194 Cellular Events Produced by Ionizing Radiations / 10 : 4 



membrane is damaged. Most of the subcellular structures, such as mito- 

 chondria, neurofibrils, and myofibrils remain unaltered at doses which 

 lead eventually to cellular death. 



Chromatin thread 

 from A does 

 not separate 



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



Chromosome A is bombarded with 

 protons in crosshatched area. 



(b) Normal metaphase 

 as in Fig. 2 (c). 



(c) Anaphase. Chromosome A 

 forms bridge. 



(d ) Telophase incomplete owing to 

 extra bridge between nuclei. 



Figure 5. Diagrammatic representation of results when micro- 

 beam of ionizing radiation strikes prophase chromosome. 

 Similar results are obtained due to irradiation during meta- 

 phase. 



The cells of muscle divide only occasionally and those of the adult 

 vertebrate nervous system not at all. Cytological changes in these types 

 of cells are very hard to demonstrate at reasonable doses of ionizing 

 radiation. In contrast, cells of most epithelial tissues (covering layers 

 such as skin) are continually dividing, as are those responsible for 

 forming erythrocytes (red blood cells) and leucocytes (white blood cells) . 

 These rapidly dividing cells are sensitive to all types of ionizing radiations. 

 Malignant tumor cells also divide rapidly; treatment with heavy doses 

 of radiation tends to stop this process. (It also probably induces changes 

 in the nuclei of surrounding cells which may lead to new types of 

 malignant growths.) 



