endosteum

**Structure and location** - A thin, delicate membrane, often only one cell layer thick, that covers all internal bone surfaces. These include the marrow (medullary) cavity, the trabeculae of cancellous bone, and the vascular canals (Haversian and Volkmann). - Cell types: osteoprogenitor (osteogenic) cells, osteoblasts, and flattened bone-lining cells (inactive osteoblasts). Osteoclasts sit on the endosteal surface in Howship lacunae when resorption is active. - It has little fibrous tissue and no distinct outer fibrous layer. This contrasts with the periosteum, which has an outer fibrous layer and an inner cellular (cambium) layer, and which is anchored to bone by Sharpey fibers.

**Function** - Bone remodeling: bone-lining cells help form the bone remodeling compartment (BRC) and the canopy over remodeling sites. They respond to PTH and mechanical signals, and they help recruit osteoclasts through RANKL/OPG signaling. - Appositional growth and repair: osteoprogenitor cells differentiate into osteoblasts. These lay down new bone on the inner surface, so they contribute to fracture healing. - Calcium homeostasis: bone-lining cells regulate ion exchange between bone fluid and blood. They also form a barrier between marrow and the mineralized matrix. - Hematopoietic niche: the endosteal surface supports hematopoietic stem cells (the endosteal niche), and osteoblasts there produce CXCL12 and other factors.

**Clinical significance** - Fracture healing: both periosteum and endosteum contribute osteoprogenitor cells to callus formation. The periosteum generally contributes more to the external callus. - Aging and osteoporosis: periosteal apposition continues with age, and endosteal resorption exceeds formation. This enlarges the medullary cavity and thins the cortex, so bone is weaker even though its outer diameter increases. - Bone marrow transplantation and hematologic disease: the endosteal niche influences HSC maintenance, and marrow disorders can disrupt it. - Osteomyelitis and tumors: endosteal scalloping on imaging suggests a lesion eroding the inner cortex. It is seen in enchondroma, multiple myeloma, and other lytic lesions. - Hyperparathyroidism: high PTH drives osteoclastic resorption, which causes endosteal resorption and cortical thinning. Subperiosteal resorption (radial side of the middle phalanges) is the classic radiographic finding. - Paget disease and other conditions with abnormal turnover can also alter endosteal surfaces.

**High-yield comparison** - Periosteum: outer surface of bone, has a fibrous outer layer and a cellular inner layer, richly innervated (pain) and vascularized, anchored by Sharpey fibers. It is absent at joint (articular) surfaces, which are covered by hyaline cartilage. - Endosteum: inner surface, a thin single cell layer, no Sharpey fibers.

**Mnemonic** "Endo = inside." The endosteum lines the inside of the bone, and the periosteum wraps the outside ("peri" = around).