Complexities of modeling the bone marrow microenvironment to facilitate hematopoietic research

Haematopoiesis occurs in the bone marrow (BM), within a specialised microenvironment referred to as the stem cell niche, where the haematopoietic stem cells (HSCs) reside and are regulated for quiescence, self-renewal and differentiation through intrinsic and extrinsic mechanisms. The BM contains at least two distinctive HSC supportive niches: an endosteal osteoblastic niche, which supports quiescence and self-renewal and a more vascular/peri-sinusoidal niche that promotes proliferation and differentiation. Both associate with supporting mesenchymal stromal cells (MSCs). Within the more hypoxic osteoblastic niche, HSCs specifically interact with the osteoblasts that line the endosteal surface, which secrete several important HSC quiescence and maintenance regulatory factors. In vivo imaging indicates that the HSCs and progenitors located further away, in the vicinity of sinusoidal endothelial cells, are more proliferative. Here HSCs interact with endothelial cells via specific cell adhesion molecules. Endothelial cells also secrete several factors important for HSC homeostasis and proliferation. In addition, HSCs and MSCs are embedded within the extracellular matrix (ECM), an important network of proteins such as collagen, elastin, laminin, proteoglycans, vitronectin and fibronectin. The ECM provides mechanical characteristics such as stiffness and elasticity important for cell behaviour regulation. ECM proteins are also able to bind, sequester, display and distribute growth factors across the BM, thus directly affecting stem cell fate and regulation of haematopoiesis. These important physical and chemical features of the BM require careful consideration when creating three dimensional models of the BM.