Cartilage is an unusual tissue in that it is avascular, alymphatic, and aneural. It consists mostly of extracellular matrix in which chondrocytes are dispersed at low densities on the order of 1 million cells/cc (whereas most tissues contain several hundred million cells/cc).

FIGURE 6.6 Encapsulation of islets in semi porous plastic is one promising way to protect them from attack by the immune system.

Chondrocytes can be cultured ex vivo to increase their numbers by up to tenfold. Using this technique, deep cartilage defects in the knee can be treated by autologous cell transplantation. In this case, a biopsy is collected from the patient’s knee outside the affected area (Figure 6.7) and the chondrocytes are liberated from the matrix by an enzymatic treatment. The cells are allowed to grow in a two-stage cell ex vivo culture process to increase their number. The cells are then harvested and reintroduced into the affected area within the knee joint. This type of therapy has been shown to aid in healing of cartilage defects and is the basis of the Carticel W cell-therapy product (Genzyme Inc.). Numerous patients are afflicted with knee problems that effectively leave them immobile. It is estimated that over 200,000 patients are candidates for this type of cellular therapy annually in the United States alone.