Extravascular coagulation in HSC niche regulation. HSC niches are found in osteoblast (OB)- and osteoclast (OC)-regulated endosteal regions vascularized by arterioles and in megakaryocyte (MK)-rich sinusoidal regions. HSCs are maintained by mesenchymal cell (MC)-derived VCAM, KitL (SCF), CXCL12, and matrix factors (osteopontin; OPN) that are regulated by coagulation and fibrinolysis.