Elements that influence the structural rigidity of the bone marrow microenvironment. Model shows a developing megakaryocyte migrating from the endosteal bone lining to a bone marrow sinusoid. Cellular expansion, polyploidization, and development of the intracellular demarcation membrane system (DMS) are shown, with shedding of platelets from proplatelet extensions within the vessel lumen. Factors that may influence the mechanical forces (block arrows) within bone marrow include marrow fat content, extracellular matrix components (eg, collagen, fibronectin, and vitronectin), and the hydrostatic pressure of sinusoids, which in turn is determined by blood pressure, flow rate, blood viscosity, and temperature. HPC, hematopoietic progenitor cell; HSC, hematopoietic stem cell; MSC, mesenchymal stem cell.