Fig. 1.

Endosteal, reticular, and vascular niches in the context of bone marrow. In the endosteal niche, HSCs proliferate and, while their number increases, they move toward the vascular niche following cytokine and physical gradient (mobilization) and reaching the vascular niche, where further differentiation into mature elements occurs. HSCs differentiation leads to cell types, including monocytes, which can turn into osteoclasts or macrophage. Monocytes and machrophages can then move to peripheral tissues through systemic vessels. HSCs homing is favoured by CXCL12 produced by CAR cells, particularly abundant in reticular niche. In bone marrow niche, mesenchymal stem cells can be found, which under the influence of Wnt10b and BMP2 are able to move to bone matrix and differentiate into osteoblasts. B Lym B lymphocyte, Bas basophil, BMP2 bone morphogenetic protein 2, CAR cells CXCL12-abundant reticular cells, CLP common lymphoid progenitor, CMP common myeloid progenitor, CXCL12 CXC-chemokine ligand 12, Dkk1,2 Dickkopf Wnt signalling pathway inhibitor 1,2, EB erythroblast, EC erythrocyte, Eos eosinophil, HSCs haematopoietic stem cells, IFNγ interferon-γ, IL1b interleukin-1b, IL-17 interleukin-17, JAG1 jagged1, MB myeloblast, MC monocyte, M-CSF macrophage colony-stimulating factor, MKC megakaryocyte, Neu neutrophil, OPG osteoprotegerin, PLT platelet, PTH parathyroid hormone, RANKL receptor activator of nuclear factor kappa-B ligand, ROS reactive oxygen species, T Lym T lymphocyte, TNFα tumour necrosis factor-α, VMSCs vascular smooth muscle cells, Wnt10b Wingless-related integration sites proteins 10b