[3H]thymidine uptake in cells of rat condylar cartilage

Weanling rats were injected intraperitoneally with [3H]thymidine and sacrificed from 5 min to 20 days later. Their mandibular condylar cartilages were examined histologically, by thin-layer autoradiography, and by using liquid scintillation and microscopic counting methods. Labeled DNA appeared in some of the chondrocytes of the resting zone as early as 10 min postinjection, and reached the proliferative zone by 24 hr and the hypertrophic zone by 4 days. The labeling pattern in the last zone was more disperse, being oriented toward the periphery of the cells as they became hypertrophic. The maximum number of labeled chondrocytes was reached by 2 hr postinjection. These amounted to approximately 11% of the total chondrocyte population, the majority of which were located in the resting zone (73%). It is concluded that, over this period, the mitotic index for these cells is 50-60 per thousand resulting in approximately 100 labeled chondrocytes. In addition, some of the chondroclasts at the erosion front contained labeled DNA as early as 5 min after [3H]thymidine administration. By 10 min, 65% of these cells exhibited one or more labeled nuclei, and the ratio of labeled cells remained high through 20 days. Chondroclasts were seen to contain a diffuse label within their cytoplasm after 5 days. This label was similar to that seen in hypertrophic chondrocytes that had reached the erosion front by that time. Clearly, chondroclasts exhibit nuclear division and do not form from fusion of hypertrophic chondrocytes, although which specific mononuclear cells may act as chondroclast progenitors is not clear. In addition, these multinucleate resorbing cells are capable of ingesting or phagocytizing nuclear remnants from hypertrophic chondrocytes at the eroding face of cartilage.