The periosteum is a dense, fibrous membrane that envelops the outer surface of nearly every bone in the body. This specialized connective tissue is firmly attached to the underlying bone, serving as a foundational layer for skeletal health and integrity. It is absent only where bones are covered by articular cartilage, such as within joints, and at the insertion points of some tendons.
Anatomy and Protective Role
The periosteum is organized into two distinct layers. The outermost layer is the fibrous layer, consisting of dense irregular connective tissue rich in collagen fibers and fibroblasts. This layer acts as a strong mechanical barrier, protecting the bone from physical trauma and potential infection.
The fibrous layer provides an anchor for surrounding soft tissues. Tendons and ligaments attach directly to the periosteum. These strong attachments are secured by specialized collagen bundles called Sharpey’s fibers, which extend from the fibrous layer deep into the bone matrix.
Beneath the fibrous layer lies the inner, more cellular cambium layer. This inner section is highly vascularized and contains various cell types, including osteoprogenitor cells. The fibrous structure and strong anchorage help transmit forces generated by muscle contraction efficiently to the bone.
Essential for Bone Nutrition and Appositional Growth
The periosteum sustains bone health through its extensive network of blood vessels. Numerous small arteries and veins run through the outer fibrous layer, with branches penetrating the underlying compact bone tissue. This vascular supply delivers oxygen, minerals, and nutrients to the outer cortex of the bone.
In long bones, periosteal blood vessels provide a significant portion of the blood supply to the outer one-third to one-half of the cortical bone. The inner cambium layer also contains the sensory nerves of the bone, making the periosteum sensitive to manipulation and injury.
The inner cambium layer contains osteoprogenitor cells, which are specialized stem cells that differentiate into bone-forming osteoblasts. These osteoblasts are responsible for appositional growth, the process of increasing the bone’s diameter or width. This process occurs throughout life, allowing the bone to adapt to mechanical stresses by adding new layers of bone matrix to the outer surface.
The Role in Bone Repair
The periosteum initiates and manages fracture healing following a bone injury. Upon a fracture, cells within the inner cambium layer rapidly activate and proliferate. These activated osteoprogenitor cells differentiate into two primary cell types: osteoblasts and chondroblasts.
The osteoblasts lay down new bone tissue on the outer surface through intramembranous ossification. Simultaneously, chondroblasts produce a soft callus of cartilage, bridging the gap between the fractured ends. This combination forms a temporary structure that stabilizes the injury site.
The soft cartilaginous callus is eventually replaced by hard, mineralized bone tissue through endochondral ossification. Progenitor cells from the periosteum are the major source for this repair cascade, forming the external callus that restores structural integrity. If the periosteum is severely damaged, the bone’s capacity for self-repair is significantly reduced.