Endosteumi

Endosteumi are the thin cellular membranes that line the inner surfaces of bones. They cover the medullary cavity, the inner surfaces of trabecular (spongy) bone, and the endosteal surfaces of the Haversian canals within compact bone. The endosteum consists mainly of osteogenic cells in a delicate connective tissue layer and lies between the mineralized bone matrix and the bone marrow or medullary space.

The cellular composition of the endosteum includes osteoprogenitor cells, osteoblasts, and, on the inner bone surfaces, osteoclasts involved in bone resorption. This single-layer or very thin multilayer lining is highly active in bone remodeling, supplying new osteoblasts for bone formation and providing a site for osteoclast differentiation and resorption as needed. The endosteum often contains a rich source of mesenchymal stem cells that can differentiate toward osteogenic lineages in response to mechanical signals or injury.

Functionally, the endosteum participates in growth, remodeling, and repair of bone. By housing osteoblasts and osteoclasts, it helps shape bone architecture, participates in calcium homeostasis, and contributes to healing after fractures. It is more metabolically active than the outer periosteum in some regions, particularly where remodeling is frequent, such as near trabecular bone and within the medullary cavity.

Compared with the periosteum, the endosteum is thinner and lies on internal bone surfaces. It remains a key site for adaptive remodeling throughout life and responds to mechanical stress, hormonal signals, and aging. Dysregulation of endosteal activity can contribute to changes in bone density and porosity.