Regenerative Capacity of Ovarian Tissue 51 



the sterilized ovary secretes a small amount of oestrogen even 

 when no obvious luteal cells are present, and that the threshold 

 of oestrogenic stimulation responsible for canalization of the 

 vagina is lower than that at which cyclical changes in the 

 vaginal epithelium occur. An alternative possibility is that an 

 X-irradiated ovary which is devoid of apparent luteal tissue 

 does not produce oestrogen but androgen, or some adreno- 

 cortical hormone, and that the latter is responsible for the break- 

 down of the vaginal closure membrane (see Burrows, 1949). 

 Whichever hormone it be, there is little likelihood that it 

 merely accelerates the canalization of the vagina, which would 

 otherwise take place, independently of hormonal action, as 

 development proceeds, since, according to our experience, the 

 vaginal canal remains permanently closed if rats are ovariecto- 

 mized twenty days or so before the usual time of disappearance 

 of the closure membrane. 



One further fact needs to be considered before returning to 

 the question of the dependence of the secretory function of the 

 ovary on its germinal elements. Is the secretory capacity of 

 the ovary after X-irradiation, whether slight or pronounced, 

 under the control of pituitary gonadotrophin, or is it auto- 

 nomous? The answer is not clear-cut, but inclines to the 

 latter possibility. We have found, in experiments on rabbits 

 and mice, that an ovary that has been effectively sterilized by 

 means of X-rays, and in which all oocytes and follicles have 

 been destroyed, does not undergo compensatory hypertrophy 

 when its fellow is removed (Humphreys and Zuckerman, 

 1954). This observation has been confirmed in a more exten- 

 sive series of experiments on rats (Mandl and Zuckerman, 

 1956a). These experiments have also shown that a completely 

 sterilized ovary not only fails to undergo compensatory hyper- 

 trophy, but also does not respond by an increase in size to 

 exogenous gonadotrophin. On the other hand, we also have 

 indications — they are little more than this — that the in- 

 tensity of oestrogenic secretion of the X-irradiated ovary, 

 however slight it may be, can be increased by gonadotrophic 

 stimulation. 



