Regenerative Capacity of Ovarian Tissue 49 



recurrent oestrous phases in the host animal for periods up to 

 a year (Parkes, 1956). Similarly, mice which have been steri- 

 lized by X-irradiation, and whose ovaries are devoid of oocytes, 

 may still continue to experience recurrent phases of oestrus for 

 periods up to 69 days (Parkes, 1927). On the other hand, it 

 is a matter of common observation that the postmenopausal 

 human ovary, when depleted of its oocytes, ceases to secrete 

 sex hormone. And it is also believed that it cannot do so, 

 even when subjected to the influence of gonadotropin (see 

 Burrows, 1949). Because of the apparent conflict between 

 these two sets of observations, the whole subject has recently 

 been re-investigated in my own laboratory. 



The first point which we (Mandl and Zuckerman, 1956a, b) 

 re-investigated was the observation that a mouse ovary that 

 has been depleted by means of X-irradiation of all oocytes 

 and of all recognizable follicular elements may still produce 

 sufficient oestrogen to induce vaginal cornification and even 

 recurrent oestrous cycles (Parkes, 1926, 1927; Westman, 1930). 

 Our technique differed from the earlier experiments in so 

 far as we irradiated the ovaries not through the body wall, 

 but directly. Only ovaries which were found on serial section 

 to be absolutely devoid of oocytes were regarded as sterile. 

 According to our observations on rats, the usual pattern of 

 oestrogenic activity after successful irradiation (as judged by 

 the criterion of a total lack of oocytes) is that short periods of 

 vaginal cornification occur at increasingly irregular intervals, 

 and that after a minimum of five and usually within 40 days, 

 the vaginal smear becomes continuously cornified. The period 

 of cornification lasts from 2 to 14 weeks, and during this time 

 the animals, in our experience, will not mate. The vaginal 

 smear then becomes anoestrous in type. The behaviour 

 of the X-ray sterilized mouse also follows this pattern. 

 Signs of oestrogenic stimulation are not confined to the 

 vaginal epithelium, but can also be observed in the uterus, 

 which while smaller than normal, remains significantly larger 

 than in spayed control rats as long as 26 weeks after treat- 

 ment. On the other hand, changes occur in the weight of 



