THE RESPONSE OF TISSUES TO CONTINUOUS 



IRRADIATION 



L. F. LAMERTON 



Physics Department, Institute of Cancer Research, Royal Cancer Hospital, London, 



England 



INTRODUCTION 



The topic I am going to discuss very briefly is the response of the rat gut and 

 bone-marrow to continuous irradiation, with particular reference to the 

 extent of maintenance of cell population and function. This topic perhaps 

 does not fall directly within the subject matter of this session, but is certainly 

 very closely related to it. 



There are great diiferences in the response of the various renewal systems 

 of the body to continuous irradiation. From the point of view of maintenance 

 of function the extremes, in the rat, may be represented by the epithelium of 

 the small intestine (and possibly of other parts of the gut) which can main- 

 tain function under a dose-rate of at least 400 rads/day until death supervenes 

 from other causes, and the testis, which shows progressive cellular depletion 

 at doses of a few rads per day. 



RESPONSE OF THE EPITHELIUM OF THE SMALL INTESTINE 



Work on the epithelium of the small intestine of the young rat, 6 to 8 

 weeks old (Quastler et al., 1959; Lamerton et al., 1961) shows that at a dose- 

 rate of 415 rads/day (from a ^^'^Cs source) the mitotic index in the crypts falls 

 during the first day or so and then rises to a value about 60% of normal. At 

 the same time the cell population in the crypts drops to about one-half of 

 normal at which value it remains steady until death occurs from bone- 

 marrow failure (see Fig. 1). One would expect that this apparent adaptation 

 was brought about by an increase in the mean proliferation rate of the crypt 

 cells, as a homeostatic response to cell death. However, measurements using 

 tritiated thymidine labelling, with the method of percentage-labelled mitoses 

 (Lamerton et al., 1962) indicate no substantial difference either in minimum, 

 or mean, generation tune in the cr}'pts of control rats, or of rats which have 

 been exposed for 5 days at 415 rads/day. Another possibility which must be 

 considered is that the crypt cells, for some reason, develop an increased 

 radio-resistance — a suggestion that was made some time ago by Margaret 

 Bloom (1950), when studying the effect of fractionated exposure on the small 



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