DEATH RATE AFTER IONIZING RADIATION 295 



berenbltjm: Would it be fair to put it this way, tliat with ageing a whole number of 

 processes, all of them lethal in the end, are likely to kUI, but only one of them will kill 

 in one animal. The earliest one is the cause of death in that animal. When you then have 

 a form of stress lilie X-rays, you may then introduce one of the others. Obviously as an 

 index it is hopeless because it is a summation of all sorts of things; they all contribute to 

 it, but only one of them in any one animal is the real cause. Isn't that it? 

 mole: Oh yes, but I think if you take post-mortems seriously you will find it very 

 difficult not to say that there are a lot of contributory lesions. 



muller: I thuik just because mortality is influenced by so many things, aU of which 

 show an increase with age, or practically all of which, maybe I was wrong about accident 

 proneness, that such a multiplicity makes for better criteria. 



BACQ: I want to take up the point that Prof. Berenblum raised before. One should take 

 as a test of ageing some kind of continuous, biochemical process known to appear in a 

 practically continuous way. Something not discontinuous such as death or the appearance 

 of an infection or sterility or an accident or things of that kind. So far as I know there are 

 two tests and one is calcification of cartilage. Then there is another one which is not so 

 regular, and that is the cholesterol content of the arteries, also a generally rather con- 

 tinuous process. I am quite certain that if we could get the turnover of these macromo- 

 lecules we spoke of this morning, these mucopolysaccharides and so on, we should also 

 fiiid that this represents a continuous process. If we could investigate this kind of 

 process we might get a better idea of what happens. 



UPTON: I would sympatliize with tliis approach too, but it seems to me that when we 

 talk about senescence we really are talking about a jjrocess which is not a constant 

 function of time. When we plot survivorship against age, we don't refer to random loss 

 of individuals. We refer instead to a type of mortahty which is relatively abrupt in its 

 onset late in life, and I would wonder if necessarily any of these functions wUl be found to 

 have mechanisms in common with mechanisms that delimit the life-span and cause the 

 evolution of tumours and so on. This is not to argue that we shouldn't look at aU age- 

 dependent changes, because a time-dependent change which is constant m rate may 

 eventually be found to be related to senescence. They may be strongly correlated but 

 then again they may not be. 



BACQ: But even if they are not correlated they may be more characteristic of the process 

 of slow senescence than any accidents occurring at the end. 



ALEXANDER: The mere fact that the death curve rises like this doesn't mean that it is the 

 result of progressive change, because this change will be a tln-eshold phenomenon, and 

 in fact the animal doesn't die until the damage has reached a certain level. The interest- 

 ing thing about radiation ageing is that in the few cases where tests have been made there 

 has been absolutely no effect of radiation; calcium deposition has already been referred 

 to and then of course the mucopolysaccharides to which you referred have been studied 

 rather indirectly. The strength of the rat tail tendon increases with age. This is almost 

 certainly due to the fact that the amount of mucopolysaccharide decreases as the amount 

 of coUagen increases; it has something to do with cross-linking too, if you cross-link you 

 don't get this affect at aU. The progressive'strengthening of that rat taU tendon parallels in 

 fact a progressive fall in turnover of mucopolysaccharides. Now the uiteresting thing is that 

 progressive strengthening of rat tail tendon is not accelerated m the slightest by radiation. 

 berenblum: I don't think I should really participate in this discussion at all because I 

 have never worked in the field and I merely want to give my impression as an onlooker. 

 I must say that the earlier discussions struck me as being somewhat unreal, and unreal in 

 this sense that what we were interested in was ageing, what we were talkmg about was 

 death rates, and the reason for that, I suggest, is that we haven't got a means of studying 



