FIG. 1.

BM microenvironment organization. The BM is spatially organized into different niches. The osteoblastic niche (left) resides in the inner part of BM, lining on the endosteum. It includes the most primitive stem cells and a frame of stromal cells, principally osteoblasts. The vascular niche (right) is constituted by committed progenitor elements that are embedded in a frame of several kinds of stromal cells (ECs, macrophages, perycites, etc.). In both niches, stem and progenitor cells establish cell–cell contacts that are instrumental to reciprocal control of cell function. Ang-1, angiopoietin 1; BM, bone marrow; cKit, stem cell factor receptor; EC, endothelial cell; MMP-9, metalloproteinase 9; N-Cad, N-cadherin; SCF, also known as kit-ligand, stem cell factor; SDF-1, stromal cell-derived factor-1; SDF-1 receptor, CXCR4, C-X-C chemokine receptor type 4; Tie2, angiopoietin receptor 2; VCAM-1, integrin receptor vascular cell adhesion molecule-1; very late antigen-4, Vla-4, integrin alpha4beta1. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars