What Is Periosteum?

Periosteum is a sturdy connective tissue envelope that covers the bone surface except for the joints. At the bone end and tendon attachment site, it is very densely attached to the bone. The periosteum in other areas is thick and easily peeled from the bone.

Periosteum is a sturdy connective tissue envelope that covers the bone surface except for the joints. At the bone end and tendon attachment site, it is very densely attached to the bone. The periosteum in other areas is thick and easily peeled from the bone.

Chinese name

periosteum

Foreign name

periosteum

Subject

biology

Constitute

Outer layer

Histological characteristics of periosteum

Although the tissue structure of the periosteum differs depending on the anatomical location and age, the periosteum is traditionally divided into a superficial fibrous layer and a deep cambrium layer. The two layers are not clearly delimited. The fibrous layer is thicker and has fewer cellular components. It is mainly thick collagen fiber bundles that are intertwined into a network. Some fibers penetrate into the bone, called sharpey fibers or perforating fibers, and serve to fix the periosteum and ligaments. The germinal layer is close to the outer surface of the bone. It has few fiber components, loose arrangement, abundant blood vessels and cells, and has osteogenic ability. It is also called osteogenic layer. Its cell components include osteoprogenitor cells, osteoblasts, and osteoclasts. And vascular endothelial cells, the tissue composition of the germinal layer changes with age and functional activity.

During embryonic and postnatal growth periods, the germinal layer is composed of several layers of cells, the outer layer is fibroblast-like osteoprogenitor cells, and the inner layer is osteoblasts, both of which have the ability to proliferate and are related to periosteal osteogenesis . After adulthood, bones are in a relatively stationary phase of slow reconstruction, with thinner germinal layers and relatively fewer bone progenitor cells. They are no longer arranged in layers, but are scattered and attached to the surface of the bone. They continue to participate in slow bone reconstruction activities and fractures throughout life. Repair activities. After the periosteum's fibrous layer is peeled off, osteoblasts and osteoclasts can still be firmly attached to the bone surface.

In recent years, some authors have put forward the idea that the periosteum is divided into three layers based on the function and anatomical basis of the periosteum, namely the superficial fibrous layer, the middle vascular undifferentiated area and the deep germinal layer. The middle layer is loose. The main cellular component is undifferentiated cells, which can provide progenitor cells for the germinal layer and fibrous layer. There are also a small number of monocytes in this layer, which play a role in the local regulation of bone reconstruction. The collagen in the extracellular matrix is arranged in an orderly manner, which is suitable for supporting functions. It also plays a viscoelastic role with non-collagen components in the matrix to cushion hair growth Changes in stress within the physiological range of the layer. The characteristics of middle layer looseness are also beneficial to the germinal layer to effectively transport nutrients and metabolites during the active growth period.

