Long bones, such as the femur, tibia, and humerus, form the structural framework of the body. The shaft, known as the diaphysis, is composed of dense compact bone that must withstand significant mechanical stress during daily activities. To maintain its integrity, grow, and repair itself, the bone requires a specialized membrane that completely encases its exterior. This protective covering is a highly active biological layer that manages the nourishment and regenerative capacity of the underlying bone tissue.
Defining the Periosteum
The outermost covering of the bone shaft is the periosteum, a tough, dense layer of vascular connective tissue that acts as a tight sleeve over nearly the entire surface of the bone. This membrane is absent only where the bone is capped by articular cartilage at the joints.
It is firmly anchored to the underlying compact bone by bundles of strong collagen fibers known as Sharpey’s fibers, which penetrate the bone matrix. This deep attachment ensures the membrane remains in place despite the constant pull from muscles and tendons. The periosteum also serves as a point of insertion for many ligaments and tendons.
The Dual Layers of the Covering
The periosteum is organized into two distinct layers. The outermost layer is the fibrous layer, composed primarily of dense irregular connective tissue. This layer contains fibroblasts and is richly supplied with blood vessels and nerves.
The fibrous layer provides mechanical protection for the bone and acts as a strong anchoring site for surrounding soft tissues. The underlying inner layer is called the cambium layer, or sometimes the osteogenic layer, due to its high concentration of active cells. This cellular layer contains osteoprogenitor cells, which are mesenchymal stem cells capable of differentiating into bone-forming cells. These cells lie adjacent to the cortical bone surface and are responsible for the membrane’s regenerative capabilities.
Key Roles in Bone Health and Nourishment
The periosteum plays a fundamental part in the ongoing maintenance and shaping of the bone. One of its primary functions is to provide nourishment to the compact bone that makes up the shaft. The fibrous layer contains an extensive network of blood vessels that supply oxygen and nutrients, which then penetrate the underlying bone tissue.
These periosteal vessels are responsible for a significant portion of the blood flow to the outer third of the compact bone cortex. This vascular supply is accompanied by sensory nerve fibers that innervate the membrane, making it a sensitive structure.
Furthermore, the cambium layer is directly responsible for appositional growth, the process of increasing the bone’s diameter or width. Throughout development and into adulthood, the osteoprogenitor cells in this inner layer differentiate into osteoblasts. These new osteoblasts then deposit new bone matrix onto the outer surface of the shaft, allowing the bone to thicken and strengthen in response to mechanical demands.
The Periosteum and Fracture Healing
The membrane’s cellular potential is most apparent in its rapid response to acute injury, such as a fracture. When the shaft of a long bone breaks, the osteogenic layer of the periosteum is immediately mobilized near the site of the break. The progenitor cells quickly proliferate and differentiate to initiate the repair process.
These activated cells serve as a major source of the cells required for the initial formation of the callus. The periosteum forms an external bridge of tissue around the fracture gap, providing scaffolding for new bone to grow. This rapid mobilization and differentiation of cells is critical to establishing the initial soft and hard callus that stabilizes the two broken ends.