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Living, mineralized connective tissue that forms the skeleton, supports the body, protects organs, enables movement, and stores minerals.
Medically reviewed & updated
Bone is a specialized, living connective tissue and the basic building block of the skeleton. Although it looks inert, bone is constantly being broken down and rebuilt (remodeled) throughout life. Human infants are born with roughly 270 bones, many of which fuse during growth so that the typical adult skeleton contains about 206 bones.
Most bones have a dense outer shell of compact (cortical) bone surrounding a lighter, mesh-like interior of spongy (trabecular) bone. In healthy adults the ratio of cortical to trabecular bone is roughly 80:20. The cortical layer is organized into cylindrical units called osteons, while the trabecular meshwork helps absorb impact and houses bone marrow. A tough membrane called the periosteum covers the outer surface of bones, supplying blood vessels, nerves, and nutrition, especially to the outer third of the bone.
Bones are commonly classified by shape: long bones (such as the femur), short bones (such as wrist carpals), flat bones (such as the sternum and skull bones), and irregular bones. The principal bone-building cell is the osteoblast, which secretes the protein-rich matrix called osteoid that later mineralizes with calcium and phosphate.
According to StatPearls, bones serve several core roles: they provide a framework for soft tissue, permit locomotion by acting as levers for muscles, protect vital organs (the skull shields the brain; the rib cage shields the heart and lungs), facilitate breathing, contribute to electrolyte and mineral homeostasis (storing about 99% of the body's calcium), and house hematopoietic (blood-forming) sites in the marrow.
Because bone is metabolically active, its health depends on a balance between formation and resorption. When resorption outpaces formation, bones lose density and become porous, a condition called osteoporosis that raises fracture risk, especially in the hip, spine, and wrist. Adequate calcium and vitamin D, weight-bearing activity, and hormonal balance all support bone strength. Fractures, infections (osteomyelitis), and tumors are other common bone problems clinicians evaluate. This page is educational and is not medical advice; concerns about bone health should be discussed with a qualified clinician.