Therefore, in addition to providing nutrition and progenitor cells, the middle layer is also a buffer zone that regulates the interaction between bone and surrounding soft tissues. After the periosteum is under stress, its fibroelastic component leaves or approaches the germinal layer through the adjustment of the middle layer. As a result, tension stimulates periosteal osteogenesis and pressure induces bone resorption. The periosteum is thick in growth and has an ideal structure. It can quickly and sensitively respond to changes in stress and start adaptive reconstruction of the bone surface. The three-layer structure of the periosteum changes significantly with age. After birth, the osteoblasts in the germinal layer are slender; the middle layer is thicker, poorly differentiated, and has fewer blood vessels. During rapid growth, the osteoblasts in the germinal layer are cubic; the blood vessels, undifferentiated cells, and monocyte phagocytic cells in the middle layer develop to a peak, and the blood vessels are clearly visible. After adulthood, the germinal cells are in a flat resting state; the middle layer structure begins to degenerate and gradually disappear, and the sensitivity of the periosteum to the stress response decreases. The periosteum is firmly attached to the bone and is generally not easy to peel off. Squier et al. Proposed another three-layer method based on the ratio of cells, fibers, and stroma within the periosteum. The first layer is composed of a supra-osteoblastlayer composed of osteoblasts immediately adjacent to the bone surface and its superficial fibroblast-like cells, which may be bone progenitor cells. The second layer is a relatively transparent area with rich capillaries, which may represent the traditional germinal layer, and most of the vascular components of the periosteum are located in this layer. The third layer is composed of collagen fibers and a large number of fibroblasts, which is equivalent to a traditional fibrous layer. It is still debated whether the periosteum contains elastic fibers in addition to abundant collagen fibers. Murakami and Emery [7] believe that there are elastic fibers in the periosteum, which are synthesized by deep bone progenitor cells. The elastic fibers are arranged in parallel along the longitudinal axis of the bone to form 5-6 layers. The inner part is included in the hair growth layer, and the outer part is integrated into the fiber layer. The cells located in the innermost part of the elastic fibers may be undifferentiated cells and can differentiate into osteoblasts or elastic fibroblasts. Electron microscopy of Tonna [8] also confirmed that both layers of the periosteum had elastic fibers. However, Chong et al. Believe that the periosteum has no elastic fibers, and the elastic fibers found by other authors may be reticular fibers.

Ultrastructure of Periosteum

Under the light microscope, there is no clear boundary between the fibrous layer and the germinal layer of the periosteum; at the ultrastructural level, there is a clear boundary between the two layers. In juvenile animals, the fibrous layer of the periosteum is composed of large collagen fiber bundles and fibroblasts. A large number of small elastic fiber bundles pass through the collagen fiber bundles, which are often arranged in parallel. Germinal bone progenitor cells have abundant swallow bodies, but lack endoplasmic reticulum, filamentous pseudopods, and cell junction complexes. Bone progenitor cells are surrounded by collagen fibers, elastic fibers, and a unique mucopolysaccharide matrix. Collagen fibers are woven into a network. The elastic fibers have fewer layers than the fiber layer. The mucopolysaccharide matrix has a high electronic density and is easily distinguished from the fiber layer. Below the bone progenitor cells are osteoblasts at different stages of differentiation. Osteoblasts are large in volume, oval in shape, rich in cytoplasm, developed with rough endoplasmic reticulum, Golgi complex, and mitochondria, with large, elliptical nuclei, and granules. Extracellular collagen fibers are abundant, and elastic fibers are rare. The osteoblasts immediately adjacent to the bone surface are separated from the bone surface by a layer of osteoid. Actively active osteoblasts are the largest cells in the mononuclear bone cell line, with a large number of filamentous pseudopods on the side facing the bone surface, which are connected to adjacent bone cells. Osteoclasts are confined to the area around the cartilage. After adulthood, as the age increases, the electron density of the elastic fibers in the fibrous layer increases, the collagen fibers become thicker, the number of cells in a unit area gradually decreases, and the remaining cells show obvious ultrastructural changes. Active fibroblasts become Slender fibroblasts. Germinal cells are reduced, cell volume becomes smaller, organelles and swallowing bodies disappear, and osteoblasts on the bone surface are similar to fibroblasts. As cell degeneration worsens, lipofuscin appears in aging fibroblasts, osteoblasts, and bone cells. Even in very old animals, a few viable cells remain in the periosteum, and these cells can be reactivated to proliferate. Bone progenitor cells of the periosteal germinal layer are different in morphology and function from fibroblasts. During skeletal growth, bone progenitor cells can continue to differentiate into osteoblasts and osteocytes. The cell processes of these three cells are closely interlocked with each other and connected to form a network. When osteoblasts are embedded in bone tissue, they become osteocytes. The space around the cell body is called bone pit, and the space around the cell process is called bone tubule. Therefore, these cellular networks not only connect cells together, but also play an important role in dragging subsequent cells into bone tissue, inducing the formation of bone tubules, and maintaining cell communication. Periosteum in different parts often presents some similar ultrastructures. Squier et al. Found in the ultrastructure and stereology of the periosteal and periosteal membranes that both periosteal layers were layered. The first layer is close to the bone surface and is mainly composed of osteoblasts. Osteoblasts account for 90% of the total number of cells in this layer, and there is often an upper layer of osteoblasts composed of small and dense cells above it. Collagen fibers make up 15% of the layer's volume and are another major component of the layer. The main component of the second layer is the matrix (32%), so it has a translucent appearance under a light microscope. The remaining components are fibroblasts and collagen (25% each) and blood vessels (mainly capillaries, accounting for 12% to 17% of the volume). Collagen fibers accounted for 46% and fibroblasts accounted for 34% in the third layer.

