Millions of Americans suffer from articular cartilage injuries every year. Unfortunately cartilage has very poor regenerative capability. Researchers and medical professionals therefore rely on various surgical procedures to relieve symptoms and restore function to joints. Unfortunately these procedures have less than satisfactory results, hence the need for better methods such as tissue engineering.
Articular cartilage has a fluid phase, consisting of chondrocytes and on extracellular matrix. The extracellular matrix is composed of collagen, proteoglycans, and noncollagenous proteins.
Articular cartilage has varying organization throughout. It is organized according to its distance from chondrocytes and its distance from the surface of cartilage.
Immediately surrounding the chondrocytes is the pericellular region composed primarily of proteoglycans. This region provides some cushioning for the cells. Surrounding this is the territorial region composed of collagen. The collagen acts as a sort of barrier preventing the chondrocytes from being damaged or significantly deformed. Everything outside of this comprises the intercellular region.
The intercellular region is divided into four zones from the surface down. Immediately on the surface and the collagen is also parallel to the surface. Below this is the transitional layer in which the chondrocytes are more spheroidal and collagen is randomly oriented. Next is the radial zone. The chondrocytes in this layer are stacked in a columnar type fashion and the collagen is oriented perpendicular to the surface. The lowest zone of cartilage is the calcified cartilage zone. This zone is the transition between cartilage and bone and it is in this layer that collagen inserts itself into bone.
Before chondrocytes become mature they produce the extracellular matrix in cartilage. In vitro studies have shown that chondrocytes will produce matrix in varying amounts depending on the mechanical forces it experiences. In vivo, chondrocytes experience different mechanical forces depending on where it is located within the cartilage, i.e. in which zone it is located in. Currently researchers are trying to understand how to replicate the different zones found in natural cartilage.
Chondrocytes are also affected by the amount of diffusion occurring in its local environment. Studies have shown that stirring of fluid in which cartilage is being grown (in vitro) increased the amount of ECM produced by chondrocytes.
Natural articular cartilage is composed primarily of type II collagen and some type I collagen. Type I collagen is found primarily in fibrocartilage in skin, bone, etc., whereas type II collagen is very characteristic of hyaline cartilage.
Proteoglycans are an extremely important component of articular cartilage. It is composed of a core protein with multiple sulfated glycosaminoglycans bound to it. This structure is then attached to hyaluronic acid via link proteins. The glycosaminoglycan groups are...