The intercellular substance of bone consisting of collagenous fibers embedded in an amorphous ground substance and inorganic salts. Contains collagen type I (90%) and noncollagenous protein (about 10%): osteonectin, osteopontin, bone sialoprotein, and growth factors (cytokines) such as insulin‐like growth factors (IGF1 and IGF2), transforming growth factor beta 1 (TGF‐β1), platelet‐derived growth factor (PDGF), acidic and basic fibroblast growth factors (aFGF and bFGF), and bone morphogenetic proteins (BMPs).