Capillaries in the endosteum supply nutrients to bone; the endosteum lines the inner surfaces of the bone, including the medullary cavity. Which surfaces are lined by the endosteum?

• A. Medullary cavity and trabeculae surfaces
• B. Outer bone surface only
• C. Joint cartilage surfaces
• D. Periosteum surfaces

Answer: (A)

Endosteum is a thin membrane that lines the inner surfaces of bone and contains osteogenic cells and small blood vessels to support remodeling from the inside. Because of this internal lining role, it covers the medullary cavity along the inner shaft and the surfaces of the trabeculae in the spongy (cancellous) bone. It does not cover the outer surface, which is the job of the periosteum, nor does it cover joint cartilage surfaces, which are covered by articular cartilage. So the surfaces lined by the endosteum are the medullary cavity and the trabecular surfaces.