PHYSIOLOGICAL EFFECTS 251 



Microirradiation of Cells 



So far, the discussion has been on whole-cell irradiation. However, by 

 microirradiation techniques, in which just a small volume within a single cell 

 receives radiation, it has been found that not all parts of the cell are equally 

 sensitive. In fact, a is much higher if the nucleus (in particular the cnromo- 

 somes within the nucleus), rather than any other part of the cytoplasm or 

 cell membrane, is irradiated. 



Microirradiation is not easy experimentally, but it has now been done 

 with proton and alpha particles, and with X and far ultraviolet electro- 

 magnetic rays. Production of the micro beam is done by a colinear series of 

 apertures in a number of absorbents (e.g., lead bricks). Sometimes it is done 

 by passing the radiation through a glass or platinum capillary mounted in a 

 lead shield. Thus any X rays falling on the wall of a Pt capillary at an angle 

 of 0.6 deg or less to the axis of the capillary are completely reflected, and are 

 propagated unchanged to the exit and thence to the target. The position of 

 the target cells can be set by means of apparatus which is not essentially 

 different from the traveling stage of a microscope: by means of a micro- 

 manipulator with worm gears the target can be moved into any desired posi- 

 tion within a limited space. 



Results with protons, alphas, X, and ultraviolet have all shown that the 

 nucleus, and specifically the nucleolus which begins to become more prom- 

 inent as mitosis begins, is far more radiation-sensitive than the rest of the 

 cell. For example, in a specific case, irradiation through an area 2.5/i in 

 diameter on a chromosome (~5ju x 30yu) with 36,000 rads of proton energy 

 (60 protons, ~1.5 Mev) caused the chromosome to become sticky (to cross- 

 link?) and the cell to die in the attempt to divide, while irradiation else- 

 where in the cell with up to 1.7 million rads caused no change in speed or 

 reliability of division, nor did it have any effect on the several observed suc- 

 ceeding generations. 



However, indirect effects on the chromosomes by irradiation elsewhere in 

 the cell have been demonstrated. Nor should one infer that irradiation else- 

 where does no permanent damage to the cell or its progeny. For such spec- 

 tacular things as blistering of the cell wall, and coagulation of cytoplasm and 

 of the mitochondria, as well as death to all the progeny of cells irradiated 

 generally elsewhere than the chromosomes, have been observed. Considera- 

 tion of the cell as "a bag of enzymes," each subject to irradiation isomeriza- 

 tion, gives one an idea of how complex this question can be. 



Unfortunately the important microirradiation studies have not, yet yielded 

 any case in which irradiation of a certain part of the cell has caused an in- 

 creased rate of reproduction of modified or cancerous cells. Hence, just how 

 absorbed radiation induces cancer at the cell level remains unanswered. It 

 is now generally assumed to be irradiation of the DNA of the chromosomes, 



