L. F. LAMERTON AND E. H. BELCHER 



(Lamerton, Elson, Harriss and Christensen^). Figure 1 is a typical 

 example of weight and blood changes in a young rat (colony-bred albino 

 strain) given 450 r whole body X-irradiation. The weight drop immedi- 

 ately following irradiation gives a measure of the severity of the initial phase 

 of the radiation response, while the second or ' acute ' phase of the response 

 is characterized by an anaemia and a second weight drop. Whether or 

 not a second weight drop occurs, and its severity when it occurs, is correlated 

 with the severity of the post-irradiation anaemia. In the case of the young 

 growinff rat, we have found the anaemia to be uncomplicated by the infec- 

 tion that a number of workers have reported m various species. Cultures 

 have been made of heart blood from a number of rats in the second phase 

 of the radiation response, and have been negative. 



It is probable that the initial weight drop of the rat is a manifestation of 

 the ' radiation sickness ' which is observed clinically, of which the origin is 

 so much in doubt. The work with rats suggests that important factors in 

 the production of the initial weight drop are the products of damaged tissue 

 (probably lymphoid tissue), together with direct radiation damage to the 

 gut, but the type of study described by Rajewsky in the present Conference 

 may throw much more light on the processes at work. 



So far as the second phase of the radiation response is concerned, the 

 important factor appears to be the severity of the post-irradiation anaemia, 

 which is not the result of gross local bleeding, but of a generalized haemor- 

 rhage characterized by a considerable leakage of red cells into the lymph. 

 We have attempted to distinguish the two factors controlling the severity of 

 the radiation-induced anaemia — the mechanisms responsible for the bleed- 

 ing, and the capacity of the animal for compensatory erythropoiesis. 



A series of studies have been made on platelet changes following irradiation 

 (Lamerton and Baxter-), and the findings are in agreement with those of 

 Cronkite and other workers that the platelet fall is a very important and 

 possibly the essential factor in initiating the severe bleeding. There is 

 evidence from these studies, though not conclusive, that there is a critical 

 platelet count of about 20,000 to 30,000 per mm^ below which it is necessary 

 for the blood platelet level to fall before a very severe bleeding and anaemia 

 is produced. 



With regard to the capacity of the animal for compensatory erythropoiesis, 

 measurement of the radioactive iron turnover in various tissues has proved 

 to be a valuable technique for a quantitative determination of the pattern 

 of erythropoietic activity in the animal (Lamerton, Belcher and Har- 

 Riss^ ; Baxter, Belcher, Harriss and Lamerton^ ; Belcher, Gh.bert 

 and Lamerton^). An example of an application of this technique is 

 shown below. 



the effect of shielding procedures 



At the 1953 Radiobiology Conference in Aarhus we reported on some 

 results of head shielding during radiation exposure. Such a procedure 

 does not affect appreciably the extent of the initial weight loss, but reduces 

 very considerably the severity of the radiation-induced anaemia. These 

 investigations have been continued (Lamerton and Baxter^) and it has been 

 found that hind-leg shielding is just as effective as head shielding. Even 



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