The articular cartilage is a type of hyaline cartilage which along with the elastic and fibrous cartilage belong to the group of connective tissue. Its only living elements are chondrocytes, which produce the paracellular matrix, together forming the chondron as the basic cartilage-metabolic-functional unit. Chondrocytes are nourished with nutrients from synovial fluid passing to it from blood vessels due to a double diffusion process.
Damage and reconstruction of the collagen matrix of the superficial layer arises in experimental models of induced cartilage degradation first. This suggests that changes in this layer may affect the development of osteoarthritis by changing the tissue response to a mechanical stimulus. A densely packed net of collagen fibers limits the penetration of both large molecules (including antibodies) from the synovial fluid as well as the loss of large molecules from the cartilage. Its damage affects not only the mechanical properties of cartilage but also the stimulation of inflammatory and immune responses. The healing of the cartilage lost is different than in other tissues. When the cartilage fails, the collagen fibers are broken and the proteoglycans decrease. Chondrocytes lead to increased production of collagen and proteoglycans to fill the cavity with the matrix. In the surgical treatment of small cartilage defects after drilling the subchondral layer of the bone, effective filling of the cavity with the cartilage is noticeable already after a few months. This effect is due to the availability of substrates for regeneration from the blood and medullary cavity and the inflow of stem cells to superficial layers of articular cartilage, which initially differentiate into chondrocytes and fill the cavity with newly synthesized cartilage.
