The Structure of Articular Cartilage

Articular cartilage consists of relatively small numbers of chondrocytes in a matrix of proteoglycans compressed by and trapped within a collagen network. The negatively charged side chains of proteoglycans create an osmotic gradient across the surface of cartilage and draw water into the tissue (the water content of cartilage is between 70%–80%). The collagen fibers provide tensile resistance to the swelling caused by the water and anchor the cartilage to the underling bone.
Articular cartilage is organized as a multilayered structure made up of four zones. The most superficial zone contains ellipsoid chondrocytes and collagen fibers oriented parallel to the articular surface. Deep to this is a transitional zone made up of collagen fibers arranged in a more random fashion. The next layer is the radial zone, made up of large collagen fibers and cells arranged in columns. The radial zone has the highest proteoglycan content and the lowest water content. Finally, there is the calcified zone, which separates the articular cartilage from the subchondral bone. The tidemark is the boundary between the radial and calcified zones.

References

• Goodwin DW. MRI appearance of normal articular cartilage. Magn Reson Imaging Clin N Am. 2011 May;19(2):215-27.
• Wong M, Carter DR. Articular cartilage functional histomorphology and mechanobiology: a research perspective. Bone. 2003 Jul;33(1):1-13.