after bone fracture, blood cells accumulate adjacent to the injury site. soon after fracture, blood vessels constrict, stopping further bleeding. within a few hours, the extravascular blood cells form a clot called a hematoma that acts as a template for callus formation. these cells, including macrophages, release inflammatory mediators such as cytokines (tumor necrosis factor alpha (tnfα), interleukin-1 family (il-1), interleukin 6 (il-6), 11 (il-11), and 18 (il-18)) and increase blood capillary permeability. inflammation peaks by 24 hours and completes by seven days. through tumor necrosis factor receptor 1 (tnfr1) and tumor necrosis factor receptor 2, tnfα mediates the differentiation of mesenchymal stem cell (originated from the bone marrow) into osteoblast and chondrocytes. stromal cell-derived factor 1 (sdf-1) and cxcr4 mediate recruitment of mesenchymal stem cells. il-1 and il-6 are the most important cytokines for bone healing. il-1 promotes formation of callus and of blood vessels. il-6 promotes differentiation of osteoblasts and osteoclasts. all cells within the blood clot degenerate and die. within this area, the fibroblasts replicate. within 7-14 days, they form a loose aggregate of cells, interspersed with small blood vessels, known as granulation tissue. osteoclasts move in to reabsorb dead bone ends, and other necrotic tissue is removed.