10 : 4/ Cellular Events Produced by Ionizing Radiations 193 



The visible cellular effects of irradiation may be divided into two types : 

 those concerned with mitosis (or lack thereof) and those producing 

 degeneration, often leading to cellular death. The sensitivity to irradia- 

 tion varies markedly from one cell to another. By and large, the cells 

 of higher animals and plants are more sensitive than those of the lower 

 ones. Also, faster growing cells are altered by lower doses of irradiation 

 than more mature cells. 



The most sensitive part of most cells is the nucleus. Direct hits in a 

 very narrow region can alter the mitotic figures or the progress of 

 mitosis. This has been demonstrated most convincingly by Zirkle and 

 Bloom and their co-workers, who have used pinpoint beams of protons 

 and ultraviolet (uv) photons on single cells. They exposed single cells 

 of different types to these microbeam radiations. The beam cross 

 section was of the order of 8 /z in diameter. The apparatus was arranged 

 so that the area exposed could be located simultaneously with an optical 

 microscope, and also so that the cell could be followed after irradiation. 

 The entire progress was recorded on a motion picture film, with intervals 

 of several seconds between pictures. 



At low doses, no cytological changes were observed when the beam 

 passed through the cytoplasm only. However, when the same types of 

 cells were irradiated with the proton or uv beam passing through the 

 nucleus, the process of mitosis was often altered. If irradiation occurred 

 during the resting phase, when distinct chromosomes cannot be observed, 

 a variety of abnormal effects were produced during the next mitosis, 

 including broken chromosomes, pairs of chromosomes stuck together, 

 and uneven division of chromosomes. During mitosis, when one particu- 

 lar region of the chromosome, the centromere, was hit by as few as a 

 dozen protons, the chromosome no longer lined up with the others. 

 Eventually, it was forced into one of the two daughter cells forming 

 either an auxiliary nucleus or a lobe of the existing one. Higher doses 

 were needed on any other part of the chromosomes to alter mitosis, 

 although these doses were small compared to those necessary to produce 

 damage when used on the cytoplasm only. Several abnormal mitoses 

 are shown in Figures 5-7. 



During mitosis, a spindle of fine threads forms and appears to pull the 

 chromosomes apart. Irradiation of the spindle or cytoplasm by protons 

 had little or no effect on the spindle. However, irradiation of any part 

 of the cytoplasm with doses of uv photons several times those used on 

 chromosomes did alter the spindle. The arrangement of the chromo- 

 somes was changed ; they split into two groups of chromosomes instead 

 of each pair splitting in two. 



The nonmitotic visible cellular changes observed are much less pro- 

 nounced. In extreme cases of high doses to single cells, the cell 



