tissueskeletal

Bone tissue, or osseous tissue, is a rigid connective tissue that forms the skeleton of most vertebrates. It provides structure, protects internal organs, anchors muscles, and serves as a major reservoir for minerals such as calcium and phosphate. It also houses hematopoietic tissue in the marrow cavities.

The tissue is composed of cells embedded in a mineralized extracellular matrix. The main cell types are osteoblasts (bone-forming), osteocytes (mature, embedded cells), and osteoclasts (bone-resorbing). The matrix is predominantly type I collagen with inorganic hydroxyapatite crystals that confer rigidity and strength.

Bone exists in two structural forms: compact bone and cancellous (spongy) bone. Compact bone forms dense shafts and contains osteons with central Haversian canals and concentric lamellae. Cancellous bone forms a porous network of trabeculae and houses marrow spaces. The surfaces of bone are covered by the periosteum externally and the endosteum internally, and the tissue is well vascularized by nutrient arteries and periosteal vessels.

Remodeling and growth are ongoing processes driven by mechanical loading and hormonal signals. Osteoclasts resorb old or damaged bone, while osteoblasts lay down new matrix that becomes mineralized. Developmentally, bone forms through intramembranous ossification (mesenchyme directly forms flat bones) and endochondral ossification (long bones develop from a cartilage model).

Abnormal bone remodeling underlies diseases such as osteoporosis, osteogenesis imperfecta, and rickets, highlighting the critical balance of formation and resorption in skeletal health.