Bone is a specialized connective tissue that provides structural support, protects vital organs, and serves as a vital mineral reservoir for calcium and phosphate. The unique strength of bone comes from its extracellular matrix, which is divided into organic and inorganic components. The organic part, mainly osteoid and type I collagen fibers, gives the bone tensile strength and a degree of flexibility so it does not shatter easily. The inorganic part consists of mineral salts, primarily hydroxyapatite crystals made of calcium and phosphate. These minerals deposit around the collagen fibers, hardening the tissue to provide rigid compressive strength. Without the organic fibers, bones would be brittle like chalk; without the minerals, they would bend like rubber.
Under the microscope, compact bone is organized into repeating structural units called osteons or Haversian systems. Each osteon resembles a set of concentric rings called lamellae, which surround a central Haversian canal that houses blood vessels and nerve fibers. Running perpendicular to these are Volkmann's canals, connecting adjacent osteons and supplying blood from the periosteum. Trapped inside tiny cavities called lacunae between the lamellae are mature bone cells. Fine, branching channels known as canaliculi radiate outward from the lacunae, linking the bone cells together like a tiny subway system so they can share nutrients and waste.
Four main types of cells maintain bone tissue, each with a distinct role in building, maintaining, or breaking down the matrix:
Intramembranous ossification builds flat bones, such as the skull bones, mandible, and clavicles, directly from a sheet of embryonic connective tissue (mesenchyme). The process unfolds in four key stages:
Most bones in the human body develop through endochondral ossification, replacing a hyaline cartilage model with bone tissue. The process begins during the second month of embryonic development. First, a cartilage model grows and a primary ossification center develops in the middle of the shaft as chondrocytes die and blood vessels invade. Osteoblasts then build a collar of bone around the diaphysis while the internal cartilage breaks down. Secondary ossification centers later develop in the epiphyses around the time of birth. Hyaline cartilage remains only at the articular surfaces and within the epiphyseal plates, where it fuels longitudinal bone growth throughout childhood.
Bones grow in two directions: length and thickness. Longitudinal growth occurs at the epiphyseal plate through the proliferation of cartilage cells, which are subsequently replaced by bone on the diaphyseal side. Appositional growth increases bone thickness and width as osteoblasts beneath the periosteum deposit new bone matrix while osteoclasts in the endosteum resorb bone along the medullary cavity. Throughout life, bones undergo continuous remodeling, replacing old tissue with new matrix to repair micro-fractures and regulate blood calcium levels. This balance between osteoblast and osteoclast activity ensures bones stay strong yet responsive to physical stress.