What is the endosteum?

• A. Lines medullary cavity
• B. Covers the outer surface of the bone
• C. Builds bone
• D. Maintains bone

Answer: (A)

The endosteum is the thin membrane that lines the inner surfaces of bone, most notably the medullary (marrow) cavity. This inner lining separates the hollow cavity from the bone tissue and houses cells involved in bone formation and remodeling, such as osteoprogenitor cells, osteoblasts, and osteoclasts. Because of its location and cellular makeup, the endosteum’s primary role is to participate in growth, repair, and remodeling at the inner surfaces of bone, rather than covering the outer surface or simply maintaining bone. The outer surface of bone is covered by the periosteum, so the correct description is that the endosteum lines the medullary cavity.