Which lining structure lines the marrow cavity inside a bone?

• A. Periosteum
• B. Endosteum
• C. Visceral peritoneum
• D. Articular cartilage

Answer: (B)

Endosteum is the thin, cellular membrane that lines the inner surfaces of bone, including the medullary (marrow) cavity. This lining houses osteoprogenitor cells that can become osteoblasts, supporting bone growth and remodeling, and contains osteoclasts involved in resorption. The outer surface of bone is covered by periosteum, joints are lined by articular cartilage, and the visceral peritoneum lines abdominal organs—none of these encase the marrow space. Therefore, the marrow cavity inside a bone is lined by the endosteum.