Cytogenetic studies in mouse oocytes irradiated in vitro at different stages of maturation, by use of an early preantral follicle culture system

In vivo studies on X-irradiated mice have shown that structural chromosome aberrations can be induced in female germ cells and that the radiation-induced chromosomal damage strongly depends on the stage of maturation reached by the oocytes at the time of irradiation. In the present study, the sensitivity of oocytes to radiation-induction of chromosome damage was evaluated at 2 different stages, using a recently developed method of in vitro culture covering a crucial period of follicle/oocyte growth and maturation. A key feature of this system is that growth and development of all follicles is perfectly synchronized, due to the selection of a narrow class of follicles to start-off culture. This allows irradiation of well characterized and homogenous populations of follicles, in contrast to the situation prevailing in vivo. Follicles were X-irradiated with either 2 or 4 Gy, on day 0 of the culture (early preantral follicles with 1-2 cell layers) or on day 12, 3 h after hormonal stimulation of ovulation (antral Graafian follicles). Ovulated oocytes, blocked in MI by colchicine, were fixed 16 h after hormonal stimulation and analyzed for chromosome aberrations. The results confirmed the high radiosensitivity of oocytes 2 weeks prior to ovulation and the even higher radiosensitivity of those irradiated a few hours before ovulation, underlining the suitability of the in vitro system for further studies on the genetic effects of ionising radiation in female mammals.