A simplistic model of hematopoietic stem cell niches. In adults, hematopoiesis occurs in the bone marrow where hematopoietic stem cells (HSCs) are engaged in a constant cross talk within specific niches at the proximity of the endosteal surface (endosteal niche) and of the perivascular space (vascular niche). These niches are composed of (1) stromal cells derived from mesenchymal stem cells (MSCs), able to generate fibroblasts, osteoblasts/osteocytes, and adipocytes, (2) osteoclasts derived from HSCs, and (3) endothelial cells derived from endothelial stem cells (ESCs). Regulatory signals emanating from these different niches include cell-intrinsic regulatory mechanisms, adhesion molecule interactions, extracellular matrices (ECMs), and environmental components such as calcium (Ca⁺⁺), oxygen (O₂) concentration, proteases, as well as humoral factors including cytokines and chemokines.