Bone is a living tissue that forms the body’s framework, providing shape and support. It is a type of connective tissue with a mineralized matrix. Compact bone, also known as cortical bone, is the dense, hard outer shell of most bones. This tissue makes up approximately 80% of the human skeleton, contributing to its strength.
Structure of Compact Bone
Compact bone has a highly organized structure built around microscopic units called osteons, or Haversian systems. These cylindrical units are the fundamental components of compact bone. Each osteon consists of concentric layers of calcified matrix, known as lamellae, arranged like rings of a tree trunk, surrounding a central canal.
At the center of each osteon lies a central canal, also called a Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. These canals supply nutrients and oxygen to the bone cells and remove waste products. Mature bone cells, called osteocytes, reside in small spaces (lacunae) between the concentric lamellae. Tiny channels, called canaliculi, radiate from the lacunae to the central canal, allowing osteocytes to connect and exchange nutrients. Horizontal perforating canals, or Volkmann’s canals, connect adjacent central canals and link them to the bone’s outer blood supply.
Where Compact Bone is Located
Compact bone forms the hard external layer of all bones. This dense tissue provides the outer casing that protects inner bone structures. It is prominently found in the shafts, or diaphyses, of long bones.
In these long bones, compact bone forms a thick, rigid wall around the central medullary cavity. Even in flat and irregular bones, compact bone is present as a relatively thin outer layer. This placement ensures the body’s framework is equipped with supportive tissue where mechanical stresses are highest.
Functions of Compact Bone
Compact bone serves multiple roles for the body’s mechanical integrity and function. A primary function is providing structural support and strength. Its dense nature allows it to withstand significant weight and resist bending or fracturing under stress. This mechanical strength is important for weight-bearing bones and those subjected to considerable forces during movement.
Compact bone also protects internal organs. For instance, the skull’s compact bone safeguards the brain, and the ribs protect the heart and lungs. It facilitates movement by providing rigid levers for muscle attachment. When muscles contract, they pull on these stable bone structures, enabling body motions. Compact bone also acts as a reservoir for minerals, notably calcium and phosphate, which are released into the bloodstream to maintain mineral balance.
Compact Bone Versus Spongy Bone
Bone tissue exists in two forms: compact bone and spongy bone. Compact bone is dense and solid, forming the outer layer of most bones, while spongy bone is porous with a honeycomb-like structure inside.
Structurally, compact bone is characterized by its organized osteons, which are absent in spongy bone. Spongy bone consists of a network of thin, interconnecting plates and rods called trabeculae, arranged along lines of stress. Compact bone has a lower surface-area-to-volume ratio and fewer blood vessels compared to spongy bone. Spongy bone contains red bone marrow, responsible for producing blood cells, while compact bone primarily contains yellow marrow in its central cavities.
Functionally, compact bone offers rigidity and resistance to bending. Spongy bone provides flexibility and shock absorption, and houses marrow for blood cell production.