X-ray stage sensitivity of mouse oocytes and its bearing on dose-response curves.

A detailed cytogenetic study of maturing mouse oocyte radiosensitivity was performed. Oocytes were collected at various intervals ranging from 1.5 days to 28.5 days after irradiation with 50, 100, 200, and 300 R of acute X-rays. The observed sensitivity to chromatid aberration induction varied greatly over this time span. Sensitivity was lowest at the shortest time interval before ovulation and gradually increased up to 9.5 days; it then remained constant until insufficient numbers of oocytes could be collected. The data were analyzed in three ways. First, the data from all time intervals at each dose were pooled; second the data from the least sensitive time intervals, at each dose, were pooled, and third, the data from the period of uniform sensitivity, at each dose, were pooled. Dose-response regression analyses were done on these pooled data and the best fits obtained were to the models Y = a + bD + cD2 and Y = a + cD2 for both deletions and interchanges. This result is interpreted as indicating that the aberrations result from a predominantly two-track process. The cytogenetic data were compared to specific-locus mutation induction data in comparable oocyte stages, and qualitative similarity in dose-response characteristics were observed. This similarity is interpreted to mean that both events result from the same mechanism, and that the large dose-rate effect, observed for both events, is a reflection of the two-track component in the dose-response curves.