4 p. ALEXANDER AND Z. M. BACQ 



Most of these chemical reactions are biologically trivial and do not 

 harm the cell. But some of these occur at vital points and act as a focus 

 for the development of damage by subsequent cell processes. These we 

 have called the Initial (Chemical) Lesions, and their nature forms the 

 subject of this paper. We have very little definite information about the 

 early chemical events. The U.N. Scientific Committee on the effects of 

 atomic radiation (UNSCEAR) concluded in its report (1958): "The 

 nature of the initial step of radiation damage remains to be deter- 

 mined."' 



After a certain period, the duration of which depends on the intensity 

 of the metabolism, biochemical lesions can be observed and these lead 

 to anatomical lesions (i.e. biological end effects). At the cellular level 

 the immediate biological effects can be conveniently classified into (i) 

 physiological ; (ii) cell lysis or interphase death ; (iii) delayed death often 

 requiring mitosis before becoming manifest. It is improbable that the 

 same radiochemical reactions initiate all these different biological 

 effects. Also, a number of these may act in conjunction to produce one 

 end effect, such as mitotic death, and their relative contribution may 

 vary from one type of cell to another. 



Numerous methods are suitable for approaching the problem of the 

 nature of the initial lesions, such as various biophysical techniques, 

 biochemical techniques, comparison between the effects of in vivo and 

 in vitro irradiation of key molecules (enzymes, proteins, RNA, DNA, 

 etc.) and the study of the mechanisms of action of chemical protectors. 

 A study of the physiological effects j of radiations may be particularly 

 useful in this connection since they occur almost immediately after 

 irradiation and the initial lesion must therefore be more closely related 

 to the biological effect observed than in the case of such phenomena as 

 cell death, chromosome abnormalities, etc., where many hours of active 

 metabolism intervene. 



I It has ber-ome clear within the last few years that numerous disturbances induced 

 by ionizing radiations occur almost immediately after irradiation (i.e. seconds to minutes) 

 and must be attributed to an interference with the ph>-siological function of nerve fibres 

 and cells, and to changes in membranes. Particularly striking demonstration of svich 

 immediate effects have been provided by Brinkman and Lamberts (1060) and by Hug 

 (1960) with snails, echinoderms and isolated mammalian organs. Very rapid effects on 

 the retina of vertebrates have also been reported at the earlier UNESCO symposium at 

 Venice (1959) the proceedings of which have been published as Supplement No. 1 (1960) 

 to the InterndtionalJournal of Radiation Biology. All these changes are physiological in the 

 sense that they are characterized by extremely rapid repair and are therefore very dose- 

 rate dependent and may have to be studied while the irradiation is going on. Very rapid 

 changes in the transjjort of the ions have been encountered following irradiation of plant 

 roots (see Baccj and Alexander, 1961) but we know of some negative unpublished 

 experiments with animal membranes. 



It is not a paradox to say that for certain biological systems, the ionizing radiation 

 at small doses or dose-rates may be a stimulus comparable to an electric current or 

 visible light and that no permanent lesion is inflicted on the organism. 



