Mechanisms of spontaneous and chemically-induced aneuploidy in mammalian oogenesis: Basis of sex-specific differences in response to aneugens and the necessity for further tests

Abstract

The basis for sex-specific differences in chemically-induced and age-related aneuploidy of mammalian germ cells is still unknown. We have analysed the maturation of isolated mouse oocytes to characterize the mechanisms underlying drug-induced aneuploidy produced by two compounds, chloral hydrate (CH) and diazepam (DZ). When administered in vivo both drugs increase hyperploidy in male but not in female germ cells. In the assay presented here we show that both CH and DZ caused meiotic delay in in vitro maturing mouse oocytes. CH blocked meiotic progression irreversibly, affecting even dictyate stage oocytes. DZ-exposure slowed down maturation but many oocytes may eventually develop to metaphase II. Under the influence of CH asymmetric spindles were formed. Often chromosomes failed to align properly. This appeared to be responsible for triggering a meiotic checkpoint which arrests oocytes in meiosis I. Many oocytes escaping the block became ‘diploid’. Lagging of chromosomes at anaphase I may contribute to significant rises in hypoploidy, while the scattering of chromosomes at metaphase II and the premature decondensation of chromatin may also predispose oocytes to the formation of structural and numerical chromosomal aberrations during meiosis II. In contrast, diazepam appeared to enhance the resumption of meiosis in immature oocytes at pharmacologically relevant doses, and only at high concentrations lead to a prominent meiotic arrest/delay. Importantly, several oocytes matured for 16 h in 25 μg/ml DZ displayed scattered chromosomes on the spindle and were hyperploid. Concomitantly, precocious separation of homologues occurred after DZ, and oocytes contained uneven numbers of chromatids, suggesting equational division at anaphase I. Cytoplasmic maturation, e.g., association of mitochondria with the spindle was disturbed by DZ. We compared the potential of the in vitro test to evaluate the aneugenic potential, the targets, threshold concentrations and long-lasting effects of relevant environmental pollutants on mammalian oogenesis with the in vivo findings, and evaluated the basis for sex-specific responses to aneugens.