THE EFFECT OF SINGLE AND FRACTIONATED DOSES OF 

 IRRADIATION ON THE HAEMATOPOIETIC SYSTEMS OF 



C57BL MICE 



P. C. KOLLER AND VALERIE WALLIS 

 Chester Beatty Research Institute, Institute of Cancer Research, London, England 



INTRODUCTION 



Kaplan and his co-workers (1953a) have shown that in 85 to 95% of mice of 

 C57BL strain, lymphoid leukaemia develops in the thymus 50 to 100 days 

 after periodic total-body irradiation. Evidence has been obtained by these 

 investigators and others (cf. Miller, 1961) showing that the target-organ is the 

 thymus, and that the haematoj^oietic tissue is also involved in the leukaemo- 

 genic process. When isogenic bone-marrow cell suspension is injected into 

 irradiated C75BL mice, the development of leukaemia can be prevented 

 (Kaplan et al, 1953b; Miller, 1961). 



One of our aims is to study the nature of this haematopoietic "principle" 

 and its mode of operation. The present report deals with the single and cumu- 

 lative effects of X-irradiation on the haematopoietic system of C57BL mice, 

 durmg the pre-leukaemic period. 



METHODS 



28 to 30-day-old C57BL mice (males and females) were irradiated with 

 four, weekly doses of 180 r (total-body) at a dose-rate of 60 r/min. The mice 

 were divided into four groups, one received only one dose of 180 r, another 

 two, a third group three, and the fourth, four doses of 180 r. Mice were 

 sacrificed 1 and 6 days after each irradiation. Peripheral blood samples were 

 taken and total white-cell counts were made on all mice sacrificed. For the 

 study of radiation-induced cell injuries, bone-marrow material was prepared. 

 Mice were injected with colcemid (dose: 2% of the body weight of an 0-02% 

 solution) intraperitoneally and sacrificed 60 minutes after injection. Bone- 

 marrow cells in metaphase were divided into two classes: normal and 

 abnormal the latter included cells which either showed chromosome frag- 

 ments or contained more than the normal diploid number of chromosomes 

 (40). By scoring cells in metaphase, not all radiation injuries can be detected, 

 and the figures obtained must be considered as an underestimate. 



35 



