Studies on the adaptive response in mouse female germ cells X-irradiated in vitro at two different stages of maturation.

Background: Radioadaptation is a phenomenon whereby cells exposed to a low dose of ionizing radiation are more resistant to a much higher dose delivered some time thereafter. This phenomenon could result from the activation of damage repair and/or antioxidant defense systems by the low dose. Material s and Methods: The existence of a cytogenetic adaptive response in female germ cells was investigated using a recently developed in vitro system. Mouse ovarian follicles were cultured from an early preantral stage up to ovulation. The follicles were X- irradiated with either 2 or 4 Gy ("challenge dose") preceded or not by 50 mGy ("conditioning dose", 5 h earlier), on days 0 or 12 of the culture. Ovulated oocytes were collected on day 13, fixed and analyzed for the presence of chromosome aberrations. Results: Irradiation with 2 or 4 Gy on days 0 or 12 did not influence ovulation but had dose-dependent effects on the germinal vesicle breakdown of the oocytes. It also caused dose-dependent chromosome damage, with a greater sensitivity of oocytes to this effect when irradiation occurred on day 12 than on day 0. Prior irradiation of oocytes with the dose of 50 mGy led to a reduction in the yield of chromosome aberrations when irradiation occurred on day 12 but not on day 0. Conclusion: These results suggest that pre-irradiation of mouse pre-ovulatory oocytes with a low conditioning dose could confer on them some protection against radiation- induced chromosomal damage by a subsequent challenge dose of a few Gy. Radioadaptation is a phenomenon whereby cells exposed to a low dose (conditioning dose) of ionizing radiation are more resistant to a dose in the order of a few Gy (challenge dose) delivered some time thereafter, from a few hours up to 1 month or even more. As a result, cell lethality (1) and the frequencies of chromosomal damage (2-4), gene mutation (5) and neoplastic transformation (6-9) are reduced compared with the effects of the challenge dose alone. This inducible and transient protective effect also seems to occur in humans (10-12) and could result from a stimulation of cell defense and DNA repair systems (13-14).