Relationship between bone growth rate and hypertrophic chondrocyte volume in New Zealand white rabbits of varying ages.

The dynamics of longitudinal bone growth are very complex, and potential targets for control could be any one of a number of cells or physiologic processes. Previous studies have demonstrated a linear relationship between hypertrophic cell morphology and bone growth rate in various growth plates and animals. It is unclear whether this relationship varies with age or growth plate. This study tested for significant correlations between mean terminal hypertrophic cell volume and bone growth rate as a function of age and growth plate in the New Zealand White rabbit. Three male rabbits in each of five age groups (2, 3, 5, 8, and 12 weeks old) were used to analyze growth plates from the proximal femur, proximal tibia, and proximal and distal radius. With use of tetracycline labeling and stereological techniques, bone growth rates and hypertrophic chondrocyte volumes were measured. The data were stratified by age and growth plate location and were analyzed using linear regressions. Analyses of covariance were used to test for significant differences. First, there were significant differences due to age. Linear relationships between bone growth rate and hypertrophic chondrocyte volume existed only for the older age groups (all r2 > 0.8), not for the 2 or 3-week-old groups. Also, the slope of the relationship was significantly higher in 5-week-old rabbits than in the 8 and 12-week-old groups. Second, there were significant differences between species. A comparison of the rabbit data with pig and rat data in the literature showed significant differences between all three species. Third, significant differences between growth plates were found. Although hypertrophic chondrocyte volume plays an important role in bone growth, its function may be dependent on age, species, and location.