**3. Articular cartilage homeostasis**

**2. Alteration in cartilage composition in Osteoarthritis (OA)**

extracellular matrix [5, 6]

366 Regenerative Medicine and Tissue Engineering

The chondrocyte is responsible for both the synthesis and the breakdown of the cartilaginous matrix but the mechanisms that control this balance are poorly understood [4]. The distribution of load across the joint is an important function of the articular cartilage for avoid excessive load affecting both cartilage and bone. It has been demonstrated that articular chondrocytes are able to respond to mechanical injury where biological stimuli such as cytokines and growth and differentiation factors contribute to structural changes in the surrounding cartilage matrix. It has been demonstrated that many non-mechanical and mechanical factors such as load clearly have a role in the initiation and propagation the processes of OA. The OA is the most common joint disease allowing dysfunction and pain. The OA is characterized by changes in chondrocyte metabolism that leads to elevated production of proteolytic enzymes, cartilage damage and loss of joint function. It have been described several mechanisms that can lead to OA, among of these mechanisms are mechanicals, bone changes and changes in the cartilage

Aging, cartilage senescence and reactive oxygen species (ROS) are normal changes in the musculoskeletal system that contribute to the development of OA, but the mechanisms are poorly understood [5]. Inflammation is considered as a very early event in OA perhaps induced by joint trauma affecting chondrocytes in the cartilage and synovial cells (fibroblasts and macrophages) to produce cytokines as interleukin-1-beta (IL-1β) and tumoral necrosis factoralpha (TNF-α), and other signaling molecules as proteoglycans to switch to or increase catabolic processes [6]. Obesity has been described as a risk factor for OA by increased mechanical load factors and degenerative knee pain. The mechanisms between obesity and OA are not completly understood but, it has been found the release of fat molecules that can affect the processes in the joint, including adipokines as visfatin and leptin, perhaps affecting the inflammatory response [7, 9]. Malalignment of the knee joint plays an important role in the development of early osteoarthritis changing the center of pressure of articular cartilage and subchondral bone. Varus or valgus malalignment of the lower extremity results in an abnormal load distribution across the medial and lateral tibiofemoral compartment and being increased in patients with knee osteoarthritis and is increased in patients with overweight. However, studies examining the relationship between malalignment and early knee osteoarthritis have produced conflicting results. The association between malalignment and OA changes is based on radiographic changes mainly and different multicenter OA studies [10-12]. Meniscus is an important tissue in the system of the knee. It is function is the load transmission and absortion shock. Complete or partial loss of meniscal tissue alters the biomechanical and biological of the knee joint modifying the pattern of load distribution and the instability of the knee. Meniscal narrowing, cartilage loss and chondral lesions increase the risk of secondary OA with cartilage degeneration. This secondary OA is associated to chondral damage, ligamentous instability, and malalignment with reduction in the shock absorption capacity of the knee [13-15]. Extrussion has been associated with articular changes according to their depth into partial-thickness and full-thickens defects. Partial-thickness lesions are considered less symptomatic with little evidence of progression on osteoarthritis. Full-thickness chondral and osteochondral lesions frequently cause symptoms, and they are considered to predispose to

Articular cartilage is composed of four distinct regions and they differ in their collagen fibril orientation: (a) the superficial or tangential zone (200 μm), (b) the middle or transitional zone, (c) the deep or radial zone and (d) the calcified cartilage zone. The superficial zone is composed of thin collagen fibrils in tangential array parallel to surface with a high concentration of decorin and lubricin and a low concentration of aggrecan. The middle zone is composed thicker collagen fibrils more random organized. The deep zone is composed the collagen bundles thickest and arranged in a radial fashion, orthogonal to the surface, and the calcified cartilage zone, located above subchondral bone and the tidemark that persists after growth plate closure and is composed of matrix vesicles, vascularization and innervation from the subchondral bone. The collagen type in the calcified zone surrounding the cells is type X as in the hypertrofic zone of the growth plate [21, 22], [23]. From the superficial to the deep zone, cell density progressively decreases. The chondrocytes in the superficial zone are small and flattened. The chondrocytes in the middle zone are rounded, and the deep zone chondrocytes are grouped in columns or clusters and they are larger and express markers of the hypertrophy as well. Differences in expression of zonal subpopulations may determine the zonal differences in matrix composition and in the mechanical environment [24, 25].

