Spongy bone, also known as cancellous or trabecular bone, is a porous tissue within the skeletal system that provides structural support and biological function. Unlike dense compact bone, which forms the outer shell, spongy bone is lighter and appears honeycomb-like. This tissue is designed to withstand stress and adapt to forces while housing the body’s blood cell production machinery. Its composition of cells and matrix components allows it to perform mechanical and metabolic duties.
Location and Macroscopic Structure
Spongy bone is found in the interior of most bones, beneath the layer of compact bone. Primary locations include the ends (epiphyses) of long bones (e.g., femur and humerus), and the interior of flat and irregular bones (e.g., vertebrae, ribs, skull, and pelvis). Here, the tissue forms a delicate, interconnected network of thin plates and bars called trabeculae.
The trabeculae create numerous open spaces, giving the bone its spongy appearance. This lattice-like arrangement is organized along the lines of stress the bone regularly experiences. This organization allows the bone to distribute weight and absorb shock efficiently, providing strength without excessive mass. The porous structure significantly reduces the total weight of the skeleton, which aids in mobility.
Cellular and Matrix Composition
The structural components of spongy bone are similar to compact bone, consisting of a mineralized matrix and living bone cells. The non-cellular matrix provides both flexibility and hardness. The organic portion is mostly Type I collagen fibers, which grant the tissue tensile strength and elasticity.
The inorganic component provides hardness and compressive strength. It is made up of mineral salts, primarily calcium phosphate, arranged as hydroxyapatite crystals. These crystals are deposited around the collagen framework, creating a strong yet light composite material.
Within this hardened matrix, the tissue contains three main types of cells that constantly maintain and remodel the structure. Osteocytes are mature bone cells embedded within small spaces called lacunae; they monitor the bone’s health and mineral content. Osteoblasts synthesize and secrete new bone matrix (osteoid), which later mineralizes.
Osteoclasts are large, multinucleated cells that break down and resorb old or damaged bone tissue. This balanced process allows the spongy bone to adapt its structure to changing mechanical demands. The open spaces between the trabeculae are also filled with bone marrow.
Primary Physiological Functions
Spongy bone serves multiple physiological roles beyond mechanical support. The most recognized function is hematopoiesis, the process of producing all blood cells. The red bone marrow housed within the trabecular spaces contains hematopoietic stem cells that generate red blood cells, white blood cells, and platelets.
The architecture provides biomechanical advantages, acting as an internal shock absorber. The lattice-like arrangement of the trabeculae helps dissipate forces applied to the bone, protecting the joints. This porous design ensures the skeleton remains lighter than if it were composed only of dense compact bone, which aids in locomotion.
Spongy bone also functions as a reservoir for essential minerals, aiding in systemic mineral homeostasis. It stores the majority of the body’s calcium and phosphate. These minerals can be released into the bloodstream when needed to maintain stable levels for nerve function, muscle contraction, and other metabolic processes. The constant remodeling activity of osteoblasts and osteoclasts regulates the balance of these circulating minerals.