Compact bone, also known as cortical bone, forms the hard, dense outer layer of bones throughout the human body. This tissue provides mechanical strength, support, and protection to internal organs and bone marrow. It is a highly organized tissue, constantly undergoing formation and breakdown to maintain its integrity and adapt to mechanical stresses. Compact bone makes up approximately 80% of the total bone mass in an adult human skeleton.
The Essential Components
Compact bone consists of both organic and inorganic materials that work in concert to provide its unique properties. The organic matrix, making up about 30% of the total bone mass, provides flexibility and tensile strength. This organic component is primarily composed of type I collagen fibers, which are arranged to resist stretching and provide a framework for mineral deposition. The collagen fibers are embedded within a gel-like ground substance.
The ground substance contains various large organic molecules, including proteoglycans and glycoproteins, which fill the spaces between the collagen fibers and cells. Ground substance contributes to structural integrity and provides lubrication for collagen fibers. The inorganic mineral component, primarily hydroxyapatite, provides bone with its hardness and compressional strength. Hydroxyapatite is a crystalline form of calcium phosphate, and these crystals are deposited onto the collagen framework.
Organized Microscopic Structure
The dense components of compact bone are arranged into highly organized units called osteons, also known as Haversian systems. These cylindrical structures serve as the fundamental functional units of compact bone, facilitating nutrient and waste exchange. Osteons can extend several millimeters to about one centimeter in length.
At the center of each osteon lies a central canal, or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. Surrounding this central canal are concentric rings of bone matrix called lamellae. These lamellae are composed of both organic collagen fibers and inorganic hydroxyapatite crystals, with collagen fibers in adjacent lamellae arranged in alternating directions to increase resistance to twisting forces.
Small, oval-shaped spaces known as lacunae are located between the lamellae, and each lacuna houses a mature bone cell called an osteocyte. Tiny canals, called canaliculi, radiate from the lacunae, forming a network that connects individual lacunae to each other and to the central canal. This network of canaliculi allows for the transport of nutrients and oxygen to the osteocytes and the removal of waste products. Connecting adjacent central canals and the periosteum (the outer covering of the bone) are perforating canals, also known as Volkmann’s canals, which run perpendicular to the central canals and contain their own blood vessels and nerves.
The Dynamic Cellular Network
Compact bone is a living tissue because it contains several types of cells that continuously form, maintain, and remodel its structure. Osteoblasts are the cells responsible for synthesizing and secreting the organic matrix, known as osteoid, which is the unmineralized bone tissue. These cells then initiate the mineralization process by depositing calcium and phosphate minerals onto the osteoid. Once osteoblasts become surrounded and trapped by the bone matrix they have produced, they differentiate into osteocytes.
Osteocytes are the most common type of bone cell and reside within the lacunae. Their primary function involves maintaining the bone matrix and sensing mechanical stresses on the bone. These cells can signal other bone cells to initiate repair processes when micro-damage is detected. Osteoclasts are large, multinucleated cells that play a role in bone remodeling. They are responsible for bone resorption, the process of breaking down old or damaged bone tissue by secreting enzymes and acids. This continuous interplay between osteoblasts forming new bone and osteoclasts resorbing old bone ensures that compact bone remains an adaptable tissue throughout life.