Chondrocytes live at low oxygen tension within the cartilage matrix, ranging from 10% at the surface to less than 1% in the deep zones. *In vitro*, chondrocytes adapt to low oxygen tensions by up-regulating hypoxia-inducible factor-1-alpha (HIF-1α), which stimulate expression of glucose transport via constitutive glucose transporter proteins (GLUTs) and angiogenic factors such as vascular endothelial growth factor (VEGF) as well as a number of genes associated with cartilage anabolism and chondrocyte differentiation [26, 27].

It is no clear how chondrocytes maintain their ECM under normal conditions since they lack access to the vascular system but gene expression and protein synthesis may be activated by injury. The aging may affect the properties of normal cartilage by altering the content, composition and structural organization of collagen and proteoglycans. The normal function of the articular cartilage within the joint is to be elastic and have high tensile strength and these properties depend on the extracellular matrix [28]. The chondrocytes produce, in appropriate amounts, this ECM that consist of structural macromolecules of type II collagen fibers, proteoglycans, non-collagenous proteins and glycoproteins, organized into a highly ordered molecular framework. The collagen matrix gives cartilage its form and tensile strength. Proteoglycans and non-collagenous proteins bind to the collagenous network and help to stabilize the matrix framework and bind the chondrocytes to the macromolecules of the network. The matrix protects the cells from injury due to normal use of the joint, determines the types and concentrations of molecules that reach the cells and helps to maintain the chondrocyte phenotype [29, 30].

The ECM surrounding the chondrocytes has been divided into zones depending on their distance from the cell. The pericellular matrix is localized immediately around the cell, the territorial matrix is next to pericellular matrix and the most distance is the interterritorial matrix. Each matrix zone is characterized by different types of collagens as shown in figure 1.

 **Figure 1.** The organization of normal articular cartilage. The organization of chondrocytes is divided in superficial, middle or transitional, deep or radial and calcified cartilage zones with a boundary or tidemark between the first three zones and the calcified zone. The extracellular matrix is divided depending the distance from the chondrocytes. The pericellular zone is the matrix surrounding immediately the chondrocytes. The territorial zone is the next to pericellu‐ lar zone and the interterritorial zone is the most distant. Every zone has specific characteristics related with the shape of the chondrocyte as well the activity and the expression of different molecules by the cell.

The pericellular matrix is a region surrounding chondrocytes in the articular cartilage where diverse molecules as growth factors have interaction with the receptors expressed on the membrane cell of chondrocyte. This region is rich in proteoglycans as aggrecan, hyaluronan and decorin. Type II, VI and IX are collagen most concentrated in the pericellular network of thin fibrils as fibronectin. Type VI collagen forms part of the matrix immediately surrounding the chondrocytes and may help them to attach to the macromolecular framework of the matrix. This pericellular matrix enclosed cells has been termed chondron. The territorial zone contains type VI collagen microfibrils but little or no fibrillar collagen. The interterritorial cartilage matrix is composed of a collagen type II, type XI collagen and type IX collagen integrated in the fibril surface with the non-collagen domain, permitting association with other matrix components and retention of proteoglycans. These collagens give to the cartilage form, tensile stiffness and strength [31-33].

Cartilage contains a variety of proteoglycans that are essential for its normal function. These include aggrecan, decorin, biglycan, fibromodulin and lumican each proteoglycan has several functions determined. The proteoglycans are very important for protecting the collagen network. Other non-collagen molecules as the matrilins and cartilage oligomeric protein (COMP) are also present in the matrix. COMP acts as a catalyst in collagen fibrillogenesis, and interactions between type IX collagen and COMP or matrilin-3 are essential for proper formation and maintenance of the articular cartilage matrix. Perlecan enhances fibril forma‐ tion, and collagen VI microfibrils connect to collagen II and aggrecan via complexes of matrilin-1 and biglycan or decorin [34].

Throughout life, the cartilage undergoes continual internal remodeling and the chondrocytes replace matrix macromolecules lost through degradation. Therefore normal matrix turnover depends on the ability of chondrocytes to detect alterations in the macromolecular composition and organization of the matrix, including the presence of degraded molecules, and to respond by synthesizing appropriate types and amounts of new molecules. In addition, the matrix acts as a signal transducer for the cells. Loading of the tissue due to use of the joint creates me‐ chanical, electrical, and physicochemical signals that help to direct the synthetic and degra‐ dative activity of chondrocytes [22, 35].
