Hello everyone, This is the definition of endosteum from wiki, The endosteum (plural endostea) is a thin layer of connective tissue which lines the surface of the bony tissue that forms the medullary cavity of long bones. However I'm of the beleif that endosteum lines the inner surface of all bones. Does it only line medullary cavity or internal surfaces of all bones. Do all bones have medullary cavities by the way. Thanks!!