Cell proliferation within the growth plate of the tich mouse

The pattern of chondrocyte proliferation was studied in the proximal tibial growth plate of tich mice (gene symbol tch), a recessive mouse mutant, which is coisogenic with the A.TL strain. Specimens were qualitatively studied at time points of 2, 3, 4, 6, and 8 weeks of age by bromodeoxyuridine labeling of cell division and routine histology. At 2 weeks, when the lesion appeared as a full-width thickening of the growth plate, a greater proportion of cells appeared positively labeled in the proliferative zone. This concavity in the central portion of the growth plate became progressively more focal between 3 and 4 weeks to give a "tongue" of unresorbed, noncalcified cartilage in the central region of the tich growth plate. BrdUrd labeling indicated that the appearance of the cartilage tongue corresponded with increased cell division in the central region of the growth plate. At the same time, a "second" zone of cell division formed, within the zone of hypertrophy, such that labeled cells appeared to be set among chondrocytes with hypertrophic morphology. At stages after 4 weeks of age the focal feature disappeared as the growth plate returned to more normal morphology by maturity. It seems that this unique "second" zone of dividing cells may contribute to formation of an elongation of the nonresorbed tongue of cartilage. However, it is not likely to be the primary defect since growth plate changes were apparent at earlier stages.