Role of Periosteum Periosteum Fracture Healing and Articular Cartilage Defects

In fracture repair, a complete periosteum sleeve is particularly important for rapid healing of the fracture. Although the periosteum itself cannot induce new bone formation, it can generate new bone tissue upon contact with a fractured hematoma. If the periosteum sheath is damaged, fibrous tissue originating from the surrounding soft tissue invades between the fracture ends, fibrous healing may occur; at the same time, fracture hematomas may flow into the soft tissue, allowing local mesenchymal cells to diffuse, affecting fracture healing. Histological and ultrastructural studies on the fracture healing process have shown that the significant change in the periosteum after fracture is thickening, especially in the germinal layer. The osteoprogenitor cells of the germinal layer proliferate and differentiate into osteoblasts, and then form periosteum callus through intraperitoneal osteogenesis. The response is most obvious near the fracture, and the intensity of the proliferative response decreases as the distance from the periosteum to the fracture increases [12]. One day after tibia fracture in rats, osteoprogenitor cells of periosteal germinal layer began to proliferate; 2 to 3 days after fracture, proliferated osteoprogenitor cells began to form trabeculae, and periosteal callus growth stopped after 8 to 9 days, waiting Hypertrophy and calcification of cartilage; 5 days after fracture, mesenchymal cells originating from surrounding muscles gathered near the fractured end of the periosteal strip and differentiated into chondrocytes to form cartilage callus; 9 to 11 days after fracture, Blood vessels penetrate into the hypertrophic and calcified cartilage callus and initiate osteogenesis within the cartilage. The authors believe that periosteal callus and osteogenic bone tissue in cartilage work together to connect and stabilize the fracture end. The process of human long bone fracture healing is similar to experimental animals. In the first week after the fracture, the osteoprogenitor cells of the periosteal germinal layer gradually increased. On the seventh day after the fracture, calcifications appeared in the periosteum for the first time. At 2 weeks after the fracture, the periosteal germinal layer was abundant with osteoblast-like cells. On the 12th day, newly generated trabeculae appeared in the periosteal epiphysis. At 3 weeks after the fracture, the cartilage in the periosteal epiphysis was clearly visible, but the number was limited. On day 18, cartilage began to calcify. These chondrocytes are likely to originate from the bone progenitor cells of the inner layer of the periosteum. Compared with experimental animals, human intraperiosteal calcifications and cartilage appear later, and the number of cartilage is less. The reason is not clear. Periosteum has the dual potential of osteogenesis and cartilage formation. Free periosteum transplantation is used to repair articular cartilage defects. Periosteum can generate cartilage to fill cartilage defects. Two weeks after transplantation, the periosteum thickened, the cells proliferated, and the undifferentiated cells were arranged horizontally. Four to eight weeks after transplantation, the proliferating cells differentiated into juvenile chondrocytes, began to secrete neutral mucopolysaccharides, and the regenerated cartilage tended to be smooth. After 16 weeks, the structure of the periosteum regenerating tissue was close to the surrounding normal articular cartilage. 24 weeks after transplantation, scanning electron microscopy showed that the surface of regenerated cartilage was a honeycomb structure, similar to that of normal articular cartilage. Nuclear chromosome analysis showed that the cells that regenerate cartilage are either progenitor cells derived from transplanted periosteum alone, or pluripotent mesenchymal cells derived from periosteum grafts and subchondral tissue.

