CYTOGENETIC EFFECTS OF X-RAYS IN THE GUINEA-PIG FEMALE GERM-CELLS .2.THE MATURING OOCYTE

In a previous study, we showed that the guinea pig constitutes one of the best models to evaluate the genetic risk associated with an irradi ation of the human female germ cells. Herewith, experiments were under taken to evaluate the chromosomal radiosensitivity of oocytes of this species at two different stages of follicular development, separated b y only 1 week. Female guinea pigs were X-irradiated on the ovaries, at either the beginning (day 3) or the middle (day 10) of the 17-day oes trous cycle. The doses delivered were 1 or 2 Gy. Meiotically competent oocytes were collected 1 week after irradiation (day 3) or immediatel y thereafter(day 10), and they were cultured to the metaphase of the f irst meiosis (MI) and examined for the presence of chromosome aberrati ons. Our data demonstrated a dramatic increase in the radiosensitivity of the oocyte during this short time interval: oocytes irradiated at the beginning of the oestrous cycle had a low frequency of chromosome aberrations, while those irradiated at the middle of the oestrous cycl e (when growing Graafian follicles are clearly visible at the surface of the ovaries) exhibited heavy chromosome damage. However, we also fo und that oocytes irradiated at the middle of the oestrous cycle were e liminated from the ovaries in a few days, after their evolution to the MII stage. The stimulation of the first meiotic division by radiation required less than 24 h after doses of 1 or 2 Gy and was probably due to a rapid atresia of the large follicles containing the oocytes. On the basis of these results, it can be concluded that the radiosensitiv ity of the nearly mature guinea pig oocyte (1 week before ovulation) i s clearly much higher than that of the corresponding stage in the mous e, both in terms of sensitivity to killing and to induction of chromos ome aberrations.