Endosteum

Endosteum is a thin cellular membrane that lines the internal surfaces of bone, including the medullary cavity and the trabeculae of cancellous bone, as well as the lining of the inner aspects of Haversian canals. It is composed mainly of a single layer of osteogenic cells—osteoprogenitor cells—with occasional osteoblasts embedded in a sparse connective tissue matrix. The endosteum is typically thinner and less fibrous than the periosteum and has a rich vascular supply.

Function: The endosteum participates in bone growth, repair, and remodeling. Osteoprogenitor cells can differentiate into osteoblasts to form new bone on internal surfaces, while osteoclasts, derived from monocytes, resorb bone during remodeling. Through these actions, the endosteum contributes to calcium homeostasis and to the adaptation of bone to mechanical demands.

Clinical aspects: During fracture healing and in response to remodeling needs, endosteal surfaces furnish cells that participate in callus formation and replacement of damaged bone. The endosteal surface is a common site of remodeling in conditions such as osteoporosis and other metabolic bone diseases.

Development: The endosteum arises from mesenchymal tissue during bone formation and persists throughout life, maintaining a reservoir of osteogenic cells.