The adult bone marrow HSC niche. The vasculature has emerged as a key structure for the maintenance of HSCs in the bone marrow. Dormant HSCs are found around arterioles where factors such as CXCL12 and SCF secreted by perivascular, endothelial, Schwann, and sympathetic neuronal cells promote their maintenance. Less quiescent or activated HSCs are located near sinusoidal niches which are likely diverse in their influence for self-renewal, proliferation, and differentiation. Hematopoietic cells such as macrophages or megakaryocytes are examples of HSC-derived progeny that can feed back to the niche to influence HSC migration or proliferation. GFAP, glial fibrillary acidic protein; TGF-β1, transforming growth factor beta-1.