42 I. BERENBLUM AND N. TRAININ 



combined action is not additive, is shown by the fact that tumours develop 

 rapidly and in large numbers when the promoting action folloivs the initiatmg 

 action, but not in reverse (Berenblum and Haran, 1955). Urethane also acts 

 as initiator for mouse skin when it is administered systemically (Haran and 

 Berenblum, 1956); croton oil acts as promoting agent only when applied 

 topically. A complete carcinogen, such as 3.4-benzpyrene of 9, 10-dimethyl- 

 1.2-benzanthracene, applied rejjeatedly to the skin, naturally jDossesses both 

 initiatmg and promoting properties; however, when applied a single time, it 

 usually acts as initiator only. 



Initiating action is very rapid; promoting action, very slow. The initiating 

 process appears to be irreversible, the same number of tumours arising from 

 subsequent promoting action, whether the latter is given immediately 

 following the initiating stimulus or only after an interval of several months 

 (Berenblum and Shubilv, 1947a). The number of tumours resulting from the 

 two-stage process is a function of the concentration of the initiating agent; the 

 speed of their aj^pearance is dependent more on the efficiency of the jy^omoting 

 action (Berenblum and Shubik, 1947a, b). 



The nature of initiating action is not known, though a mutation-lilvc 

 change could explain the quantitative relationships observed, and would be 

 consistent with the rapid action. There is no proof, however, that the change 

 is "mutational" in the genetic sense. Promotion, on the other hand, operates 

 very differently: it is not only slow in action but, in its early stages, apparently 

 reversible. 



The most plausible explanation to account for the results obtained for 

 skin carcinogenesis is that initiation causes an irreversible change in a 

 normal epidermal stem-cell, converting it into a "dormant tumour cell", which 

 then needs a further, prolonged stimulus — promoting action — to enable it to 

 develop into a progressively growing tumour (Berenblum, 1954). Without the 

 added promoting action, the growth equilibrium, which exists in normal skin 

 epithelium and also under conditions of reparative hyperplasia, would apply 

 equally to the dormant tumour cell and its progeny. Promoting action pre- 

 sumably causes a delay in maturation of the dormant tumour cell and its 

 progeny, leading to the formation of a critical sized colony, after which, the 

 process becomes self-perpetuating (Berenblum, 1960). 



Whether ths scheme is applicable to carcinogenesis in general, and if so, 

 whether it offers a complete explanation of the mode of action of carcinogens 

 or merely an over-simplified model, could only be answered if such a two- 

 stage mechanism were also demonstrated in a number of systems other than 

 of the skin. For technical reasons, this proved to be difficult, especially for 

 carcinogenesis of internal organs. 



When, therefore, Kawamoto et al. (1958) reported that urethane had a 

 "co-leukaemogenic", or synergistic, action vis-a-vis X-ray leukaemogenesis, 



