A bone is a living connective tissue organ composed of cells, an extracellular matrix containing organic components, mineral crystals, water, blood vessels, nerves and marrow spaces. The combination of these materials gives bone its distinctive balance of strength, stiffness and toughness. The organic component is dominated by type I collagen, which forms a flexible protein framework. The inorganic component consists primarily of calcium phosphate mineral organised largely as hydroxyapatite crystals.
Collagen helps bone resist tension and prevents it from being excessively brittle, while mineral provides hardness and resistance to compression. Water is also important because it contributes to the physical properties and biochemical environment of the matrix. Bone therefore cannot be accurately described as simply calcium. Calcium and phosphate are major mineral constituents, but bone is a complex living tissue containing cells and multiple classes of extracellular material.
Type I collagen is the dominant structural protein in the organic matrix of most bone. Collagen molecules are assembled into fibres that provide a framework onto which mineral crystals are deposited. The organisation of collagen at microscopic scales contributes strongly to the ability of bone to absorb energy and resist cracking. Other organic components include non collagenous proteins and proteoglycans that influence mineralisation, cell adhesion and matrix organisation. Osteoblasts synthesise much of the organic matrix before it becomes mineralised.
The initial unmineralised matrix produced by osteoblasts is called osteoid. As mineral deposition proceeds, the tissue becomes harder and more resistant to compression. The interaction between collagen and mineral is therefore fundamental. Neither component alone reproduces the full mechanical behaviour of intact bone. The composite structure allows bone to be simultaneously strong, relatively light and capable of tolerating repeated mechanical stresses.
The mineral component of bone consists mainly of calcium phosphate crystals chemically related to hydroxyapatite. These crystals are deposited within and around the collagen framework. Bone mineral contains calcium and phosphate as major elements and can also incorporate other ions, including carbonate and smaller amounts of magnesium, sodium and other substances. The exact composition varies with tissue age and physiological conditions. Hydroxyapatite contributes substantially to bone stiffness and compressive strength.
Mineralisation is regulated by cells and depends on local concentrations of ions, enzymes and matrix molecules. The process is not equivalent to simply allowing calcium salts to crystallise randomly. Bone formation involves carefully controlled deposition and organisation of mineral within a biological matrix. Mineral can also be removed during bone resorption, allowing the skeleton to participate in mineral homeostasis.
Bone contains several specialised cell types. Osteoblasts are bone forming cells that synthesise osteoid and help regulate mineralisation. When osteoblasts become enclosed within the matrix they produced, they can differentiate into osteocytes. Osteocytes reside in small spaces called lacunae and communicate through microscopic channels called canaliculi. They act as important regulators of bone maintenance and respond to mechanical loading and changes in the local environment.
Osteoclasts are large cells derived from the monocyte macrophage lineage and are specialised for bone resorption. They attach to bone surfaces and create acidic and enzymatic environments that dissolve mineral and degrade matrix. Bone remodelling depends on coordinated activity among these cells. The composition of bone is therefore continually modified by living cellular processes rather than being permanently fixed after formation.
Bone also contains water and an extensive vascular and neural supply. Blood vessels enter bone through openings and canals and provide oxygen, nutrients and signals while carrying away metabolic waste. Nerves can accompany blood vessels and contribute to the sensation of pain associated with some bone injuries and pathological conditions. Marrow occupies internal spaces and contains haematopoietic and adipose tissues depending on location and age.
Compact bone forms dense outer regions, while cancellous or trabecular bone consists of a lattice of thin bony structures surrounding marrow spaces. The relative proportions of compact and trabecular bone differ among bones and regions because they experience different mechanical demands. Trabecular architecture can be remodelled in response to loading and metabolic requirements, demonstrating that bone's internal structure is an active component of its material properties.
Bone is commonly described in terms of compact bone and spongy bone, although the latter is more precisely called cancellous or trabecular bone. Compact bone forms dense outer layers and contains microscopic structures called osteons or Haversian systems in many regions. These systems contain concentric layers of bone around central canals containing blood vessels and nerves. Trabecular bone consists of interconnected plates and rods arranged along patterns influenced by mechanical loading.
Despite appearing less dense, trabecular bone is highly organised and can provide substantial strength while reducing mass. The outer surface of bone is covered by periosteum, a connective tissue layer containing blood vessels, nerves and cells involved in bone maintenance and repair. Internal bone surfaces are lined by endosteal tissues. These coverings provide biological interfaces between bone and surrounding tissues and participate in growth and remodelling.
The proportions and properties of bone components change with age, nutrition, hormones, physical activity and disease. During growth, newly formed bone undergoes maturation and mineralisation. Mechanical loading stimulates pathways that help maintain bone mass and architecture, whereas prolonged lack of mechanical loading can promote bone loss. Adequate calcium and phosphate availability is necessary for normal mineralisation, while vitamin D is important for calcium and phosphate absorption and regulation. Hormones such as parathyroid hormone and oestrogen influence bone turnover.
In osteoporosis, changes in bone remodelling can reduce both the amount and structural quality of bone. In conditions affecting mineralisation, bone can become abnormally soft even when collagen is present. This demonstrates that bone health depends not only on total mineral content but also on matrix quality, microscopic architecture, cellular activity and the balance between formation and resorption.
A bone is therefore a sophisticated biological composite material. Collagen provides a tough organic framework, hydroxyapatite provides hardness and compressive strength, water contributes to the biological and mechanical environment, and living cells continuously build, maintain and remove matrix. Blood vessels, nerves and marrow support the tissue and connect it with the rest of the body. Compact and trabecular architectures distribute mechanical loads in different ways, while the periosteum and endosteum provide interfaces for growth and remodelling.
The resulting material is stronger and more versatile than either pure mineral or pure collagen would be alone. Understanding bone composition is important for explaining growth, fracture repair, osteoporosis, mineral metabolism and the mechanical behaviour of the skeleton. Bone is consequently best understood as a living composite organ rather than as an inert mineral structure.
