Bone

A bone is a rigid organ that forms part of the skeleton in most vertebrate animals. It performs a wide range of essential functions, including protecting internal organs, producing blood cells, storing minerals, supporting body structure, and enabling locomotion. Bones occur in many shapes and sizes and contain complex internal and external features that contribute to their lightweight yet strong composition. Bone tissue, or osseous tissue, is a specialised connective tissue with a mineralised matrix that provides structural rigidity while maintaining a degree of elasticity.

Composition and Structure

Bone is not a uniformly solid material. Instead, it consists of a matrix made up of approximately 30 per cent flexible organic components and 70 per cent bound inorganic minerals. The organic portion is largely collagen, also called ossein, which contributes to elasticity and resistance to fracture. The inorganic portion is primarily calcium phosphate arranged in a form of calcium apatite known as bone mineral. This combination produces a material that is both strong and lightweight.
Other tissues found within bones include bone marrow, endosteum, periosteum, cartilage, blood vessels, and nerves. At birth, the human skeleton contains around 300 bones, many of which fuse as a child grows, resulting in 206 bones in the adult body (excluding variable sesamoid bones). The largest human bone is the femur, while the smallest is the stapes in the middle ear.
Bone tissue appears in two primary forms: cortical (compact) bone and cancellous (trabecular or spongy) bone. These tissues differ in density, microscopic structure, and function, allowing bones to meet both mechanical and metabolic demands.

Cortical Bone

Cortical bone forms the dense outer layer of bones and accounts for approximately 80 per cent of the total bone mass of an adult skeleton. Its smooth, solid appearance contributes to the protective and supportive roles of the skeleton. This outer cortex provides levers for movement and acts as a major reservoir for minerals, particularly calcium.
At the microscopic level, cortical bone is composed of cylindrical structures known as osteons or Haversian systems. Each osteon consists of concentric layers of bone tissue arranged around a central Haversian canal containing blood vessels and nerves. Transverse Volkmann’s canals connect different osteons. The outer surface of cortical bone is covered by the periosteum, whereas the inner surface is lined by the endosteum, which borders the cancellous bone.

Cancellous Bone

Cancellous bone, or spongy bone, is located mainly at the ends of long bones, within vertebrae, and in other areas requiring shock absorption and flexibility. It is composed of a porous network of lattice-like structures known as trabeculae. These trabeculae follow lines of mechanical stress, allowing cancellous bone to efficiently withstand multidirectional forces.
Cancellous bone has a higher surface-area-to-volume ratio than cortical bone, making it more flexible but less dense. Its internal surfaces house red bone marrow, where haematopoiesis—the production of red blood cells, white blood cells, and platelets—takes place. Although cancellous bone constitutes only around 20 per cent of total bone mass, it has nearly ten times the surface area of cortical bone and plays a vital role in metabolic regulation.

Bone Marrow and Vascular Supply

Bone marrow is found in the cavities of cancellous bone. In infants, most marrow is red and haematopoietically active. As individuals age, much of the red marrow is replaced by yellow marrow, also known as marrow adipose tissue. In adults, red marrow is mainly concentrated in the vertebrae, ribs, pelvis, and the proximal ends of long bones such as the femur.
Bone is highly vascular, receiving roughly 10 per cent of the body’s cardiac output. Blood enters through the endosteum, circulates through the marrow, and exits via small vessels in the cortex. Bone marrow has distinct oxygen tension characteristics compared with arterial and venous blood, reflecting its unique metabolic environment.

Bone Cells

Bone tissue is dynamic and metabolically active. It is constantly remodelled through the coordinated actions of several specialised cells:

• Osteoblasts are bone-forming cells responsible for producing osteoid, a collagen-rich matrix that later mineralises. They synthesise Type I collagen and secrete regulatory molecules such as prostaglandins. Once an osteoblast becomes surrounded by mineralised matrix, it transforms into an osteocyte.
• Osteocytes originate from osteoblasts that become embedded within the bone matrix. They occupy small cavities known as lacunae and extend long cellular processes through narrow channels called canaliculi. Osteocytes communicate extensively with other bone cells through gap junctions and help regulate bone maintenance and mineral homeostasis.
• Osteoclasts are large, multinucleate cells responsible for bone resorption. Derived from the same precursor lineage as macrophages and monocytes, they break down mineralised bone, allowing osteoblasts to form new tissue. This ongoing cycle of breakdown and formation underlies bone remodelling.
• Bone lining cells are modified osteoblasts that remain on the surface of bone, forming a protective layer and regulating mineral exchange.

These cell types work in balance to maintain skeletal integrity throughout life. Disruptions in their activity can lead to conditions such as osteoporosis or excessive bone growth.

Bone Matrix and Mineralisation

The bone matrix is largely composed of collagen fibres arranged in a complex, interwoven pattern. This organic framework provides tensile strength, while mineral deposition delivers compressive strength. The process of mineralisation involves the deposition of calcium phosphate crystals, which interact with carbonate, hydroxide, and other ions to form hydroxyapatite-like structures.
Bone’s ability to withstand stress arises from the complementary properties of its organic and inorganic components. Collagen contributes elasticity and resistance to stretching, while mineralisation enhances hardness and rigidity.

Function and Adaptation

Bones serve multiple essential functions:

• Structural support: providing the framework for body shape.
• Protection: shielding organs such as the brain, heart, and lungs.
• Movement: acting as levers for muscles.
• Mineral storage: maintaining calcium and phosphate balance.
• Haematopoiesis: producing blood cells within marrow.