Which statement accurately describes the periosteum and endosteum?

• A. Periosteum lines the medullary cavity; endosteum covers the external bone surface.
• B. Periosteum is a fibrous outer covering with osteogenic cells; endosteum lines the medullary cavity and inner bone surfaces; both contribute to growth and repair.
• C. Periosteum lines the medullary cavity and endosteum covers the external surface.
• D. Periosteum and endosteum are the same layer.

Answer: (B)

The essential idea is that bone surfaces are covered by two distinct membranes, each with bone-forming cells. The periosteum sits on the outside of the bone, a fibrous outer layer with an inner cellular layer rich in osteogenic (bone-forming) cells. It provides a source of osteoblasts for growth in thickness (appositional growth) and for repair after injury. The endosteum lines the inner surfaces—the medullary cavity and the internal surfaces of the bone’s trabeculae and canals—and also contains osteogenic cells that participate in remodeling and repair from the inside. Because both membranes harbor cells that can form bone, they both contribute to growth and repair. The other statements mix up locations (periosteum does not line the medullary cavity; endosteum does not cover the external surface) and incorrectly imply they are the same layer.