Figure 1.

Bone-like nodule formation. (A): Schematic overview of developmental sequences involving morphologic progressions and selective expression of genes that regulate cell proliferation and promote osteoprogenitor commitment and bone-like nodule formation. These processes were schematically illustrated by three major stages divided by two restrictive points. Downregulation of proliferation/ECM accumulation and ECM remodeling/mineral deposition corresponds, respectively, to the first and second restrictive points. Details about OB-mediated ECM mineralization (red square) are provided in (B). (B): The proposed OB-dependent mechanism of calcium phosphate nucleation includes both extracellular and intracellular matrix vesicles. Abbreviations: AM, amorphous mineral; CK, cytoskeleton; COL-I, type-1 collagen; ECM, extracellular matrix; eMV, extracellular matrix vesicle; GJ, gap junction; I, integrin; iMV, intracellular matrix vesicle; M, mitochondria; MC, mineral crystal; MSC, mesenchymal stem cell; N, nucleus; OB, osteoblast; OP, osteoprogenitor cell; RER, rough endoplasmic reticulum.