Bone

Bone is a type of rigid connective tissue found within the internal skeletal structures of vertebrate animals, including humans. Bones support the body, protect vital organs such as the brain and heart, and serve as attachment points for muscles, enabling movement.

Although bone may appear rigid and unchanging, it is living tissue with cells, blood vessels, and nerves. It is constantly undergoing remodeling: portions are removed and replaced with new material, allowing bone to adapt when stressed or damaged.

Structure

Bone consists of two distinct types: compact bone, which forms the hard outer layer, and spongy bone, which occupies the interior. The outer surface is covered by a membrane called the periosteum. Within larger bones is marrow, which produces some blood cells.

In terms of composition, bone is a composite material. Its organic component is largely made up of a protein called collagen, which gives bone flexibility and resilience. Its mineral component—consisting mainly of calcium and phosphate salts—provides hardness. This combination makes bone difficult to break.

Bone cells

Three main cell types carry out bone's primary functions. Osteoblasts build new bone tissue. Osteocytes are cells embedded within the tissue they have formed, monitoring its condition. Osteoclasts break down and reabsorb old bone. The balance between building and resorption determines bone density.

Other functions

Beyond support and protection, bone serves as a mineral reservoir: the body obtains calcium from bone or stores it there to maintain blood levels within a set range. The marrow within bone produces red blood cells, white blood cells, and platelets. The small bones of the inner ear also transmit sound.

Conditions affecting bone

Bone fracture results from external force or weakened bone. Osteoporosis is a condition in which bone density decreases, increasing fracture risk; it is often associated with age, hormones, and inadequate calcium or vitamin D intake. In children, vitamin D deficiency can cause rickets, a condition in which bones form abnormally.