Schematic representation of the medullar niches. In healthy adults, hematopoiesis occurs in the bone marrow where hematopoietic stem cells (HSCs) are engaged in a constant crosstalk within specific niches. Very schematically, these haematopoietic stem cell niches are conceptually divided into two niches: The endosteal niche in which osteoblasts, derived from Mesenchymal Stromal Cell (MSC) differentiation and contributing to bone formation and osteoclats, derived from HSCs, and participating in bone resorption played a crucial role. The vascular niche in which endothelial cells, resulting from the proliferation and differentiation of Endothelial Stem Cells (ESC), are in close contact with HSC within the perivascular spaces. Beyond cell-cell contacts through receptor-ligand and adhesion molecule interactions, extracellular matrix component (EMC), cytokines and chemokines, proteases, calcium and oxygen concentrations are essential environmental components of these niches.