THE MECHANISM OF INHIBITION OF CELL DIVISION 219 



epithelinm of the mouse under conditious of 130CA subcutaneous in- 

 jection (1-I0to4mg). The third curve from the to]) characterizes this 

 inhibiting action. Combining the DOCA injection with irradiation, it is 

 possible to observe a stinndating effect (the first curve). 



It is still necessary to show that these extracellular factors participate 

 in the changes of DNA synthesis in irradiated animals. In fact, evidence 

 for this is included in the experiments of Kuzin and Budilova. 



Taking into account the role of extracellular factors for statistical 

 purposes is one of the most important tasks in formulating a theory of 

 radiation damage of mitosis. So far, however, the very concept of extra- 

 cellular factors is far from being deciphered. One should not believe 

 that extracellular factors are represented solely by nervous and humoral 

 influences. Actually, the events are much more complicated. The 

 cytolytic systems of the organism may effect the fate of cells exposed 

 to radiation. They may play a certain role in the processes of restoration 

 facilitating the release of the cell population from the effect of the 

 perished cells. It is well known that after radiation the situation is 

 unfavourable : along with numerous perished cells, the cytolytic activity 

 of the tissue fluid decreases (Zaretskaya, 1960). 



Figure 5(a) shows the dependence of the numberoflysed rabbit bone- 

 marrow cells upon the exposure time of normal rabbit aqueous humour, 

 Fig. 5(b) shows the same process under the influence of the aqueous 

 humour in irradiated animals : the marked decrease of the lysing capabil- 

 ity of the serum, especially on the 5th and 7th days after irradiation is 

 seen distinctly from this flgure. 



It is doubtful whether the cited data cover all the possible ideas con- 

 cerned with the extracellular factors affecting the mitotic activity of 

 the tissue of exposed animals. So we should think about the complex 

 system of nervous and humoral disturbances of the regulatory mechan- 

 isms to explain the data of Gruzdev (1960. unpublished) who showed in 

 mice exposed to total radiation (750 r) the disappearance of the typical 

 diurnal rhythm of mitosis (Prof. Horizontov's laboratory). We 

 attempted to give the schematic picture of the possible additional cellu- 

 lar and extra cellular factors capable of influencing the mitotic activity 

 of the normal (left) and irradiated (right) cell (Fig. 6). These are 

 hormones, mediators of the respective nervous influences, cytometric 

 factors and some others. 



It stands to reason that this scheme is inadequate for the statistical 

 analysis of the system undergoing the extracellular effects. Indeed, 

 specific ideas about the essence of the extracellular effects are greatly 

 limited. The studies of a number of these extracellular effects reveal 

 their mainly inhibiting action on mitosis. In this case most of them 



