Hypertrophic chondrocyte volume and growth rates in avian growth plates

Abstract

In growing mammals there is a positive linear relationship between the mean hypertrophic chondrocyte volume and the rate of bone elongation. This suggests that the control of chondrocytic volume in the growth plate, is a major determinant in controlling bone elongation in mammals. In the present study the existence of such a relationship was tested for in birds. A scheme of fluorochrome labelling was devised to enable direct measurement of bone elongation per unit time. Four weight-bearing growth plates from two-week-old mallard ducklings and the corresponding four growth plates from two-week-old leghorn chicks were studied. Growth plate cartilage was fixed in the presence of ruthenium hexamine trichloride and embedded in Epon araldite. Estimates of mean cell volume, v (chondr) , and mean cubic intercept ( l 3 ) were calculated by applying the stereological relationship: v (chondr)=(π/3) × ( l 3 ). Regression analysis revealed a positive linear relationship between the two parameters, rate of bone elongation and mean hypertrophic cell volume in both species (squared correlation statistics: 65 per cent for mallards, 54 per cent for leghorns). There was a wide range in rates of bone elongation among growth plates studied (318 to 1418 μm 24 h −1 for mallards, 77 to 445 μm 24 h −1 for leghorns) and compared to mammals (such as rabbits, rats, swine and dogs), a small range in mean cell volume (2709 to 4786 μm 3 for mallards, 3663 to 5719 μm 3 for leghorns). Although there was a positive linear correlation between these two parameters, because of the small range of mean hypertrophic cell volumes, it is important to consider additional factors, other than cellular hypertrophy, that might also be related to avian long bone growth.