Both external and internal bone surfaces are covered with membranes: - the periosteum and the endosteum -. Both membranes contain osteoblasts and osteoclasts and thus are involved in bone growth, bone repair and maintenance.

THE PERIOSTEUM (Osteo =bone; peri= around) covers and protects the outer surface of the bones that are not covered by cartilage.

The periosteum is a double layered membrane:

- the outer fibrous layer, a dense irregular connective tissue proper, is the protective layer;

- the inner layer is delicate. It is the osteogenic layer (Osteo =bone; genic= beginning) and contains mainly osteoblasts (bone forming cells) and osteoclasts (destroyers of bone).

The periosteum is anchored to the bone with strands of collagen fibers (=Sharpey's fibers) that extend from its fibrous layer into the bone matrix.

The periosteum is supplied with nerve, blood and lymphatic vessels that penetrate into the bones.

The outer periosteum has to be tough because it is subjected to a lot of pulling: it is connected to tendons, ligaments and the fibrous layer of the articular capsule in synovial joints.

THE ENDOSTEUM: (Osteo =bone; endo= inside)

it is an osteogenic membrane: a delicate connective membrane containing mainly osteocytes and osteoclasts.

Endosteum covers all the bones' inner cavities that contain marrow and/or blood vessels.