Cartilage is a viscoelastic connective tissue that is found mainly in load bearing joints in mammals. It is classed broadly into 3 categories: 1) elastic cartilage, found mainly in the external ear and epiglottis. 2) fibrocartilage, which comprises the pubic symphisis, tendon and ligaments and 3) hyaline cartilage, present at the sternum-rib interface, nucleus pulposus of intervertebral discs and at the articulating ends of long bones where it is referred to as articular cartilage. Hyaline cartilage is the most widespread of all three cartilage forms.
The dry weight of cartilage consists of collagen, proteoglycans and chondrocytes (as the singular cell type present within cartilage tissue). From the articular surface to the bone interface, cartilage tissue exists in a number of distinct zonal regions; these are typified by the differences in their cell and extracellular matrix morphology and organisation (Hunziker et al., 1997, Hunziker et al., 2002, Hwang et al., 1992).
The superficial tangential zone is that closest to the synovial fluid in the joint and therefore at the forefront of changes in mechanistic dynamics. Although, comprising a relative small spatial claim to cartilage (10 – 20%), it houses the largest population of chondrocytes. Chondrocytes within this region are flatter and stiffer compared to those located in the inner zones (Darling et al., 2006, Shieh and Athanasiou, 2006). Collagen content in this zone is highest and proteoglycan content is lowest (Responte et al., 2007) imparting more tensile strength to this zone. The arrangement of collagen fibrils in this zone is almost exclusively perpendicular to the bone rather than parallel as is observed in the remaining zones.
The middle or radial zone has the most spatial occupation of articular cartilage. The chondrocyte population is sparse and the orientation of collagen fibrils and the chondrocytes themselves run parallel. Chondrocytes within this region are highly active compared to the superficial zone (Wong et al., 1996).
The deep zone is located closest to subchondral bone and earmarks the transition of cartilage tissue into bone. Within this zone chondrocytes begin to undergo hypertrophy, signifying terminal differentiation in cell development. Cell activity in this zone is markedly altered as the cells have increased alkaline phosphatase activity and begin to deposit calcium resulting in a calcified layer.
Figure 5-1 Depiction of the structure of hyaline cartilage from the articular end of a knee joint. The diagram illustrates the zonal organisation of cartilage tissue from the articular surface to the interface with bone tissue.
The fibrillar element of cartilage extracellular matrix comprises the collagen types II, VI, IX, X and XI. Of these, the most abundant is type II collagen, it is also specific to hyaline cartilage and is therefore considered a primary indication of successful cartilage development. Type IX and type XI collagen form cross-links with type II collagen and together they form the meshwork of fibres that lend cartilage its tensile strength.
The proteoglycan content imparts to cartilage the tissues ability to resist compressive loads. Whilst cartilage holds a number of proteoglycans and non-collagenous proteins:
biglycan, decorin, tenascin, cartilage oligomeric matrix protein (COMP) being a few examples. The most abundant proteoglycan, however, is aggrecan. Structurally, aggrecan constitutes a protein core to which a series of glycosaminoglycans (GAG) are attached. The GAG content of aggrecan molecules; keratin sulphate and chondroitin sulphate, are negatively charged causing repulsion between the GAG branches and allowing interaction with water molecules (Roughley et al., 2006, Urban et al., 2000, Walsh and Lotz, 2004). Aggrecan has no covalent links to the collagen mesh and is
stabilised within the fibrillar network due to this interaction with water and the inclination of aggrecan molecules to form dense aggregates.
Type II collagen, structurally, has a high number of hexose groups which facilitate interaction with water better than the other collagen types that have been mentioned (Trelstad et al., 1976). The high propensity of type II collagen and aggrecan for association with water molecules attributes cartilage with its high hydration property. This fluid content makes up approximately 80% of the total tissue weight (Temenoff and Mikos, 2000) and plays an important role in sustaining tissue integrity and survival. As an avascular tissue, nutrient and oxygen exchange occurs mainly via diffusion through the fluid phase of cartilage, which is facilitated by the force dynamics experienced by cartilage tissue.
The low chondrocyte population and avascular nature of cartilage poses as an impairment to the regeneration of tissue injuries that are beyond a superficial nature (Temenoff and Mikos, 2000). Critical defects require the synthesis of large amounts of ECM required to bridge the defect, which is not met by chondrocyte populations.
Furthermore, there is a lack of repopulation by progenitor cells that is usually supplied by the vasculature. In addition, the nature of the ECM is such that it can act as an impediment to inherent cellular repair by resident chondrocytes (Temenoff and Mikos, 2000, Responte et al., 2007). For reasons unknown, cartilage that suffers trauma and is repaired naturally does not recover the original ECM make up, synthesising fibrocartilage which characteristically contains type I collagen, and is thus not typical of hyaline cartilage structure (Poole, 1997, Volpato et al., 2013). To this end, a suitable means of repairing or replacing damaged cartilage is an area of intense research in tissue engineering.
The unsuitability of allogenic and autogenic implants means that it is preferable to try to heal damage to cartilage instead of employing replacement therapies. Cartilage injuries can be repaired using tissue grafts. However, this means of regeneration is limited by the low number of chondrocyte cells within the graft able to facilitate integration with the native tissue (Poole, 1997, Responte et al., 2007). Therefore, the use of scaffolds is particularly attractive for engineering cartilage tissue. Cells can be populated as required within the scaffold, it also acts as a major support for the cells promoting retention of chondrocyte morphology thus discouraging dedifferentiation while encouraging cell adhesion and migration (Estes and Guilak, 2011, Ma et al., 2003, vanSusante et al., 1995). Cell types housed within the constructs themselves can be fully differentiated chondrocytes but also includes the use of stem cells encouraged to develop into chondrocytes. The use of MSCs within constructs over mature chondrocytes is thought to be particularly advantageous as MSCs exhibit the ability to produce cartilage tissue as
well as enhance development of the subchondral plate when implanted in vivo used to heal osteochondral defects (Vonschroeder et al., 1991). These results suggest that the use of MSCs enables continual development of from nạve chondrocyte through to terminal differentiation.