Compact bone is the dense, rigid outer layer that forms the exterior of nearly every bone in the human body. This tissue, also known as cortical bone, accounts for about 80% of the total skeletal mass. Its structure is solid and tightly packed, which gives the skeleton its necessary hardness and strength. Compact bone forms the entire shaft of long bones and acts as a protective shell around the softer inner tissue of all other bones.
The Microscopic Blueprint
The fundamental structural unit of compact bone is the osteon, also called the Haversian system, which is a cylindrical structure that runs parallel to the long axis of the bone. This arrangement resembles a series of tightly packed drinking straws, providing a highly organized internal architecture. The osteon itself consists of concentric rings of calcified matrix known as lamellae, which surround a central Haversian canal.
The Haversian canal at the center of the osteon contains the blood vessels, nerves, and lymphatic vessels that service the bone cells. The bone cells, called osteocytes, are housed in tiny spaces called lacunae, situated between the concentric layers of the lamellae. The cells in the lacunae maintain the surrounding bone matrix.
Osteocytes are connected to one another and to the central canal by small channels called canaliculi. These microscopic canals are a network of passageways that allow for the exchange of nutrients and waste products throughout the dense bone matrix.
Adjacent osteons and the surface blood supply are connected by perforating canals, also known as Volkmann’s canals, which run perpendicular to the central canals. The collagen fibers within each lamella are arranged in alternating directions, which enhances the bone’s ability to resist twisting and bending forces.
Primary Functions of Compact Bone
Compact bone’s most prominent function is providing mechanical strength and support for the entire body. This bone tissue resists compressive forces and provides the rigid levers against which muscles pull to generate movement.
Compact bone also plays a major role in the protection of internal organs, forming the solid casings of the skull, rib cage, and pelvis. The hard matrix serves as the body’s primary reservoir for calcium and phosphate ions.
The minerals stored within the bone can be released into the bloodstream to help regulate systemic concentrations, a process known as mineral homeostasis. Compact bone also houses the yellow bone marrow in the medullary cavity of long bones, which serves as a major site for fat storage.
Compact Bone Versus Spongy Bone
Bone tissue is organized into two distinct types: compact bone and spongy bone, which is also known as cancellous or trabecular bone. The most obvious difference is their density; compact bone is solid and heavy, while spongy bone is light and porous, having a honeycomb-like appearance. Compact bone forms the outer layer of all bones and the entire shaft of long bones, whereas spongy bone is found primarily in the interior, particularly at the ends of long bones and inside flat bones.
Structurally, the two differ in their basic organizational units. Compact bone is characterized by the cylindrical osteons, which are tightly packed together to form a solid mass. Conversely, spongy bone lacks osteons and is instead composed of an irregular latticework of thin plates called trabeculae. These trabeculae are aligned along lines of stress, providing structural support while reducing overall bone mass.
The functional differences align with their structural traits, as compact bone is designed for maximum strength and weight bearing. Spongy bone, with its open spaces, contains red bone marrow, making it the main site for hematopoiesis, the process of blood cell production. While compact bone provides rigidity, the porous nature of spongy bone allows it to absorb and dampen mechanical stress, especially at joints.