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Articular cartilage, which is a form of hyaline cartilage is a biomaterial composed mainly of collagen, covers the articulating surfaces of bones . Collagen is the most important structural material of hard and soft tissues in animals and operates as the bearing surfaces of joints. Cartilage is porous and its high degree of mechanical strength comes about from the movement of fluid (synovial fluid) in and out of the tissue, when forces are applied to it via joint loading .
The avascular tissue articular cartilage is mainly composed of one cell type (chondrocytes), and the ability of articular cartilage to self-repair is very restricted. The role of chondrocytes is to synthesise and preserve the extracellular matrix . Articular cartilage also has high water content (70 to 80% of the tissue wet weight ww), and this allows cartilage to endure the compressive, tensile and shear forces related with joint loading .
In elder individuals, the loss of the physical and mechanical properties of cartilage gradually results in pain, and other clinical symptoms related with osteoarthritis. Osteoarthritis is an age-related disease that ultimately has an effect on each individual, who live onto they senior years .
The pain associated with osteoarthritis typically emerges from the degeneration of the cartilage between the joints, as a result of primary osteoarthritis, or from trauma bringing about the loss of cartilage . Given that cartilage demonstrates a poor ability to self-repair, these injuries are sustained for years and can ultimately bring about further degeneration (secondary osteoarthritis) . The degeneration of cartilage causes bone ends to become exposed, and the deposition of new osseous tissue on the bone ends. This also reduces the space in the joint cavity and limits movement . It has been estimated that roughly 36 million Americans are diagnosed with some form of arthritis . Osteoarthritis is also the main reason as to why many individuals undergo hip and knee replacement surgery , and is also the main cause of mobility impairment in elder individuals .
The typical methods that are normally used for cartilage repair (e.g. tissue autografts or allografts) do not predictably re-establish a stable articular surface to an osteoarthritic joint. Cell based therapies i.e., implantation of cells or engineered cartilage, signify a different method to articular cartilage repair .
In vivo studies for articular cartilage repair, or in vitro approaches to study cell and tissue-level responses to molecular, mechanical, and genetic manipulations using engineered cartilage can be carried out .
Aims and objectives.
In order to gain a better understanding of the properties of cartilage tissue, the present study will be carried out to observe the mechanical and histological properties of cartilage under various conditions. This will be achieved by designing an experimental procedure whereby static and dynamic test will be carried out at various loads on the cartilage tissue. Histological sectioning will then be carried out in order to examine the penetration of synovial fluid into the cartilage tissue.
2. Background on cartilage.
Cartilage can be found in many different areas of the human body, and is a durable and flexible type of connective tissue . It is avascular, meaning it does not have a blood supply and thus being the main reason why it is has difficulties in self repair. There is only cell type found in cartilage tissue matrix, the chondrocytes that are bound in lacunaes. Another main reason as to why cartilage has such difficulties in self-repair is that the chondrocytes are not capable to be transported to the site of an injury .
There are 3 main types of cartilage found in the body, and these include hyaline cartilage, elastic cartilage and fibrocartilage. These three types of cartilage are all made up of chondrocytes and extracellular matrix molecules. Elastic cartilage can be distinguished via the presence of elastin, and makes up the ear and nose. Fibrocartilage is mainly present at the ends of ligaments and tendons. A large amount of collagen is also present in the ECM of fibrocartilage in comparison to hyaline cartilage .
Articular cartilage is composed of a range of different materials that have different properties. Roughly 75 to 85% of the whole tissue is made of water in weight. The remaining weight is made up by proteoglycans and collagen . The presence of type II collagen in hyaline cartilage distinguishes it from the other types of cartilage . Articular cartilage which is a type of hyaline cartilage has a smooth, shiny appearance and also appears to be white . The articulating surfaces of long bones and sesamoid bones found in synovial joints are coated in articular cartilage, for e.g. in the knee joint. Articular cartilage has the ability to endure very large loads, in particular in the hip and knee joints. It also provides the joints with a smooth lubricating bearing material [15-16]. Other examples or hyaline cartilage include the growth plate and the larynx .
