Bone marrow vascular niche and the control of angiogenesis in multiple myeloma.

Bone marrow contains hematopoietic stem cells (HSCs) and non hematopoietic cells. HSCs are able to give rise to all types of mature blood cells, while the non hematopoietic component includes osteoblasts/osteoclasts, endothelial cells, endothelial progenitor cells and mesenchymal stem cells. All of these cells form specialized "niches" which are close to the vasculature ("vascular niche") or to the endosteum ("osteoblast niche"). The "vascular niche" is rich in blood vessels where endothelial cells and mural cells (pericytes and smooth muscle cells) create a microenvironment that affects the behavior of several stem and progenitor cells. The vessel wall serves as an independent niche for the recruitment of endothelial progenitor cells, mesenchymal stem cells and HSCs. The activation by angiogenic factors and inflammatory cytokines switch of the "vascular niche" promote tumor growth. This review article will focus on the description of the mechanisms involved in the generation of signals released by endothelial cells in the "vascular niche" that promote tumor growth in multiple myeloma.