# Nanomaterials in Bone Regeneration
## 2 Bone Biology
### Bone Extracellular Matrix  
The mechanical properties of bone tissue strongly depend on the specific structure and organization of the bony extracellular matrix (ECM). The bone tissue consists of 20% organic and 65% inorganic substances; the remaining 15% is water and fat. The bone ECMis a dynamic network composed of collagenous and non-collagenous proteins, which are the main components of the trabeculae and osteon units. The most abundant constituent of the organic matrix is collagen, a triple-helical molecule formed by three amino acid chains (two identical and one different), which form the collagen fibrils. A small amount of collagen III and V regulate the fiber diameter and fibrillogenesis of type I collagen. The crosslinking of collagen is key to its mechanical properties. The main function of collagens is to act as a scaffold for bone cells. The remaining organic part of ECM is other different non-collagenous proteins, such as OC, osteonectin (ON), osteeopontin (OPN), bone sialoprotein, fibronectin, bone morphogenetic protein (BMP), and other types of proteoglycans involved in the regulation of bone matrix mineralization, organization, and other bone cell activity [47]. Osteocalcin (OC, \(\gamma\)-carboxoglutamic acid protein) is a 46-50 amino acid residues long protein secreted primarily by osteoblasts. It is produced as a pre-pro-protein undergoing further modifications, including the conversion of glutamate residues to \(\gamma\)-carboxoglutamate. The mature OC protein is packaged into intracellular vesicles for secretion into the bone matrix. Initially, it was proposed that OC initiates the hydroxyapatite (HAp) formation by binding to calcium ions. However, other studies point to its function as an inhibitor of bone mineralization. This idea is supported by research, where warfarin treatment results in higher mineralization of bones and early closure of the growth plate [48; 49]. Based on many studies, OC is not only a protein bound in ECM; it is a regular bone-derived factor that influences glucose metabolism, reproduction, and cognition through endocrine loops between bone and the brain, pancreas, and testes [50; 51; 52; 53]. Osteonectin (ON) is a calcium-binding glycoprotein associated with the growth and remodeling of bones. Bone ON binds to Col1 andHAp, and serves as a link between the mineral and organic phases of bone tissue. With the ability to induce mineral deposition onto type I collagen and inhibit the growth of HAp crystals, it can help in the regulation of bone mineralization. Osteopontin (OPN) is expressed by osteoblasts, osteocytes, and odontoblasts. After post-translational modification, it is a highly glycosylated and phosphorylated protein with high importance as a component of mineralized extracellular matrices of bones and teeth. The binding of OPN to integrins is crucial for signal responses, which involves regulation of osteocl activity and activation of OPG expression. Further, OPN binding to integrin plays a major role in the formation of a sealing zone in osteoclast activity. OPN has a chemotactic activity on the precursors of osteoclasts and, as a part of the sealing zone, affects osteocl resorption [54]. OPN induces on the surface of osteoclasts integrin clustering and leads to intracellular signaling. The expression of OPN increases in response to mechanical stress [17; 18; 19]. Therefore, it is a critical factor in regulating bone remodeling in response to mechanical stimuli. The organic part of the bone matrix contains a plethora of other proteins with diverse and overlapping tasks in bone mineralization, strength, stability, and integrity [55]. The inorganic component serves as an ion reservoir and contributes to bone stiffness and strength. Its basic components are apatite crystals containing calcium and phosphate ions, as well as sodium and magnesium ions. The mineral part of the bone, which essentially contains HAp, contributes to 65% of the bone weight.