Histologically, the meniscus is a fibrocartilaginous tissue composed primarily of an interlacing network of collagen fibers interposed with cells. The cells of the meniscus are responsible for synthesizing and maintaining the extracellular matrix. There is still some debate as to whether the cells of the meniscus are fibroblasts, chondrocytes, or a mixture of both and whether the tissue should be classified as fibrous tissue or fibrocartilage .
The cells have been termed fibrochondrocytes because of their chondrocytelike appearance and their ability to synthesize a fibrocartilage matrix. Two basic types of fibrochondrocytes have been described within the meniscus: a fusiform cell found in the superficial zone of the meniscus and an ovoid or polygonal cell found throughout the remainder of the tissue.
Although the fusiform cells resemble fibroblasts, they are situated in well-formed lacunae and resemble the chondrocytes found in the superficial (tangential) zone of articular cartilage. Both cell types contain abundant endoplasmic reticulum (ER) and Golgi complexes.
Mitochondria are only occasionally visualized, suggesting that, as in articular chondrocytes, the major pathway for energy production for the fibrochondrocytes in their avascular surroundings is probably anaerobic glycolysis.
The extracellular matrix of the meniscus is composed primarily of collagen (60% to 70% of the dry weight). It is mainly Type I collagen (90%), although Types II, III, V, and VI have been identified within the meniscus. The circumferential orientation of these collagen fibers appears to be most directly related to the function of the meniscus. In a classic study describing the orientation of the collagen fibers within the menisci, it was noted that although the principal orientation of the collagen fibers is circumferential, a few small, radially disposed fibers appear on both the femoral and tibial surfaces of the menisci as well as within the substance of the tissue.
It is theorized that these radial fibers act as “ties” to provide structural rigidity and help resist longitudinal splitting of the menisci resulting from undue compression. Subsequent light and electron microscopic examinations of the menisci have revealed three different collagen framework layers: a superficial layer composed of a network of fine fibrils woven into a mesh-like matrix, a surface layer just beneath the superficial layer composed in part of irregularly aligned collagen bundles, and a middle layer in which the collagen fibers are larger and courser and are oriented in a parallel, circumferential direction. It is this middle layer that allows the meniscus to resist tensile forces and function as a transmitter of load across the knee joint.
In addition to collagen, the extracellular matrix of the meniscus also consists of proteoglycans, matrix glycoproteins, and elastin. The proteoglycan content of the adult meniscus is approximately 10% of the amount found in hyaline cartilage, although this has been shown to vary with age and location within the tissue. A study in the porcine meniscus has shown a higher (two to four times) content of hexosamine and uronic acid in the inner third of the meniscus as compared to the outer two thirds.
These substances have also been known to be more prevalent in the anterior horn as compared to the posterior horn in both the medial and lateral meniscus . The glycosaminoglycan (GAG) profile of the adult human meniscus has been reported to consist of chondroitin 6-sulfate (40%), chondroitin 4-sulfate (10% to 20%), dermatan sulfate (20% to 30%), and keratan sulfate (15%).
Matrix glycoproteins, such as the link proteins that stabilize the proteoglycan-hyaluronic acid aggregates, and a 116-k Dalton protein of unknown consequence, have also been identified within the extracellular matrix . In addition, adhesive glycoproteins, such as Type VI collagen, fibronectin, and thrombospondin, have been isolated from the meniscus . These macromolecules have the property to bind to other matrix macromolecules and/or cell surfaces and may play a role in the supramolecular organization of the extracellular molecules of the meniscus.
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