Bone

Bone looks rigid, yet it is living tissue constantly renewed by specialized cells. Osteoblasts lay down new matrix rich in collagen and calcium phosphate, while osteoclasts carve tunnels that recycle old material. This remodeling responds to stress, repairing microfractures from daily movements and reshaping bones to match the loads they carry.

Each bone has a dense outer layer called cortical bone and a honeycomb interior known as trabecular bone. The outer cortex withstands bending, whereas the inner lattice lightens the skeleton and provides space for marrow. Long bones such as the femur contain growth plates near their ends during childhood, allowing limbs to lengthen until puberty when the plates close.

Bone marrow performs several vital functions. Red marrow manufactures red blood cells, white blood cells, and platelets, while yellow marrow stores fat as an energy reserve. Calcium and phosphorus stored in the matrix can be released into the bloodstream under hormonal control, helping maintain steady mineral levels for nerves and muscles.

Bones meet at joints, where cartilage, ligaments, and synovial fluid reduce friction. muscle tendons anchor to periosteum—the fiber-rich outer coating—so that contractions can move the skeleton. When bones break, a blood clot forms, fibroblasts create a soft callus, and osteoblasts deposit new matrix that eventually hardens, demonstrating the tissue's remarkable healing capacity.

nutrition, weight-bearing exercise, and hormones like estrogen and parathyroid hormone all influence bone density. Adequate calcium, vitamin D, and protein fuel remodeling, while regular movement signals bones to stay sturdy. Public health campaigns encourage fall prevention, screening for osteoporosis, and early treatment of fractures to keep people mobile throughout life.