The bone matrix is the non-cellular, extracellular substance that forms the structural framework of bone tissue. It is the primary component of bone, giving it distinct properties and making up most of its mass. This material surrounds living bone cells, providing the rigid foundation for the skeletal system.
Key Components
The bone matrix is composed of two main categories: organic and inorganic components. The organic portion, making up about 30-35% of the matrix by weight, primarily consists of Type I collagen fibers. This collagen is the most abundant protein, providing bone with flexibility and tensile strength, allowing it to resist stretching and twisting forces.
Beyond collagen, the organic matrix includes various non-collagenous proteins, which play specific roles in bone formation and mineralization. Osteonectin, for example, is a protein that binds to both collagen and the mineral component, helping to initiate and regulate the mineralization process. Osteocalcin, another non-collagenous protein, is produced by bone-forming cells and actively participates in binding calcium and controlling the deposition of minerals within the matrix. Proteoglycans, which are proteins with attached sugar chains, help organize the extracellular matrix, regulate collagen fiber formation, and attract water, contributing to the bone’s hydration and mechanical properties.
The inorganic component makes up about 60-70% of the bone matrix by weight and is primarily hydroxyapatite. This mineral, a crystalline form of calcium phosphate, is responsible for bone’s hardness and rigidity. Hydroxyapatite crystals are small, plate-like or needle-like structures deposited within and around collagen fibers, forming a highly organized composite material.
Essential Functions
The bone matrix performs several functions. It provides structural support, forming the body’s rigid framework, anchoring muscles, and protecting internal organs. This framework allows for movement and maintains overall body shape.
The hardness from inorganic minerals also protects organs. For instance, the skull shields the brain, and the rib cage safeguards the heart and lungs from external forces. This protection results from the bone matrix’s dense, rigid nature.
The bone matrix serves as a reservoir for minerals, particularly calcium and phosphate. It helps maintain the body’s mineral balance by storing and releasing these ions into the bloodstream when needed. The combination of flexible organic components and rigid inorganic minerals gives bone its mechanical strength. This blend allows bone to withstand various stresses, including compression, tension, and torsion, making it strong and resilient to fracture.