Discussion 127 



cycling apparently normally, you can find absolutely no form of oocyte 

 throughout the whole serial sections of the entire animal's ovaries. 



Parkes: Dr. Krohn, you are suggesting that the ovary will continue 

 to produce oestrogen, or cyclically to produce some semblance of an 

 oestrus cycle for many months after the last egg has disappeared. 



Krohn: I am indeed, exactly. 



Parkes: I have been reading a number of papers which imply that 

 the disappearance of eggs from the ovary, as by radiation, necessarily 

 means that the cycles would stop fairly soon. 



Krohn: The situation is quite different when you irradiate the ovary. 

 I am referring here to work by Mandl and Zuckerman (1956, J. Endocrin., 

 13, 243). Following irradiation of the ovary you can destroy all the 

 oocytes very rapidly. Such animals go on having irregular oestrus 

 cycles for up to 6 weeks and then go into constant vaginal oestrus for 

 as long as 100 days. What you are doing in irradiation is damaging every 

 component of the ovary and you are destroying all those tissues which 

 could go on secreting oestrogen for very long. Once the lifespan of 

 anything that you have left behind after irradiation is over, in 60 

 days or so, then there is nothing left from the ovary to go on producing 

 oestrogen ; and I imagine that in a very old mouse you would get the 

 same situation : first of all there are no ova left, then there is no luteal 

 tissue or granulosa cell tissue, or whatever it is, left and finally there is 

 nothing left and you would find permanent dioestrus or anoestrus. 



Parkes: What is your proof that, after irradiation with a moderate 

 dosage, anything would have been destroyed except the eggs ? 



Krohn: I have no proof of that. That is what I would presume has 

 happened. I cannot believe that the dose of radiation can be so finely 

 adjusted, and certainly no attempt has been made to adjust it finely, 

 so that it would just kill the oocytes and not kill any of the other tissue 

 in the ovary. 



Parkes: By using a strict dosage you can avoid damaging almost any 

 other cellular bodies. 



Krohn: Dr. Miihlbock, when you are doing your breeding to get an 

 inbred strain I think it is important that your choice of breeding pairs 

 in each generation should be at random. In selecting pairs from one 

 generation to another one often says "That is a good-looking mouse, and 

 that one is, and we shall take those as our breeding pair"; one is then, 

 in effect, selecting residual heterozygosity and it will take far longer to 

 get homozygosity than just taking a pair entirely at random. I believe 

 that the fact that the strain was not inbred sufficiently has proved to 

 be an important factor in some work on skin grafting. 



Miihlbock: Yes, I agree, but pregnancy is another factor in selectivity. 

 All our strains are inbred for one hundred generations or more and so we 

 are relatively sure that we have really pure strains. I would like to 

 stress a point that most people do not realize : there must be no relaxation 

 of inbreeding, that is an essential point. 



Best: You use parabiosis as a test for purity of strain? With what 

 do you correlate the ability of the animal to live in the parabiotic state ? 

 What do you look at in the animals ? 



