat formation. This work reports the presence of two pericyte subpopulations in the skeletal muscle and characterizes their specific roles. Skeletal muscle from Nestin-GFP/ NG2-DsRed mice show two types of pericytes, Nestin-GFP-/NG2-DsRed + (type-1) and Nestin-GFP + /NG2-DsRed + (type-2), in close proximity to endothelial cells. We also found that both Nestin-GFP-/NG2-DsRed + and Nestin-GFP + /NG2-DsRed + cells colocalize with staining of two pericyte markers, PDGFRb and CD146, but only type-1 pericyte express the adipogenic progenitor marker PDGFRa. Type-2 pericytes participate in muscle regeneration, while type-1 contribute to fat accumulation. Transplantation studies indicate that type-1 pericytes do not form muscle in vivo, but contribute to fat deposition in the skeletal muscle, while type-2 pericytes contribute only to the new muscle formation after injury, but not to the fat accumulation. Our results suggest that type-1 and type-2 pericytes contribute to successful muscle regeneration which results from a balance of myogenic and nonmyogenic cells activation.

Whereas the cellular basis of the hematopoietic stem cell (HSC) niche in the bone marrow has been characterized, the nature of the fetal liver (FL) niche is not yet elucidated. We show that Nestin+NG2+ pericytes associate with portal vessels, forming a niche promoting HSC expansion. Nestin+NG2+ cells and HSCs scale during development with the fractal branching patterns of portal vessels, tributaries of the umbilical vein. After closure of the umbilical inlet at birth, portal vessels undergo a transition from Neuropilin-1+Ephrin-B2+ artery to EphB4+ vein phenotype, associated with a loss of periportal Nestin+NG2+ cells and emigration of HSCs away from portal vessels. These data support a model in which HSCs are titrated against a periportal vascular niche with a fractal-like organization enabled by placental circulation.