Periosteal blood supply and its clinical significance

Periosteum is rich in blood vessels. Within the fibrous layer, the arteries and accompanying veins form a dense network of blood vessels around the bone. The vascular network consists of short branches, circular branches and longitudinal branches. The short branch has no main running direction, the ring branch surrounds the tubular bone, and the longitudinal branch is parallel to the long axis of the bone. The germinal layer is thin and sparse, and runs along the long axis. The blood supply sources of the anastomotic branches connected to the periosteum between the two layers of blood vessels are divided into four groups: (1) Periosteum intrinsic blood vessels, which are distributed in the periosteum fiber layer. (2) Musculoskeletal vessels, where the muscle vessels and the periosteal vessels coincide. It has been clinically confirmed that periosteal detachment caused by fractures, as long as the anastomosis between the muscle and the periosteum is not damaged, the periosteum can still survive and form new bone; if the anastomosis between the muscle and the periosteum is destroyed, it may affect the collateral blood of the muscle Supply may affect the ability of the periosteum to form new bone. (3) Fascia periosteum vessels, which are branches of blood vessels in the limbs, are distributed in the periosteum through the muscle space. (4) Cortical capillary anastomosis, a large number of small periosteum vessels are distributed in the outer 1/3 or 1/4 of the cortex via Volkmann's canal. These small blood vessels not only have the function of fixing the periosteum, but also interconnect with the longitudinal blood vessels in the Hastelloy to form a vertical and horizontal capillary network to communicate the bone marrow capillary network inward. When a fracture hurts and nourishes the arteries, the periosteum can supply more cortical bone. Cortical bone blood flow in healthy adults is centrifugal, and blood from high-pressure nourishing arteries is distributed deep in the periosteum through the cortical blood circulation to supplement the periosteum blood supply. At the muscle attachment site, the cortical capillaries drain through the anastomosis branch of the periosteum and muscle vessels to the small venous periosteal vessels between the muscle bundles. They not only provide nutrition to the periosteum, but may also participate in the osteogenesis of the periosteum. Recent studies have shown that during the process of periosteal cells being activated into bone, the pericytes of the capillaries and microvenous cells can proliferate and differentiate into osteoblasts, becoming a supplemental source of osteoblasts in periosteal osteogenesis.

Periosteum innervation

Periosteum is abundantly innervated, but the properties of its nerve fibers and neurotransmitters have not been systematically documented. It is generally believed that intraperiosteal nerve fibers are mainly myelinated nerve fibers, and their free nerve endings are related to pain. Some of the nerve fibers of the periosteal plexus enter the backbone through Volkmann's canal, and the rest form nerve endings in the periosteum. Immunocytochemical localization studies show that there are nerve plexus stained by substance P in the periosteum, and there are small nerve fiber branches immediately below the periosteum surface. These peptide-containing nerves containing substance P may be related to periosteum-sensitive pain. In addition, periosteum contains vasoactive intestinal peptide (VIP) stained nerve fibers, and its role is unknown. To sum up, the periosteum is a complex and ordered organ with abundant blood vessels and nerves, which is closely related to the surrounding soft tissues and bones. Osteoprogenitor cells and osteoblasts within the periosteum are the histological basis of their osteogenic properties. Due to the lack of effective and specific research techniques, the location, origin, structural characteristics, and mechanism of proliferation and differentiation of osteoprogenitor cells in the periosteum have not yet been determined. The properties of periosteal nerve and its relationship with the periosteal cells' metabolism and osteogenic properties need further study. With the solution of these problems, it is bound to deepen the understanding of periosteum tissue structure and function, and promote the widespread clinical application of periosteum transplantation.