Periosteum

The periosteum is a dense layer of vascular connective tissue that envelopes the outer surface of bones, except at joint surfaces where articular cartilage covers. It consists of two layers: an outer fibrous layer and an inner cellular (cambium) layer. The outer layer is rich in collagen fibers and contains fibroblasts, providing an attachment site for ligaments and tendons. The inner cambium layer is osteogenic, housing progenitor cells that can differentiate into osteoblasts and participate in bone growth and repair.

The periosteum is tightly connected to the underlying cortical bone, with collagen fibers called Sharpey's fibers anchoring the periosteum to the bone matrix. It is highly vascularized and innervated, supplying blood to the outer bone and contributing to sensory perception, particularly pain, through periosteal nerves.

Functions of the periosteum include nutrient supply to the outer bone cortex, protection, and serving as an important site for bone growth and healing. The cambium layer enables appositional growth in diameter and participates in fracture repair by forming new bone tissue. The periosteum also serves as a reservoir of osteoprogenitor cells that can differentiate in response to injury or stress.

Clinical relevance includes recognition of periosteal reactions on imaging, which can indicate bone remodeling, infection, or neoplastic processes. The periosteum is sometimes used in regenerative procedures and bone grafting because of its osteogenic potential. Articular cartilage on joint surfaces lacks periosteum.