Fig. 2

Morphology of the cartilage tissue. The superficial zone consists of a high concentration of collagen fibers parallel to the articular surface. This layer is rich in type II collagen and contains small amounts of type I collagen and proteoglycans. The middle zone is the thickest cartilage layer and accounts for 40–60% of articular cartilage volume. Collagen fibers in this area are thicker and contain higher levels of proteoglycans. The deep region has the highest concentration of proteoglycans where chondrocytes and collagen fibers are arranged in vertical columns perpendicular to the surface.. In areas of calcification, a proteoglycan-free matrix surrounds round chondrocytes with hypertrophic phenotypes