DIFFERENTIAL GROWTH BY GROWTH PLATES AS A FUNCTION OF MULTIPLE PARAMETERS OF CHONDROCYTIC KINETICS

Differential elongation of growth plates is the process by which growt h-plate chondrocytes translate the same sequence of gene regulation in to the appropriate timing pattern for a given rate of elongation. Whil e some of the parameters associated with differential growth are known , the purpose of this study was to test the hypothesis that eight inde pendent variables are involved. We tested this hypothesis by consideri ng four different growth plates in 28-day-old Long-Evans rats. Tempora l parameters were provided by means of oxytetracycline and bromodeoxyu ridine labeling techniques. Stereological parameters were measured wit h standard techniques. For all four growth plates, the calculated numb er of new chondrocytes produced per day approximated the number of cho ndrocytes lost per day at the chondro-osseous junction. This suggests that the proposed equations and associated variables represent a compr ehensive set of variables defining differential growth. In absolute nu mbers, the proximal tibial growth plate produced about four times as m any chondrocytes per day as the proximal radial growth plate (16,400 c ompared with 3,700). In the proximal tibia, 9% of growth is contribute d by cellular division; 32%, by matrix synthesis throughout the growth plate; and 59%, by chondrocytic enlargement during hypertrophy. In th e more slowly elongating growth plates, the relative contribution to e longation from cellular enlargement decreases from 59 to 44%,with a re lative increase in contribution from matrix synthesis ranging from 32% in the proximal tibia to 49% in the proximal radius. This study sugge sts that differential growth is best depicted as a complex interplay a mong cellular division, matrix synthesis, and cellular enlargement dur ing hypertrophy. Differential growth is best explained by considering a set of eight independent variables, seven of which vary from growth plate to growth plate. Thus, this study confirms the importance of cel lular hypertrophy during elongation and adds to our understanding of t he importance of locally mediated regulatory systems controlling growt h-plate activity.