Articular Cartilage

Articular cartilage, also called hyaline cartilage, is the smooth, glistening white tissue that covers the surface of all the diarthrodial joints in the human body. As its name implies, articular cartilage is critical in the movement of one bone against another. Articular cartilage has an incredibly low coefficient of friction which, coupled with its ability to bear very large compressive loads, makes it ideally suited for placement in joints, such as the knee and hip.

Articular cartilage is not a homogeneous tissue. Instead, it has a very complex composition and architecture that permits it to achieve and maintain proper biomechanical function over the majority of a human lifespan. Articular cartilage is composed mainly of water (70-80% by wet weight). The solid phase of articular cartilage consists primarily of type II collagen and aggrecan, a chondroitin and keratan sulfate proteoglycan. Collagen forms a network of fibrils, which resist the swelling pressure generated by the proteoglycans. Aggrecan, because of its tendency to noncovalently interact with hyaluronic acid, forms huge aggregates that become trapped in the collagen network. Because of their numerous negatively charged sulfate groups, these proteoglycan aggregates attract cations, which in turn bring in water to minimize differences in osmotic pressure. Thus, type II collagen and proteoglycans create a swollen, hydrated tissue that resists compression.

The distribution and arrangement of these components, however, is not uniform. Instead, articular cartilage is divided into four zones: superficial, middle, deep, and calcified. The superficial zone is characterized by flattened chondrocytes, relatively low quantities of proteoglycan, and high quantities of collagen fibrils arranged parallel to the articular surface. The middle zone, in contrast, has round chondrocytes, the highest level of proteoglycan among the four zones, and a random arrangement of collagen. The deep zone is characterized by collagen fibrils that are perpendicular to the underlying bone, and columns of chondrocytes arrayed along the axis of fibril orientation. The calcified zone is partly mineralized, and acts as the transition between cartilage and the underlying subchondral bone.

With this knowledge in hand, our lab hypothesizes that the zonal arrangement of articular cartilage is a critical component in providing its functional capabilities. We believe that it is crucial to recapitulate this architecture by using phenotypically distinct cells from each zone, and exposing these cells to biomechanical forces appropriate to their zone. We are also interested in the role of adhesion peptides and growth factors in modulating the regenerative potential of chondrocytes. We believe that a multifactorial approach, incorporating scaffolds modified with peptides and growth factors, zonal chondrocytes, and biomechanical forces, will provide us with the best opportunity to engineer functional articular cartilage.

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