A tetracycline study of cyclic longitudinal bone growth in the female rat.

This study shows that bone growth in the young female rat is cyclic. Using tetracycline labeling, measurements were taken for every possible 2-day growth interval during the 4-day estrous cycle. The slow phase of bone growth occurs when estrogen and glucocorticoid serum levels are elevated, whereas the rapid phase of growth occurs when estrogen serum levels are minimal. Parathyroidectomy or 1-microgram injections of exogeneous 17 beta-estradiol retard the rapid phase of growth. Adrenalectomy alters the rate of growth in that the slow phase no longer occurs but rather increases to a rate equal to the rapid phase. The results show that parathyroid hormone is an essential component of the cyclic growth pattern in the young adult female rat and that estrogen, which reduces the rate of longitudinal bone growth, has its effect primarily through its stimulation of glucocorticoid production. These findings are discussed in relation to other recent studies which show that there are no estrogen receptors in bone and that high levels of estrogen stimulate 1,25-dihydroxyvitamin D3 production in the kidney. A hypothetical model, interrelating these hormones to the cyclic longitudinal bone growth pattern found in the female rat, is proposed.