# Bioactivity and Delivery Strategies of Phytochemical Compounds in Bone Tissue Regeneration
## 3 Bone Regeneration Process  
The bone healing process is a complex mechanism of bone regeneration, which involves inflammation, bone production, and bone remodeling phases [34]. Inflammation is usually observed immediately following the fracture at the injury site since tissues swell, bone cells die, and blood vessels break, with consequent formation of hematoma, a source of hematopoietic cells capable of secreting growth factors. The injury to bone leads to the secretion of pro-inflammatory factors like tumor necrosis factor-alpha (TNF-\(\alpha\)), bone morphogenetic proteins ( BMPs), and interleukins (IL-1, IL-6, IL-11, IL-23). These molecules attract, at the fracture site, macrophages, monocytes, and lymphocytes able to take out damaged necrotic tissue and secrete factors (i.e., vascular endothelial growth factor, VEGF) to stimulate angiogenesis and healing [35]. MSCs migrate to the fracture site and, under BMPs control, start to differentiate into fibroblasts, osteoblasts, and chondroblasts. As a result, chondrogenesis begins to occur, and a fibroratilaginous callus (also called \"soft callus\") forms within two weeks. The soft callus undergoes endochondral ossification, which converts fibroratilaginous callus to bony callus (also identified as \"hard callus\"). The expression of RANKL promotes further differentiation of chondroblasts, osteoblasts, and osteoclasts [36; 37; 38]. Bone regeneration is thinly modulated by several signaling pathways and transcriptional factors. During the early stages of bone healing, the Wnt pathway suppresses the differentiation of mesenchymal stem cells into osteoblasts, while in the later stages it controls the commitment of the undifferentiated cells to the osteoblasts [39]. Notch signaling is another potential pathway with osteoinductive properties. Notch receptors, through their ligand (Jag-1), increase the expression of genes related to osteoblasts as ALP and BSP, inducing osteoclastogenesis [40]. An interesting study demonstrates that the functionalization of titanium implant surface with Jag-1 contributes to the enhancement of osteoblast differentiation, improving osteogenic properties [41]. Indeed, the activation of the Notch pathway leads to an increase of osteogenic differentiation, as highlighted by an upregulation of osteogenic markers, in particular BSP and OCN; bone differentiation proteins as BMP2 and BMP6; and growth/differentiation factor 15 (GDF15) [42]. Several other factors are implicated in bone regeneration processes, such as mitogen-activated protein kinase (MAPK) associated pathways, phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/Akt, growth factors as fibroblast growth factors (FGFs), insulin-like growth factors (IGFs), and VEGF [43]. MAPKs pathway takes part in bone formation and bone healing post-fracture through the transduction of signals induced by numerous growth factors or adhesion molecules. The (PI3K)/Akt signaling pathway promotes the expression of OPG, Runx2, \(p\)-Akt, and BMP-2 proteins, as well as the proliferation, differentiation, and osteogenesis of osteoblasts [44]. [PERSON] and [PERSON] demonstrated that the release of rh-VEGF, rh-IGF-I from a polylatcle-polyglycolide acid membrane and \(\beta\)-tricalcium phosphate bone graft led to better clinical results such as bone pocket reduction and bone filling simultaneously with respect to growth factors used alone [45]. Furthermore, FGF signaling plays a pivotal role in the intramembranous and endochondral signaling pathway regulating process in osteoprogenitor cells [46]. It has been reported that treatment with hydrogel-bFGF after a fracture has a higher rate of mineralization, as well as an upregulation of Runx2 and osteoclastin in mice [47; 48]. Normal bone development requires the downregulation of Runx2 to form mature bone [49].