392 CELLS, TISSUES, AND ORGANISMS 



decrease in the rate of tibial growth. The rate of regression of cartilage 

 plate associated with sexual maturity is similar to that observed after 

 hypophysectomy (cf. Geschwind and Li, 1955). 



It would appear from the above study that estrogen elaborated at 

 the beginning of sexual maturity in the normal animal is able, by some 

 mechanism, to cause the decrease in epiphyseal activity. Exogenous 

 estrogens, as first demonstrated by Gaarenstroom and Levie (1939) 

 and subsequently by others, produce a definite decrease in the width 

 of epiphyseal cartilage. 



If the physiological amount of estrogen elaborated from the ovary 

 of the mature rat is sufficient to influence the epiphyseal cartilage, 

 ovariectomy of the animals should alter the response. Such an ap- 

 proach was tried by Geschwind and Li ( 1955 ) , utilizing animals 

 ovariectomized between the ages of 28 and 35 days. Their data, sum- 

 marized in Table II, show first the decrease in tibial epiphyseal width 



TABLE II 



Effect of Ovariectomy on Width of Proximal Epiphyseal 

 Cartilage of Tibia 



** Data adapted from Geschwind and Li, 1955. 



in normal rats that occurs with age. Eight days after ovariectomy of a 

 30-day-old rat the tibial epiphyseal width was not significantly different 

 from that of normal animals, as the 38th day of age is just prior to the 

 beginning of sexual function in the rat. The data for days 56 through 

 65 demonstrate that approximately one-half of the decline in epiphyseal 

 width that occurs with age in the normal rat can be prevented by 

 ovariectomy. Since ovariectomy does not completely abolish the change 

 occurring in normal rats, other factors also must be operative. The 

 data do, however, demonstrate the importance of ovarian function in 

 inhibiting the epiphyses— an effect probably due to the removal of 

 estrogens. 



