The morphology and selected biological properties of articular cartilage

The purpose of this article is to present the current state of knowledge regarding the structure and functions of articular cartilage. Articular cartilage is constructed with hyaline cartilage tissue. It is composed of chondrocytes located in lacunae and the extracellular matrix. The chondrial matrix contains water, collagen, proteglycans, non-collagenous matrix proteins, and lipids. Articular cartilage is devided into four zones - superficial, intermediate, deep, and calcified - on the basic of morphology, the orientation of collagen fiber, and the proteoglycan content. The dominant collagen of this tissue is Type II collagen, which, together with smaller quantities of other collagens (i.e. Types IX and XII), forms a network of fibers, with large, aggregating proteoglycans and smaller, non-aggregating proteoglycans. Proteoglycans are proteins that contain covalently attached glycosaminoglycans (GAGs), with water between them. The large aggregating proteoglycans, called "aggrecans", form aggregates that bind hyaluronic acid, and together with collagen they are responsible for the mechanical properties of cartilage. The smallnonaggregating proteoglycans, decorin and fibromodulin, limit the formation of collagen fibres. Other proteins in the cartilage matrix - chondrocalcin and the N-propetide of Type II collagen - participate in fiber formation. Yet other proteins - chondronectin, fibronectin, vitronectin and thrombospondin - take part in the interaction between the chondrocytes and the matrix. Cartilage oligomeric matrix protein (COMP) prevents the vascularization of the cartilage and, perhaps, is responsible for the repair process. The proteins known as Cart-1 and CEP-68 participate in chondrogenesis, while tenascin and Mgp are considered to be cartilage calcification inhibitors. Apart from the structural elements, chondrocytes produce substances that fulfill purely physiological functions: enzymes and cytokines. The enzymes - which include metalloproteinases, adamalysins, serine and cysteine proteases and their inhibitors - participate in cartilage matrix reconstruction. The cytokines - IL-1, TNF-alfa, IL-6, IL-8, and LIF - stimulate the chondrocytes to produce an increased amount of enzymes, while IL-4 inhibits this process. Human articular chondrocytes exibit the constitutive expression of class I molecules of the major histocompatibility complex (MHC), molecules regulating the activation of the complement, and after activation (e.g. under the influence of IFN-alfa, IL-1, TNF-a or in the course of arthritis), also MHC class II and ICAM-1 intracellular adhesion molecules. Numerous studies have shown that chondrocytes also have tissue-specific antigens, which induce the production of antibodies in patients with cartilage grafts, as well as those with rheumatoid arthritis and osteoarthritis. Some of these antibodies react with type II collagen, others are directed against other proteins i.e. anchorin CII and CH65. the role of these diverse molecules, which are present in cartilage cells and separated from the immune system by the matrix, remains unclear.