Articular cartilage is mainly present in diarthrodial joints . Diarthrodial/ synovial joints which are found for e.g. in the knee, hip and shoulders are different from other joints found in the body (fibrous and cartilaginous), as they allow for a greater variety of movement (see Figure 1) . The diarthrodial joint is surrounded by a fibrous capsule, and the inner layer is formed of synovium. The synovium is an important feature as it is involved in the removal of waste produced by cellular components in the joint. The synovium secretes the synovial fluid which is also a vital feature of diarthrodial joints, as it acts as a lubricant reducing the amount of wear and friction in the joint, and carries nutrients to the cells in articular cartilage .
Figure1. Diagram of a normal Diarthrodial joint.
2.1. Structure and function of articular cartilage.
The articular cartilage in diarthrodial joints is crucial in absorbing and minimising large amounts of loads encountered in personââ‚¬â„¢s everyday life, and also essential in reducing the amount of surface friction. It does so by increasing the contact area, absorbing the force, and thus reducing the applied strain . Another unique property of articular cartilage is that it can withstand various cyclic loading conditions, and compressive loads that can be up to three to six times the body weight without any considerable wear . These qualities of articular cartilage come about from the presence of interstitial fluid. The various loads applied to the joint are supported by interstitial fluid and this protects the solid matrix from the forces of the load applied . The frictional coefficient is thus reduced to 0.0075 due to the pressurisation of the tissue fluid , and this is further decreased via the presence of a lubricating film e.g. synovial fluid .
The only cell type found in human articular cartilage is chondrocytes, and these cells makeup about 1-10% of the total tissue volume . Chondrocytes play an important role in the preservation of the size, and mechanical properties of the articular cartilage tissue, by restoring degraded matrix molecules. Some chondrocytes are thought to be involved in sensing the mechanical environment of the cell, due to these chondrocytes having cilia that expand from the cell into the extracellular-matrix, and because chondrocytes are involved in the modification of the matrix properties in reply to loading .
Chondrocytes are derived from mesenchymal stem cells (MSCs) which originate in the bone marrow of fully grown individuals. The development of chondrocytes begins during embryogenesis, where mesenchymal stem cells differentiate into chondrocytes, and a cartilaginous matrix is secreted. These cells carry on dividing 
Because cartilage is an avascular tissue i.e. does not have a bloody supply or any lymphatic vessels, chondrocytes do not have any physical cell-to-cell contact amongst them. Therefore there must be a diffusion and connective transport of nutrients, waste, signal molecules and oxygen through the matrix, in order for chondrocytes to survive [15, 21].
2.1.2. Collagen and proteoglycans.
Type II collagen is the most abundant type of collagen present in articular cartilage, and accounts for about 90%of the collagen in the matrix. The remaining 10% includes various other types of collagen i.e. type V, VI, IX and XI . Collagen type X is only found in calcified areas of cartilage .
In contrast to the other types of collagen, type II collagen contains a larger amount of bound carbohydrate groups, and this enables it to have greater amounts of interactions with water compared to the other types of collagen. The arrangement of type IX, XI and type II collagen in articular cartilage, provides the tissue with tensile strength, and allows it to physically entrap various macromolecules. This arrangement comes about from the different types of collagen developing a mesh that is made via the collagen types forming fibrils that interweave .
Type X collagen is deemed to a proteogylcan and thus dissimilar from other types of collagen, due to the attachment of its collagen chain to an added GAG chain . Type II and IX collagen also have an important role in the matrix function as they are markers of differentiated chondrocytes .
Roughly 20-30% of the dry weight of articular tissue is made up of proteoglycans . Proteoglycans are large complex biomolecules, of which 95% is made up of polysaccharides and the remaining 5% is protein. The protein core of proteoglycans is linked with one or few different types of glycosaminoglycan (GAG) chains [3, 25]. Aggrecan is the main proteoglycan found in articular cartilage, as well other proteoglycans such as versican, biglycan and decorin .
A fibrous network is formed in articular cartilage, by the collagen weaving together, and in which proteoglycan aggregates are trapped, thus producing a cohesive porous organic matrix . Within this matrix there are significant molecular interactions between the collagen and proteoglycan aggregates. These interactions can have an effect on chondrocyte metabolism, collagen network organisation and collagen fibrillogenesis. Other forces within and outside of the tissue, i.e. swelling pressure and external loading can also have an effect on these .
2.1.3. Cartilage Nutrition.
2.1.4. Interstitial fluid.
2.1.5. Noncollagenous proteins.
2.1.6. zones of organisation.
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