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.[1] The outer surface of a bone is lined by a thin layer of connective tissue that is very similar in morphology and function to endosteum. It is called "periosteum".
References
^Netter, Frank H. (1987). Musculoskeletal system: anatomy, physiology, and metabolic disorders. Summit, New Jersey: Ciba-Geigy Corporation ISBN 0914168886, p.171
