GIANT CELL FORMATION PRODUCED BY LASER MICROBEAM IRRADIA TION OF CHROMATIN IN CHINESE HAMSTER CELLS

A pulsed laser microbeam of wavelength 532 nm was used to produce visible smalliesions in the nucleoplasm or in the cytoplasm of V79 Chinese hamster cells. Transmission electron microscopy (TEM) of microirradiated nuclei showed that the lesions were produced within the nucleus and comprised between 0.2 and 0.5 % of the total chromatin. Serial sections above and below the lesion site did not reveal any detectable chromatin damage, indicating that a visible lesion was restricted to the focal point of the beam. Whereas cells microirradiated anywhere in the cytoplasm showed normal donal growth with few exceptions, the cells containing nudear lesions did not enter mitosis at the time of unirradiated controls. Instead they formed giant cells in a high percentage of cases (72/99). The DN A content of these cells was considerably increased suggesting polyploidization. In some cases, division of giant cells was observed resulting in non-viable daughter cells containing micronuclei. Further evidence that the induction of giant cell formation depends on chromatin damage was obtained by microirradiation of chromosomes in anaphase. Here, giant cell formation was observed in the daughter cell which received microirradiated chromatin, whereas microir-radiation of cytoplasm between the moving sets of chromosomes did not affect subsequent divisions of both daughter cells. Our data point out that loss of reproductive integrity and giant cel