Oocyte aneuploidy screening using superovulating prepubertal mice: effect of methotrexate

Numerical chromosomal aberrations (aneuploidy) are a major factor in pregnancy wastage, birth defects, and mental retardation. Consequently, effective cytogenetic procedures in mammalian gamates are required for studying mechanisms and causes of chromosomal nondisjunction. Our aims were to determine for oocytes from superovulating prepubertal mice: (1) the yield of chromosomally scorable ova, following application of a double fixation procedure: (2) the background level of aneuploidy, and (3) the sensitivity to induction of aneuploidy (by methotrexate). Superovulation was induced in C129F1 hybrid mice, 22-24 days old, with pregnant mare serum and human chorionic gonadotropin (HCG). Diurnal photoperiodicity and injections were scheduled to assure HCG-induced ovulation of known timing. Methotrexate (200 mg/kg) was given at 3 hr and ova were recovered at 15 hr after HCG. We describe the adaptation of a double fixation procedure to mouse oocytes. Methotrexate led to significantly increased hypoploidy (2 1/2-fold) but not to the hyperploidy reported by others for adult mice. There was a high yield of ova with exactly countable chromosomes (average of 9.5 ova per mouse). Concomitantly, the background level of aneuploidy was very low (0/465 scorable ova were hyperploid). Given the additional advantages of economy and convenience, the superovulating 3-week-old mouse could be an effective source of ova for testing environmental agents for their aneuploidy-inducing potential; however, further studies are needed to establish the degree to which such ova are susceptible to aneuploidy induction.