Zone I of normal young adult human articular cartilage is a highly modified layer consisting of tangentially oriented bundles of closely spaced unit collagen fibers 320 Å in diameter with characteristic 640 Å periodicity, separated by little or no intervening amorphous ground substance. The surface cells in Zone I show a relatively small cytoplasmic volume, a poorly developed granular endoplasmic reticulum, and a small Golgi complex. Degenerative changes are infrequently observed, and no more often than in the chondrocytes of other zones. The surface cells appear to be a relatively dormant or quiescent cell population in which protein synthesis is reduced and metabolic activity is at a low level.

Beneath Zone I, collagen fiber bundles disappear and are replaced by randomly oriented, widely spaced, individual collagen fibers with characteristic periodicity and of increasing diameters. In the matrix separating unit collagen fibers there runs a delicate network of smaller fibers which lack the characteristic periodicity of collagen. Large numbers of these smaller fibers are distributed throughout the matrix immediately surrounding chondrocytes. The chondrocytes of Zones II and III show a relatively large cytoplasmic volume, and elaborately developed granular endoplasmic reticulum and an extensive Golgi complex, characteristic of cells actively engaged in protein synthesis.