CELL DIVISION, MORPHOLOGY, VIABILITY 805 



cells following large doses of radiation (2500 r or more). She recognizes 

 also "mitotic degeneration" resulting from the unsuccessful attempt of 

 irradiated cells to undergo mitosis. In this type "the chromatic material 

 (chromatopycnosis) is assembled in bands (hyperchromatosis) which 

 later tend to accumulate at the nuclear membrane and finally (chromato- 

 lysis), undergo shrinkage and lysis. The cytoplasm of these cells under- 

 goes fatty or colloquative degeneration at an early stage, shrinks, and 

 disappears." 



The formation of large intranuclear vacuoles surrounded by irregular 

 layers of desoxy pentose nucleic acid within a few hours of the injection of 

 P 32 is described for the intestinal cells of rats by Warren, Holt, and 

 Sommers (1951). Such changes lead to eventual degeneration. 



On the basis of direct observations of living chick fibroblasts in culture, 

 Strangeways and Oakley (1923) described the "breaking down" of cells 

 after X irradiation and concluded that this resulted from damage to cells 

 about to divide. Some cells were not affected until telophase, when one 

 or both of the daughter cells broke down. Their observations have been 

 confirmed and extended by more recent investigations based mainly on 

 material fixed and stained at intervals following treatment. Degenerate 

 cells in irradiated tissues seem to belong to one of three classes: 



(1) After doses of ionizing radiation sufficiently small for mitotic 

 recovery to take place within a few hours after treatment, the maximum 

 number of degenerating cells coincides more or less closely in time with 

 the high point of mitotic activity that marks recovery from the mitosis- 

 depressing effect of radiation (Strangeways and Fell, 1927; Tansley et al., 

 1937; Spear and Glucksmann, 1938; Glucksmann and Spear, 1939; 

 Lasnitski, 1940, 1943a; Tansley et al., 1948). The close correspondence 

 between rise in mitotic activity and the degenerate cell count suggests 

 that the effect of the irradiation has been to render cells about to enter 

 division incapable of completing it, so that they degenerate during or 

 after division. If the radiation dose and other conditions are such that 

 a large proportion of the dividing cells complete mitosis instead of or 

 before degenerating, the degenerate cell high will follow the mitotic high ; 

 if they are such that few of the cells that undertake division complete it, 

 undergoing degeneration instead, the degenerate cell high may precede 

 the mitotic high (Glucksmann and Spear, 1939; Lasnitski, 1943a). 

 Glucksmann and Spear (1939), who altered the mitotic activity of the 

 cells of the tadpole eye by fasting and by exposure of the animals to 

 low temperatures, were able to correlate the amount of cell degeneration 

 following 7 irradiation with the amount of mitotic activity of the tissues. 

 Since the degenerate cell counts in many of their experiments greatly 

 exceeded the mitotic counts, they reached the conclusion that it was net 

 the cells in division at the time of treatment, but those approaching 

 division, that subsequently constituted the bulk of the degenerating cells. 



