bone

Bone is a rigid organ that forms part of the endoskeleton of most vertebrates. It consists of osseous tissue, a mineralized connective tissue made primarily of collagen type I and hydroxyapatite crystals, and contains cells such as osteoblasts, osteocytes, and osteoclasts within a mineralized matrix.

Bones are organized into cortical (compact) bone and trabecular (spongy) bone. Compact bone provides strength and protects the inner compartments, while spongy bone reduces weight and houses marrow. The outer surface is the periosteum, and the inner surfaces are lined by the endosteum. The medullary cavity contains bone marrow, red in children and yellow with age. Blood supply comes from nutrient arteries and additional vessels at the ends and surfaces; nerves accompany vessels, enabling sensation of pain after injury.

Functions include support and protection of soft tissues, leverage for movement, mineral storage (primarily calcium and phosphate) and a role in hematopoiesis. Bone also participates in endocrine regulation through signaling molecules such as osteocalcin, which can influence metabolic processes.

Development and remodeling: bones form by intramembranous or endochondral ossification. Ongoing remodeling by osteoclasts and osteoblasts adapts bone structure to mechanical load and repairs microdamage, forming osteons in cortical bone and trabeculae in spongy bone. The rate of turnover and the balance between formation and resorption vary by age, anatomical site, and health status.

Clinical notes: osteoporosis and fractures are common bone disorders. Adequate intake of calcium and vitamin D, physical activity, and targeted medications influence bone health and healing.