Compact bone, also known as cortical bone, forms the hard, dense outer layer of most bones. It provides structural support, protects delicate internal organs, and bears the weight of the body. While it appears solid to the naked eye, microscopic examination reveals a highly organized network of tunnels and channels. This tissue makes up about 80 percent of the total bone mass in an adult human skeleton, with the remaining 20 percent consisting of spongy bone.
The functional unit of compact bone is the osteon, or Haversian system. Each osteon is a cylindrical structure that runs parallel to the long axis of the bone.
Compact bone contains four main types of specialized cells, each fulfilling distinct roles in formation, maintenance, and resorption.
The exceptional hardness of compact bone comes from its unique extracellular matrix, which is divided into organic and inorganic portions.
The organic part consists mainly of collagen fibers, providing flexibility and tensile strength. The inorganic part is composed of hydroxyapatite crystals, formed from calcium phosphate and calcium hydroxide. These mineral crystals deposit around the collagen fibers, hardening the tissue so it can withstand heavy compressive loads without breaking.
Because the calcified extracellular matrix prevents nutrient diffusion, compact bone requires a dedicated transport network. Blood vessels and nerves travel from the periosteum into the bone through perforating canals, also known as Volkmann's canals. These canals run at right angles to the central Haversian canals, connecting the blood supply of the medullary cavity and periosteum with the interior osteons.
Compact bone and spongy bone differ significantly in structure, location, and function.
| Feature | Compact Bone | Spongy Bone |
|---|---|---|
| Structure | Dense and solid | Porous trabeculae network |
| Location | Outer layer of bones | Interior of bones and epiphyses |
| Primary Function | Weight-bearing and protection | Red marrow housing and lightness |
Despite these differences, both tissues share the same types of bone cells and extracellular matrix components.
Compact bone is built to withstand mechanical stress, particularly compression and torsion forces encountered during movement. Beyond mechanical support, the cortical bone acts as a vital storage depot for minerals like calcium and phosphorus, releasing them into the bloodstream when systemic levels drop. The outer periosteum covering this tissue also serves as an attachment site for tendons and ligaments.