10 : 4/- Cellular Events Produced by Ionizing Radiations 



195 



Most of the effects on rapidly dividing cells are associated with 

 alterations in the chromosomal material or spindle. The microbeam 

 experiments of Zirkle and co-workers indicate that chromosomal changes 

 are extremely local. This suggests they are direct effects associated with 

 a sensitive volume. Dosage studies likewise show that only a single 



(a) Prophase as in Fig. 2 (b). 

 Chromosome B' is bombarded 

 with protons at the centromere. 



(b) Metaphase. Chromosome B' 

 does not line up 

 on equatorial plate. 



(c) Anaphase. Chromosome B' drifts 

 to one side without separating. 



(d) Telophase. Chromosome B' forms extra lobe on 

 upper nucleus. Lower cell lacks B . 



Figure 6. Diagrammatic representation of abnormality result- 

 ing from bombarding centromere of one chromosome with 

 ionizing radiation. Similar results are obtained if the 

 centromere is irradiated during metaphase. The reader is 

 reminded that the chromosome shapes and numbers are purely 

 diagrammatic. 



ionization is necessary in this critical volume. In contrast, the changes 

 induced by uv photons in the spindle were not direct effects ; they could 

 be induced by irradiation of any part of the cytoplasm. Thus, both 

 direct and indirect effects are important. 



There are several exceptions to the rule that rapidly dividing cells 

 are more sensitive to ionizing radiations than are other cells. Some types 

 of rapidly growing tumors are quite insensitive, whereas lymphocytes 

 which divide only occasionally are among the most sensitive. The 

 reasons for these differences are not known. 



In all cases tested, a decrease of oxygen tends to decrease the effect 

 of the ionizing radiation. Furthermore, certain substances such as the 



