CELL-PROLIFERATION WITHIN THE GROWTH-PLATE OF THE TICH MOUSE

The pattern of chondrocyte proliferation was studied in the proximal t ibial growth plate of tich mice (gene symbol tch), a recessive mouse m utant, which is coisogenic with the A,TL strain, Specimens were qualit atively studied at time points of 2, 3, 4, 6, and 8 weeks of age by br omodeoxyuridine labeling of cell division and routine histology, At 2 weeks, when the lesion appeared as a full-width thickening of the grow th 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 a ppearance of the cartilage tongue corresponded with increased cell div ision 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 h ypertrophic morphology, At stages after 4 weeks of age the focal featu re disappeared as the growth plate returned to more normal morphology by maturity, It seems that this unique ''second'' zone of dividing cel ls may contribute to formation of an elongation of the nonresorbed ton gue of cartilage, However, it is not likely to be the primary defect s ince growth plate changes were apparent at earlier stages.