The periosteum is a dense connective tissue membrane that covers the outer surface of nearly every bone in the body. It has two distinct layers, contains multiple cell types, supplies blood to the underlying bone, is richly innervated with nerves, and plays a central role in both bone growth and fracture repair. These are the core statements that accurately characterize the periosteum, and each one deserves a closer look.
It Has Two Distinct Layers
The periosteum is not a single uniform sheet. It consists of an outer fibrous layer and an inner cellular layer, each with a different structure and job.
The outer fibrous layer is made of dense, irregular connective tissue packed with collagen fibers and relatively few cells. It subdivides further into a superficial portion that is more vascular (receiving periosteal blood vessels) and a deeper portion that is fibro-elastic. The collagen in this layer is firm and insoluble, giving the periosteum its toughness and its ability to protect the bone beneath it.
The inner layer is called the cambium layer (also known as the osteogenic layer). It is far more cellular and is where the bone-building action happens. The zone closest to the bone surface, sometimes called the germinative zone, is packed with osteoprogenitor cells and osteoblasts. Just outside that sits a thicker, more transparent zone containing capillaries, fibroblasts, and pericytes embedded in an amorphous matrix. Together, these two zones make up the cambium layer.
It Contains Bone-Forming Cells
The cambium layer houses two key cell populations: osteoprogenitor cells and osteoblasts. Osteoprogenitor cells are stem-like cells that can differentiate into osteoblasts whenever the body needs new bone tissue. In children and adolescents, osteoblasts are abundant and active in the inner layer. In adults, osteoblasts may be largely absent from the periosteum under normal conditions, but they reappear rapidly when a fracture or other injury signals the need for repair.
This ability to “wake up” and produce bone-forming cells on demand is one of the periosteum’s most important features. It means the membrane serves as a reservoir of regenerative potential throughout life, even when it appears relatively quiet in a healthy adult skeleton.
It Drives Bone Growth in Width
Bones grow longer at their growth plates, but they grow wider through a process called appositional growth, and the periosteum is directly responsible for this. Osteoprogenitor cells in the cambium layer are activated by signaling molecules like insulin-like growth factor 1 (IGF-1) and bone morphogenetic proteins (BMPs), then differentiate into osteoblasts. Those osteoblasts deposit new bone tissue on the outer surface of the existing bone, gradually increasing its diameter.
The new bone tissue forms around periosteal blood vessels, initially appearing as periosteal striae. Over time, these striae merge around the vessels to create Haversian canals surrounded by concentric rings of bone matrix, ultimately becoming the compact cortical bone that gives the skeleton its strength. This is why the periosteum is sometimes described as the structure responsible for making bones thicker and more structurally robust during growth.
It Is Essential for Fracture Repair
When a bone breaks, the periosteum is one of the first tissues to respond. Within three days of a fracture, expression of periostin (a key signaling protein) increases roughly fourfold in the periosteum near the injury site. Osteoprogenitor cells in the cambium layer rapidly begin differentiating into both osteoblasts and cartilage-forming cells.
Research using genetic tracing in animal models has shown that periosteal cells are the major source of the soft callus that bridges a fracture gap. These cells give rise to the chondrocytes (cartilage cells) and osteoblasts found in both the soft and hard callus. The periosteum also promotes the growth of new blood vessels at the fracture site by activating vascular endothelial growth factor (VEGF), which helps deliver oxygen and nutrients needed for healing. In short, the periosteum orchestrates the early stages of bone repair: forming a temporary cartilage scaffold, converting it to new bone, and restoring blood supply.
It Is Vascular and Highly Innervated
The periosteum carries a rich network of blood vessels, particularly in its outer fibrous layer. These periosteal vessels supply nutrients to the outer third of the cortical bone beneath. This is one reason that stripping the periosteum during surgery can compromise blood flow and slow healing.
The membrane is also densely supplied with nerve fibers. Immunohistochemical studies have identified sympathetic nerve fibers running through the periosteum in multiple mammalian species. These nerves are one reason bone injuries hurt so much: the periosteum is far more sensitive to pain than the bone tissue itself. Beyond pain signaling, the sympathetic nerve fibers in the periosteum appear to play a role in regulating bone mineralization, linking the nervous system directly to skeletal health.
It Attaches to Bone via Sharpey’s Fibers
The periosteum is anchored to the underlying bone by collagen bundles called Sharpey’s fibers (also known as perforating fibers). These fibers extend from the fibrous layer of the periosteum into the bone matrix, holding the membrane in place. The strength of this attachment varies by location. Near the ends of long bones and at sites where tendons and ligaments insert, the attachment is firm. Along the shaft of a long bone, the connection through Sharpey’s fibers tends to be looser and more easily separated.
It Does Not Cover Every Bone Surface
One commonly tested characteristic of the periosteum is where it is absent. The periosteum covers the shafts and most outer surfaces of bones, but it does not cover articular surfaces (the smooth ends of bones inside joints, which are instead lined with articular cartilage). It is also absent at sites where tendons and ligaments attach directly to bone, and at sesamoid bone surfaces within tendons. Recognizing these exceptions is important because areas lacking periosteum have different healing and blood supply characteristics than periosteum-covered bone.
It Changes With Age
The periosteum is thickest and most active during childhood and adolescence, when bones are growing rapidly. The cambium layer in young bones is rich with osteoblasts, and appositional growth is at its peak. As a person ages, the cambium layer thins and its osteoblast population diminishes. In adult bones, osteoblasts are largely absent from the periosteum under resting conditions. However, the osteoprogenitor cells remain, and they can still be recruited during fracture healing or other repair processes. This declining but persistent regenerative capacity is why fractures in older adults heal more slowly but still heal, in part thanks to the periosteum’s retained cell population.