THE MECHANISM OF INHIBITION OF CELL DIVISION 215 



These considerations are adequate in regard to the exponential 

 population i.e. a particular situation where cell propagation is deter- 

 mined only bj^ intracelhdar events on the basis of our calculations. We 

 have used in our considerations the notion of only two intracellular 

 phenomena: processes of synthesis and the removal of metabolites. 

 HoA\ever, we are aware that the intracellular factors for mitotic 

 activity are not limited only by the inhibition of synthesis of the 

 definite biochemical precursors and the removal of the resulting meta- 

 bolites. Expeinments show the importance of kinetics in the formation 

 of toxic products in the irradiated cell, as was demonstrated by Ilyina 

 and Petrov (1960). I wish to remind you that according to these authors 

 the toxicity of mitochondria and microsomes in the small intestinal 

 mucosa of the irradiated animals increases sharply. It is not excluded 

 that the transfer of these toxins towards the nucleus affects the mitotic 

 processes. 



Besides the toxic products arising in mitochondria and microsomes 

 in the irradiated cells there are other intercellular factors changing the 

 mitotic activity. This is not enough. Actually we must admit the 

 existence of a great number of extracellular factors that are active 

 in cell division, even in the case when in the purposes of symplifying 

 one chooses the simplest biological object — the corneal epithelium 

 devoid of a blood supply system. 



x\s to this and some other objects it is easy to demonstrate that the 

 notion about the real events in the cellular population may be created 

 only by taking into account the existence of extracellular factors. 



We supposed above that the j)robability of cell division in the basal 

 layer per time unit depended upon the concentration of the metabolites 

 in the media surrounding the cell. In the first approximation we may 

 assume that we should speak chiefly about the DNA synthesized in 

 cells. 



The experimental data of Skovronsky, Fradkin and Borisova (1961), 

 may serve as new evidence in favour of this view. They showed that, 

 against the background of stimulating nucleic acid synthesis and in- 

 hibition of protein synthesis with the aid of small concentrations of 

 levomycin (1 mg/ml), the number of cells capable of division in irradi- 

 ated (60Co) suspensions of ^. coli increased (Fig. 2). And on the contrary, 

 the number of dividing cells is considerably less against the back- 

 ground of almost total blockade of protein and inhibition of nucleic 

 acid synthesis initiated by the very same antibiotic at large concentra- 

 tions (60 mg/ml, Fig. 3). 



There are other instances of inhibition of DNA synthesis caused by 

 radiation and of the role of this inhibition in the destruction of processes 



