The periosteum is a specialized membrane that envelops the outer surface of most bones, acting as a protective covering for the underlying tissue. It is found across nearly all bone surfaces, except where articular cartilage covers joints or where tendons and ligaments attach directly.
Its Structure and Composition
The periosteum is a thin, fibrous membrane structured into two distinct layers. The outermost layer, the fibrous layer, consists of dense, irregular connective tissue rich in collagen fibers. This layer provides mechanical strength and protection to the bone, containing blood vessels and nerves.
Beneath this protective outer shell lies the inner layer, often referred to as the cambium or osteogenic layer. This cellular layer is highly active and contains various cell types essential for bone processes. Within this inner layer are osteoprogenitor cells, which are specialized stem cells capable of differentiating into bone-forming cells. These also include osteoblasts, the cells responsible for synthesizing new bone matrix, and sometimes mesenchymal stem cells, which can differentiate into both bone and cartilage cells.
The periosteum is rich in blood vessels and nerves. Blood vessels within both layers, particularly the outer fibrous layer, deliver oxygen and nutrients to the underlying bone tissue. Many sensory nerves make it highly sensitive to pain. Strong collagen fibers, known as Sharpey’s fibers, extend from the periosteum into the bone, firmly anchoring the membrane to the bone’s surface.
Essential Roles in Bone Health
The periosteum performs several functions for bone health. One significant role is in bone growth, particularly appositional growth, which increases the diameter of bones. This process is especially active during childhood and adolescence, as the osteoblasts within the inner periosteal layer continuously lay down new bone tissue on the existing surface. Even in adulthood, the periosteum contributes to bone remodeling, helping bones adapt to mechanical stresses.
Another primary function is its involvement in bone repair after an injury or fracture. When a bone breaks, the osteoprogenitor cells in the cambium layer become activated. These cells then differentiate into osteoblasts, which produce new bone material to form a callus, bridging the fracture site and facilitating the healing process. Without an intact periosteum, bone regeneration can be impaired.
Beyond growth and repair, the periosteum provides nourishment to the outer layers of the bone. Its vascular network delivers blood, rich in oxygen and nutrients, directly to the bone tissue. This supply is important for maintaining the health and metabolic activity of bone cells. The periosteum serves as an attachment point for various soft tissues, including muscles, tendons, and ligaments, integrating them with the skeletal system.
When the Periosteum is Affected
Given its location and functions, the periosteum can be involved in several conditions and injuries. Periostitis refers to the inflammation of the periosteum, which can cause discomfort and tenderness. A common example is medial tibial stress syndrome, often known as “shin splints,” experienced by runners when the periosteum along the shinbone becomes irritated.
Because of its dense nerve supply, the periosteum is sensitive to direct trauma. This sensitivity is why bone injuries, even without a complete fracture, are painful. The periosteum’s role in bone healing also means that damage or infection to this membrane can complicate recovery from bone injuries or procedures. The periosteum can also form benign bone growths, such as periosteal chondromas.