THE DOMAIN OF HYPERTROPHIC CHONDROCYTES IN GROWTH PLATES GROWING AT DIFFERENT RATES

In this study, we tested the hypotheses that (a) both the domain volum e (volume of the cell and the matrix it has formed) and matrix volume of juxtametaphyseal hypertrophic chondrocytes in the growth plate is t ightly controlled, and that (b) the domain volume of juxtametaphyseal hypertrophic chondrocytes is a strong determinant of the rate of bone length growth. We analyzed the rate of bone length growth (oxytetracyc line labeling techniques) and nine stereologic and kinetic parameters related to the juxtametaphyseal chondrocytic domain in the proximal an d distal radial and tibial growth plates of 21- and 35-day-old rats. T he domain volume increased with increasing growth rates, independent o f the location of the growth plate and the age of the animal. Within a ge groups, the matrix volume per cell increased with increasing growth rates, but an identical growth plate had the same matrix volume per c ell in 21- and 35-day-old rats. The most suitable regression model (R- 2 = 0.992) to describe the rate of bone length growth included the mea n volume of juxtametaphyseal hypertrophic chondrocytes and the mean ra te of cell loss/cell proliferation. This relationship was independent of the location of the growth plate and the age of the animal. The dat a suggest that the domain volume of juxtametaphyseal hypertrophic chon drocytes, as well as the matrix volume produced per cell, may be tight ly regulated. In addition, the volume of juxtametaphyseal hypertrophic chondrocytes and the rate of cell loss/rate of cell proliferation may play the most important role in the determination of the rate of bone length growth.