238 E. Y. GRAYEVSKY, et Cll. 



causes bringing about the death of cells and conditions for tissue re- 

 generation. 



Some questions concerning the causes of cell damage, tissue and cell 

 repair and the mechanism of action of some protective agents are 

 discussed in this paper. 



I. PROCESSES OF CELL INJURY 



The death of irradiated cells that are able to undergo division is due 

 to chromosome unbalance or to gross sti'uctural damage to the chro- 

 mosomes which often stops the cells from completing mitosis. 



1. Most of the irradiated cells of mouse cornea with bridges and 

 acentric fragments were found to have died in the first generation. 

 A complete elimination of such damaged cells takes place in the first- 

 third generation ; some cells die during mitosis (Shapiro and Kon- 

 stantinova, 1959). 



When irradiated cells do not enter mitosis, chromosome injuries are 

 preserved in them during the whole interkinetic period. Thus about the 

 same percentage (60 per cent) of liver cells with anaphase bridges and 

 acentric fragments was observed in rats exposed to 500 r independent 

 of the time between irradiation and hepatectomy (1 day, 2 or 4 months). 

 The number of cells with chromosomal aberrations was only 25 per 

 cent when partial hejiatectomy was performed twice: 1 day and 1 

 month after total-body irradiation (Shapiro, 1959). Similar results were 

 obtained on the liver of irradiated mice (xAlbert, 1958). 



The data shoA\' that some chromosomal injuries, which become appa- 

 rent during mitosis and lead to cell death, are preserved for a long time 

 during interkinesis and have no visi])le effect on cellular viability. 

 This mechanism of cell death, however, is not unique. Mass destruction 

 of cells takes place in radiosensitive systems. This process develops 

 within a few hours of exposure and, as will be shown below, it is not 

 related to mitosis. 



2. Mass destruction of cells proceeds only in directly irradiated parts 

 of radiosensitive systems and does not occur in shielded parts. 



This thesis may be illustrated by the results of experiments on local 

 X-irradiation with 700 to 1,000 r of different portions of the haemato- 

 poietic system of mice. Cell destruction takes place only in the irra- 

 diated areas (Fig. la, b). The number of cells did not change signifi- 

 cantly in the shielded areas. (Grayevsky, et aJ.. 1958; Barakina, 

 1959a). 



3. The radiosensitivity of cells in vivo and in vitro is almost the same. 

 Puck (1900) and Puck et al. (1957) have shown that various normal 



