CELL DAMAGE IN THE MAMMALIAN RADIATION SYNDROME 243 



houi-a after X-irridiation. This decrease is a result of distant action as 

 it is almost the same in the directly irradiated and in the shielded parts 

 of the bone-marrow (Brodsky et (iL. I!)51)). Taking into consideration 

 that destructive processes and characteristic changes in the rate of 

 mnltipHcation occur mainly in dii-ectly-irradiated tissues it may be 

 suggested that these changes are not a consequence of disturbances 

 in nucleic acid synthesis. 



Delaj^ of division and cell destruction were found by some other 

 authors to be not directly connected with the concentration and the 

 synthesis of nucleic acid substances (Howard and Pelc, 1953; White- 

 held, 1959; Biagnini et al., 1958; Hjort, 1960). 



II. THE REPAIR OF TISSUE DAMAGE 



The viability of an irradiated organism depends first of all on the 

 rate of regeneration of its haematopoietic and digestive systems. 



1. The repair of these systems is beyond any doubt due to the multi- 

 plication of either residual or injected viable cells. The following data 

 support this concept: (a) preservation in an irradiatec^ organism of 

 intact portions of haematopoietic tissue (to a great extent indepen- 

 dently of their volume) strongly accelerates the regeneration of haema- 

 topoietic organs dej^leted by irradiation (Grayevsky, et al., 1958; 

 Barakina, 1959a); (b) injection of an irradiated organism with intact 

 bone-marrow cells has the same effect (Lorenz etcd., 1951; Nowell, et 

 al., 1956). Injection of disrupted or injured cells is without effect 

 (Barakina, 1959b; Soska, 1958; Cole et al., 1953). Other things being 

 equal, cells of isologous bone-marrow exert a greater therapeutic effect 

 than homo- and heterologous cells. Better results can be obtained after 

 intravenous than after intraperitoneal injection (Van Bekkum and Vos, 

 1957; Barakina, 1960, Fig. 7); (d) Experiments in which cytological, 

 cyto-chemical and immunological methods were used are the most 

 convincing proof that the repopulation of haematopoietic organs in 

 irradiated animals proceeds at the expense of dividing intact donor cells 

 even if the latter belong to a species other than the recipient (Ford et al., 

 1957; Vos et al, 1956; Makinodan, 1956). 



The role of humoral factors in the restoration of haematopoietic 

 organs especially in some later period after exposure remains thus far 

 obscure. 



2. Chromosome aberrations and an inhibition of mitotic activity are 

 the factors limiting the regeneration of irradiated tissues. 



The more cells without lethal chromosome aberrations that are 

 preserved after irradiation the more rapid will be the regenex'ation of 



s 



