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Figure 1. Organization of the mammalian growth plate. The growth plate is classically divided into four major zones. The approximate extent of each of these regions is shown in this longitudinal section of a rat growth plate. The most superficial zone, resting cartilage, contains chondroprogenitor cells. These cells serve as the stem chondrocytes or chondroblasts of the growth plate. In the proliferative cartilage zone, chondrocytes divide rapidly in a direction that is parallel to the long axis of the bone, thereby providing longitudinal appositional growth. In the hypertrophic cartilage zone, chondrocytes cease to divide and begin to increase their intracellular volume, therefore achieving some interstitial growth. Toward the bottom of the growth plate, the hypertrophic chondrocytes induce calcification of the extracellular matrix. It is in this zone that apoptotic cells are evident.
