CYTOGENETIC EFFECTS OF X-RAYS IN THE GUINEA-PIG FEMALE GERM-CELLS .1.THE IMMATURE OOCYTE

In female mammals, the immature oocyte is the germ cell most at risk, from the genetic point of view. We have previously shown that ovaries of newborn guinea pigs contain a great majority of immature oocytes an d small numbers of maturing oocytes. At this time, all immature oocyte s are in a typical diplotene stage, comparable to that of the human re sting oocyte. Furthermore, the LD50 of the guinea pig oocytes is about 4 Gy, like that of human oocytes. Consequently, performing genetic st udies on guinea pig oocytes irradiated just after birth constitutes a very useful approach to predict the mutational response of the human r esting oocyte. Female guinea pigs were X-irradiated with 1 or 2 Gy wit hin 24 h of birth, and their meiotically competent oocytes were collec ted and cultured to the metaphase I stage, 1 year after treatment, ens uring that all oocytes were at the immature stage at the time of irrad iation. The proportion of oocytes that showed chromosome aberrations ( breaks, fragments or interchanges) was only 1-2% Gy(-1). Although not definitive, these results provide support for a low risk in women from an exposure of the immature oocyte to low doses of radiation.