Bone tissue is an active living material that continuously remodels itself throughout life. It is composed of cells embedded within a mineralized matrix, providing both flexibility and rigidity. Bone is broadly categorized into two major structural types, each adapted to different mechanical roles within the skeleton. These types are distributed across the body to maximize both strength and lightness.
Compact Bone The Concentric Structure
The type of bone that is arranged in concentric layers is compact bone, also known as cortical bone. This dense, solid tissue forms the hard outer shell of all bones and is the primary component of the shafts of long bones, accounting for about 80% of total bone mass. Compact bone offers robust support, protection for internal organs, and resistance against bending and compression forces. The concentric layering is organized into microscopic cylindrical units called osteons, or Haversian systems. These osteons run parallel to the long axis of the bone, contributing to the bone’s ability to withstand stress along its length.
The Detailed Anatomy of the Osteon
The osteon is the fundamental structural unit of mature compact bone, and its arrangement creates the concentric pattern. At the center of this cylindrical unit is the Haversian (Central) Canal, a microscopic channel that runs longitudinally. This canal serves as a passageway for blood vessels, nerves, and lymphatic vessels, supplying the living bone cells with nutrients and oxygen.
Encircling this central canal are the lamellae, which are concentric layers of mineralized bone matrix. These layers are composed of collagen fibers and inorganic salts. The collagen fibers in each successive layer run in slightly different directions to enhance the bone’s resistance to twisting forces. Between the lamellae are small, lake-like spaces called lacunae.
Within each lacuna resides an osteocyte, a mature bone cell responsible for maintaining the bone matrix. These cells maintain contact with each other and the central blood supply through an intricate network of tiny channels called canaliculi. The canaliculi radiate outward from the lacunae, allowing for the exchange of nutrients and wastes between the osteocytes and the central canal. Adjacent osteons are interconnected by perforating canals (or Volkmann’s canals), which run perpendicular to the central canals, ensuring a continuous blood supply throughout the compact bone structure.
Spongy Bone An Alternative Arrangement
The alternative bone tissue is spongy bone, also known as cancellous or trabecular bone. Spongy bone does not contain osteons, and therefore lacks the concentric layers that define compact bone. Instead, its structure is characterized by a lattice-like network of delicate, interconnecting bony plates called trabeculae.
The trabeculae are precisely organized along the lines of mechanical stress within the bone, providing strength without excessive weight. This porous, open structure is found primarily in the interior of bones, such as the ends of long bones (epiphyses) and inside flat bones like the skull and vertebrae. The spaces within this meshwork are filled with bone marrow, where blood cell production (hematopoiesis) occurs.
The trabecular structure allows the bone to distribute stress efficiently and provides cushioning for absorbing impact. Although it lacks central canals, spongy bone still contains osteocytes residing in lacunae. These cells are nourished by blood vessels that permeate the marrow-filled spaces surrounding the trabeculae. This architecture complements the rigid mechanical strength provided by the surrounding compact bone.