The endosteum is a thin membrane that lines the inner surfaces of bones. This connective tissue plays a fundamental role in bone health by lining the internal cavities of bones. It is a dynamic layer involved in maintaining skeletal integrity. This inner membrane is less widely known than the outer periosteum, yet its contributions to bone physiology are equally significant.
Anatomy and Cellular Makeup
The endosteum lines various internal bone compartments. It covers the medullary cavity, which is the central canal of long bones containing bone marrow. It also lines the network of trabeculae within cancellous (spongy) bone, found at the ends of long bones and inside other bone types. The endosteum also lines the Haversian canals (osteonal canals) and Volkmann’s canals, which are microscopic tunnels within compact bone that house blood vessels and nerves.
It consists of specialized cells. Among these are osteoprogenitor cells, stem cells residing within the endosteum that can differentiate into other bone cell types. Also present are osteoblasts, which are responsible for forming new bone tissue. These cells synthesize and secrete osteoid, the organic matrix of bone, which then undergoes mineralization. It also houses osteoclasts, cells that break down old or damaged bone tissue.
Essential Functions
The endosteum is involved in bone remodeling, a continuous process where old bone is removed and new bone is formed. This cycle ensures the skeleton remains strong, repairs damage, and adapts to stresses. Osteoclasts, regulated by endosteal cells, initiate this process by dissolving bone, creating small depressions. Subsequently, osteoblasts, also present in the endosteum, move into these areas to deposit new bone matrix. This balanced activity, regulated by the endosteum, maintains bone mass and overall bone health.
Beyond remodeling, the endosteum contributes to bone growth and repair. During bone growth, particularly in width, endosteal osteoclasts resorb bone from the inner surface, preventing excessive thickness as new bone is laid down by the periosteum on the outer surface. In a fracture, endosteal osteoprogenitor cells are activated. These cells multiply and differentiate into osteoblasts, laying down new bone to repair the damaged area and form a reparative callus.
The endosteum also supports hematopoiesis, the process of blood cell production. Adjacent to bone marrow, the endosteal surface provides a specialized microenvironment (niche) that supports hematopoietic stem cells. Endosteal cells, particularly osteoblasts, contribute to this niche by producing signaling molecules and growth factors that regulate the maintenance, self-renewal, and differentiation of these blood-forming stem cells. This interaction ensures a continuous supply of new blood cells.