CYTOPLASMIC DNA IN IRRADIATED xNEURAL TUBE 81 



Maximow (1925) recognized that the peculiar granules occurring in tissue 

 cultures of young rabbit embryos were inclusions within the cytoplasm of 

 otherwise normal appearing cells, and that, except for their enormous in- 

 crease in number, they were identical wdth the inclusions of normal em- 

 bryos. They were especially abundant in the neural tube and mesenchyme 

 and were usually more numerous in the central part of an explant. He stated 

 that they always first appeared as small granules in close proximity to the 

 nucleus, and he assumed that the large granules resulted from the growth of 

 small ones. 



Temperature changes ( Chevremont it ai, 1958 1 and chemical agents 

 (Dustin. 1947; McLeish, 1954: Chevremont ct ai, 1958) may evoke DNA- 

 containing cytoplasmic bodies. Explanations have varied. Dustin (1947) saw 

 evidence that dividing cells respond to colchicine and a series of other 

 mitotic poisons by nuclear pyknosis. followed by fragmentation and engulf- 

 ing of the debris by the cytoplasm of surrounding cells. "Micronuclei" may 

 result from chromosome breakage and be the source of DNA-containing 

 bodies in the cytoplasm. Chromosome breakage may occur from intracellular 

 metabolic disturbances resulting from changes in temperature or o.xygen ten- 

 sion (Roller, 1954) and following exposure to chemicals (McLeish, 1954; 

 Frederic it al. 1959). Chevremont vt ai, (1958; Baeckeland it ai. 

 1957) found that fibroblasts cultivated in the presence of DNase contained 

 numerous Feulgen-positive granules in their cytoplasm. This DNA, which 

 may amount to 90''r of the normal diploid nuclear value, was newly syn- 

 thesized, as demonstrated by labeling with tritiated thymidine (Chevremont 

 ('/ al., 1959). Since other nonphysiologic agents, including chilling to 20°C, 

 gave similar results (Chevreinont irt al.. 1958), and in \iew of the evidence 

 that cytoplasmic DNA may mean only that dead nuclear fragments have 

 been phagocytized, the authors" interpretation that the bodies are altered 

 mitochondria must be accepted with caution. 



Extranuclear DNA following irradiation may be the result of cell death by 

 nuclear pyknosis, ''micronuclei" resulting from chromosome breakage, or a 

 manifestation of other mechanisms. 



According to Spear and Gliicksmann i 1938) and Gliicksmann i 1951 ). cell 

 death from radiation injury is by pyknosis, and the Feulgen-positive bodies 

 represent pyknotic degeneration, which they subdivided into three stages: 

 chromatopycnosis, consisting of the separation of the chromatic from the 

 nonchromatic material and the precipitation and coalescence of the chro- 

 matin into granules; hyperchromatosis of the nuclear membrane, in which 

 the chromatin, having united into a single body, lies against the nuclear 

 membrane as a deeply staining rim or partial rim; and chromatolysis, with 

 loss of the Feulgen reaction. The entire process may take place in about an 

 hour. Since onset of prophase in itself effects separation of chromatic from 

 nonchromatic elements, cells degenerating in mitosis omit the first stage. 



