What are the four zones of cartilage in diarthrodial joints?
The different molecular components of cartilage are highly organized into a structure that varies with the depth of cartilage. From top to bottom, these four zones include:
1. Superficial (tangential) zone (10%)—smallest zone (also called lamina splendens ). Collagen fibers are thin and oriented horizontally to subchondral bone. This zone has a low GAG content but is enriched with lubricin (proteoglycan 4) that is important for lubrication.
2. Middle (transitional, intermediate) zone (50%)—largest zone. Collagen fibers are thicker and start to be arranged into radial bundles. This zone has high proteoglycan and water content.
3. Deep (radial) zone (20%)—largest collagen fibers arranged radially (perpendicular) to subchondral bone. This zone contains many chondrocytes.
4. Calcified zone —separates cartilage from subchondral bone. Collagen fibers penetrate into this zone and anchor the cartilage to the bone.