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.
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