Experimental Modification of Radiosensitivity of Embryonic Cells

Experiments in which the radiosensitivity of cells has been studied indicate that different cells and tissues respond to x-irradiation in various ways. Other experiments in which the radiosensitivity of cells has been altered experimentally by changing their physiological activity show that the same modifying agent does not always change the sensitivity of different types of cells and tissues in the same way. Thus, some investigators (Holthusen, 17; Dognon, 8; Politzer, 27; Packard, 25) have found that low temperature during irradiation increases resistance, while others (Mottram, 24; Crabtree and Cramer, 7; Petry, 26; Ancel, 1; Ancel and Vintemberger, 2; Henshaw and Francis, 15), using other types of cells, have shown that low temperature either increased or had no effect on radiosensitivity. Lambert and Firket (18–21) studied oxygen consumption, water uptake, and mitotic activity of pea seedlings at different stages of development but were unable to correlate radiosensitivity with any of these factors. Henshaw and Francis (15, 16) obtained similar results with wheat seedlings, and in further experiments found that, when the development of germinating seedlings was inhibited by dehydration or by anaerobiosis, the radiosensitivity, as measured by subsequent growth rate, was decreased. Thus it seems obvious that, while radiosensitivity may be modified experimentally, the results obtained with different cells and organisms vary even when the same experimental method is used. The present experiments were undertaken in an attempt to obtain information concerning the effect of low temperature and dehydration on the radiosensitivity of embryonic cells of the grasshopper and to determine whether or not these effects were constant at different stages in the development of the organism. Material and Methods The egg of the grasshopper, Melanoplus differentialis, was chosen as the experimental material, since the changes in its radiosensitivity at different developmental stages have been studied by Evans (9–12) and because various aspects of its physiological activity have been examined by Bodine and others (3, 4, 5, 14, 30, 35, 36). Eggs of this species, when kept at a constant temperature of 25° C., develop for approximately twenty-one days (pre-diapause period), at the end of which time they enter a period of developmental block (diapause). If they are then placed at a low temperature (below 10° C.) for two to three months, upon return to constant high temperature (25° C.) they will undergo further development (post-diapause) and hatch on about the eighteenth day. The morphological changes taking place during development have been carefully recorded by Slifer (31), and the developmental stages studied in the present experiments are expressed as days at 25° C., since Slifer has described the embryos in these terms.