Periosteum

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Bone is the basic unit of the human skeletal system.

The periosteum refers to a fibrous connective tissue membrane that covers the external surfaces of all bones with the exception of joint surfaces, which are covered by articular cartilage. It consists of two layers: an outer fibrous layer and an inner cellular layer.

The periosteum also bears thick collagen fibers called Sharpey’s fibres or perforating fibers which penetrate perpendicularly into the outer cortex, anchoring the periosteum to the underlying bone. 

The periosteum is much like the perichondrium, which is a sheath of dense connective tissue that covers cartilage at most regions of the body, with the exception of articular cartilage.

The periosteum separates the bone from surrounding structures. It contains small blood vessels that provide nourishment to the outer cortex of bones. Thus, a bone stripped of its periosteum will die.

Additionally, the periosteum has a rich supply of sensory nerves and is therefore very sensitive to pain, making it the principal structure responsible for the acute pain experienced during fractures.

The periosteum is unique in that it is able to form bone. The inner layer of periosteum contains stem cells (osteoprogenitor cells and osteoblasts) which are responsible for bone growth, continuous remodeling and fracture healing.