The irradiation of the chromosomes of single cells in metaphase with a microbeam ofα-particles 5μin diameter

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Abstract
α-particle microbeam irradiation of single chick fibroblasts in metaphase has been carried out with a collimating hole 5μin diameter and the results have been compared with whole cell irradiations of single cells with a collimating hole 80μin diameter. An attempt has been made to examine the dependence of the immediate cytological effects upon theα-particle dose. At low doses, 0 to 10α-particles/μ2, the main effects observed are largely cytoplasmic with excessive bubbling at all stages of division; about half the irradiated cells show abnormal effects but the other half appear to continue normally through cell division. With inter­mediate doses of 10 to 50α-particles/μ2about two-thirds of the irradiated cells show abnormalities and the remainder continue apparently normally through division. Three different mechanisms for the production of binucleated cells have been recognized. At 100 to 200α-particles/μ2all the cells show clumping of nuclear material in metaphase and failure to divide. The cells die shortly after irradiation. Irradiation of the cytoplasm alone produces little effect, but it appears that irradiation of the cytoplasm and nucleus together produces similar effects to those of irradiation of the nucleus alone. A few irradiations were made on Chinese hamster cultures from which evidence was obtained that in order to produce comparable cytological abnormalities the doses ofα-radiation needed were higher than those used for chick fibroblast cultures.

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