EFFECTS OF TESTOSTERONE ON SKELETAL GROWTH IN LAMBS AS ASSESSED BY LABELING INDEX OF CHONDROCYTES IN THE METACARPAL BONE-GROWTH PLATE

The effects of testosterone on the epiphyseal growth plate of metacarp al bones of growing sheep were evaluated in 20 rams, 20 wethers, and 2 0 wethers receiving subcutaneous testosterone replacement therapy. Two animals from each testosterone treatment group were slaughtered at 14 -d intervals from 49 to 133 d, and then at 28-d intervals until 217 d, for a total of 10 slaughter ages. Immediately after slaughter, the ca nnon bones were dissected of extraneous tissue, weighed, and their len gths measured. Growth plates from the metacarpal bones were isolated a nd explants were cultured for 24 h in medium containing [H-3]thymidine . After autoradiography, labeling index was calculated as the ratio of labeled to total nuclei in the resting and proliferative zones of the growth plate. Testosterone increased (P < .03) weight and length of t he metacarpal bone. Increased bone length due to testosterone was asso ciated, in part, with a higher (P < .05) labeling index in chondrocyte s of the proliferative zone of the growth plate. Labeling indices in t he resting zone chondrocytes of rams were higher near the time of pube rty. Accelerated growth followed by cessation of growth occurs concurr ently with puberty in males of several species and is accompanied by a n increase in the blood concentration of testosterone. Testosterone ma y mediate this accelerated growth by first increasing bone growth and then depleting the source of stem cells in the cartilage growth plate, the site where growth in length of long bones occurs.