FIG. 2.

Temporal sequence of growth factors (GFs) and cytokines expression during bone regeneration. The solid dash line represents the inflammatory sequence of events, peaking early upon the injury with high expression of cytokines, such as tumor necrosis factor (TNF)-α, interleukin (IL)-1, -6, -11, and -18, and angiogenic molecules Angiopoietin-1 (Ang-1) and Ang-2. These molecules are involved in the stimulation of cell migration, production of cartilaginous callus, and vascular endothelial growth factor (VEGF) production. The square dot line represents the chondrogenic route that leads to the generation of a cartilaginous callus peaking between 7 and 14 days after injury. Transfoming growth factor (TGF)-β1 and Growth differentiation factor-8 (GDF-8) act early in this step, with other isoforms responsible for chondrogenesis and endochondral ossification peaking later in the cycle. The long dash double dot line represents the osteogenic sequence of molecules acting on the formation of a bony callus. Simultaneously to the expression of chondrogenic markers, bone morphogenetic protein (BMP)-2, BMP-7, and stromal cell-derived factor-α (SDF-α) are responsible for inducing the migration of mesenchymal stem cells (MSCs) with fibroblast growth factor (FGF)-2 stimulating their proliferation. As callus chondrocytes proliferate, they become hypertrophic and express high levels of VEGF, promoting the invasion of blood vessels, leading to the tissue vascularization. During this step, TNF-α initiates chondrocyte apoptosis and promotes the recruitment of MSCs with osteogenic potential. Moreover, this cytokine is also involved in bone remodeling, the last step in which the hard callus undergoes a resorptive phase to form the typical lamellar bone structure with a central medullar cavity.^201–203