Effect of irradiation on cell division and nucleic acid synthesis in strain U-12 fibroblasts

The relationship between cell killing, chromosome aberrations, spindle defects and mitotic delay in mouse lymphoma cells of differential sensitivity to X-rays

The ultrasensitivity of a subline of L5178Y mouse lymphoma cells to X-rays was thought to result from chromosome structural aberrations which are much more frequent in these cells than in radiation-resistant cells derived from them (Scott, Fox and Fox 1974). However, Ehmann, Nagasawa, Peterson and Lett (1974) in time-lapse photography studies of the sensitive line, concluded that the induction of multipolar mitoses by X-rays might be a more important mechanism of cell killing than chromosome aberrations. We have now shown that at survival levels above about 20 per cent, chromosome structural aberrations which lead to bridges and fragments at anaphase are about four times more frequent than spindle defects. We have confirmed the higher frequency of structural aberrations and spindle defects, and the greater mitotic delay in the X-ray-sensitive than in the X-ray-resistant cell line and have proposed a model which causally relates these end-points to cell killing and DNA repair.

The ultrastructure of male germ cells in rats x-irradiated at birth

A histological and ultrastructural study was made of the testes of rats, aged 1 to 15 days, which had received partial-body X-irradiation (ca. 100 r) on the day of birth. Nineteen serially sectioned testes were subjected to a quantitative analysis including counts of germ cells and measurements of nuclear volume. The type, position and general form of germ cells, together with the incidence of various organelles, was assessed semi-quantitatively from 533 electron micrographs of irradiated (370) and normal (163) tissues. Exposure to 100 r at birth has no marked effect on the total population of germ cells during the first 5 days. The transformation of gonocytes into transitional cells is not affected. Subsequently, the treated testes differ from those of coeval controls by the almost complete absence of germinal mitoses and, consequently, of spermatogonia type A. Many of the transitional cells grow into irregularly-shaped giant cells, usually with a highly lobed nucleus and enlarged nucleoli. Although some cells show condensation of chromatin which may indicate the onset of mitotic prophase, the majority appear to degenerate at interphase. By 10 days, the testes contain only a fraction of the original population of germ cells. No significant changes are detectable in the ultrastructure of germ cells up to 4 days after irradiation. The treatment only induces the sporadic appearance of two minor features (pinocytotic vesicles containing electron-dense material; peripheral granular bodies with some internal organization) which are normally absent from corresponding cells in the controls. Electron micrographs of giant transitional cells confirmed the cytological observations of irregularity of nuclear outline and nucleolar enlargement. The nucleoli are frequently irregular in shape and contain material of two distinct electron densities. In general, the organelles of giant transitional cells are normal and healthy in appearance. With advancing age, an increasing proportion of germ cells show degenerative changes leading to pyknosis. Dead cells appear to be engulfed by neighbouring somatic cells. In both normal and irradiated testes, the simple form of Golgi apparatus in the gonocyte changes to a more complex type in the transitional cells. The persistence of giant cells is associated with a tendency for the Golgi apparatus to revert to the simple type, or to disperse into a number of small simple units. The most striking feature is the relatively high incidence of centrioles, suggesting that radiation-induced mitotic inhibition and gigantism may be accompanied by endomitosis. Irradiation does not markedly affect the appearance and incidence of A- and B-bodies (Franchi & Mandl 1964). These organelles tend, however, to persist over a longer period in irradiated than in untreated young rats.