The periosteum is a living membrane that envelops the outer surface of most bones. This thin tissue acts as a sheath, separating bone from its surrounding structures. It plays a role in the skeletal system by providing bones with a blood supply and a sense of feeling. Understanding this structure offers insight into how bones maintain themselves, grow, and recover from injury.
Anatomy of the Periosteum
The periosteum is composed of two distinct layers. The outer, fibrous layer is primarily made of dense connective tissues, including strong collagen fibers. This durable layer provides structural integrity and protection to the inner components of the membrane and the bone itself. It also houses a significant network of blood vessels and nerves.
Beneath the protective fibrous layer lies the inner cellular layer, also called the osteogenic layer. This layer is distinguished by the presence of osteoprogenitor cells, a type of stem cell. These cells can develop into bone-forming cells known as osteoblasts. The thickness of this inner layer is most prominent during childhood and adolescence when bone development is rapid, and it thins in adulthood.
To ensure a firm connection, the periosteum is securely anchored to the underlying bone. This attachment is achieved through thick collagenous fibers known as Sharpey’s fibres. These fibers extend from the inner layer of the periosteum and penetrate the outer part of the bone, known as the cortex. This arrangement ensures the periosteum remains tightly integrated with the bone, except at joint surfaces and at sites where tendons and ligaments attach.
Key Functions
One of the periosteum’s primary roles is facilitating bone growth in diameter, a process called appositional growth. During periods of growth, such as childhood and adolescence, the osteoprogenitor cells in the inner layer differentiate into osteoblasts. These osteoblasts then deposit new bone tissue onto the existing outer surface, effectively widening the bone.
The periosteum is also responsible for fracture healing. When a bone breaks, blood vessels within the periosteum rupture, contributing to the formation of a hematoma, or blood clot, at the injury site. Following this initial response, the osteoprogenitor cells in the inner layer multiply rapidly. These cells form a soft callus, an early scaffold of new bone that bridges the gap between the fractured bone ends.
The periosteum is also a conduit for the bone’s nutrient supply. It is highly vascularized, containing an extensive network of small blood vessels. These vessels penetrate the dense outer bone through channels known as Volkmann’s canals and connect to Haversian canals, which run lengthwise through the bone. This ensures that bone cells receive the necessary oxygen and nutrients to remain viable, and a bone stripped of its periosteum will eventually die.
Periosteum Injuries and Conditions
The rich nerve supply within the periosteum makes it highly sensitive to pain. The outer layer contains a high concentration of sensory nerve endings called nociceptors. This is why injuries to the bone, such as fractures, are intensely painful. Much of the pain signal originates from the tearing of this nerve-rich membrane rather than the bone tissue itself, which has a more limited nerve supply.
Inflammation of this membrane, a condition known as periostitis, can arise from various causes. It often results from repetitive stress or overuse, leading to irritation and swelling of the tissue. A common example of periostitis is medial tibial stress syndrome, more commonly known as shin splints. This condition involves inflammation of the periosteum along the tibia, or shin bone, frequently experienced by runners and athletes due to repeated impact and muscle strain on the bone’s surface.
The periosteum can be involved in other medical issues. For instance, bacterial infections can lead to a subperiosteal abscess, a collection of pus that accumulates between the periosteum and the bone surface. The cells within the periosteum can also give rise to tumors, which can be either benign or malignant.