A Tetracycline Study of Cyclic Longitudinal Bone Growth in the Female Rat*

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Abstract
Bone growth in the young adult 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 occurred when estrogen and glucocorticoid serum levels were elevated, whereas the rapid phase of growth occurred when estrogen serum levels were minimal. Parathyroidectomy or 1 .mu.g injections of exogeneous 17.beta.-estradiol retard the rapid phase of growth. Adrenalectomy altered the rate of growth in that the slow phase no longer occurred but rather increased to a rate equal to the rapid phase. Parathyroid hormone is an essential component of the cyclic growth pattern in the young adult female rat and 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.