DISCUSSION 
Lipmann: In biochemistry we have sometimes found that 
with a very simple system one may get answers to a problem 
that ranges over a very wide field. Have we any definite 
information, for example, about the mechanisms of greying 
of hair? If we knew exactly why and how the colour of hair 
fails, we might know more about ageing. The activity of the 
enzyme dopa oxidase obviously must go down or become 
blocked. 
Comfort: This loss of dopa oxidase certainly occurs at very 
different rates in different people. It also occurs in some 
follicles but not in others, for instance in genetic roaning in 
animals. White horses which lose their pigment start off fairly 
dark and whiten considerably as time goes on. It is not known 
exactly why it occurs, except that it is to some extent under 
genetic control. 
Huxley: There is a particular breed of horse which gets 
whiter as it grows older and which tends to develop pigment 
tumours (melanomas) in a large proportion of cases. 
Comfort: These are the greys, which have a different survival 
curve from any other horses. 
Medawar: I think the greying of hair is actually due to 
loss of pigment-producing cells, melanocytes, from the hair 
bulb; once they have been lost they cannot be regenerated. 
One cause of loss could be radiation, and the greying of hair 
is in fact used as a measure of the incidence of ionizing particles 
on mice in outer space! Whatever the cause may be, it is 
cumulative and irreversible. 
MacKay: ‘Thinking about machines might help our under- 
standing of one aspect of ageing. Suppose we had an artificial 
machine which was able to do intelligent “‘man-like” things. 
In principle such a machine could be everlasting, because 
repair men could plug in any part as soon as it needed replace- 
ment. The snag is that among other things we want it to store 
information; and a fault in an information store generally means 
an irreversible loss. Furthermore, a machine with a roughly- 
known life-span can be made vastly more effective for a given 
cost by embodying well-calculated compromises in the whole 
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