These studies have examined alterations in the in vitro growth properties of mammary epithelial cells isolated at 1, 4, and 16 weeks after in vivo irradiation with-137Cs γ rays or fission-spectrum neutrons. Altered in vitro growth potential was characterized by the proliferation of epithelial foci (EF) from irradiated animals under conditions in which mammary cells from nonexposed animals senesced. These EF were further characterized based on their ability to be subcultured. Both γ and neutron irradiation resulted in the appearance of cells capable of forming EF. Further, with increased time in situ between irradiation and cell isolation, the frequency of EF which were capable of being subcultured indefinitely (EFs) increased. Reducing the γ-ray dose rate resulted in fewer EFs while reducing the neutron dose rate resulted in increased frequencies of EFs. These data confirm earlier observations following γ irradiation and show these cellular changes are also observed following neutron irradiation. In addition, these data indicate that changes in dose rate primarily influence the emergence of immortalized cell populations.

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