Chloral hydrate induced spindle aberrations, metaphase I arrest and aneuploidy in mouse oocytes

In view of tissue- and sex-specific differences in chemically-induced aneuploidy, we analysed the effects of chloral hydrate (CH) on cell-cycle progression, spindle formation and aneuploidy in in vitro maturing mouse oocytes with cytogenetic and immunofluorescent methods. CH blocks oocyte maturation irreversibly and concentration dependently. During culture in 125 micrograms/ml CH, germinal vesicle breakdown is delayed, and most oocytes become arrested in meiosis I with bivalent chromosomes. Their spindles are asymmetric or attain fusiform poles. Oocytes which progress to metaphase II also possess astral instead of barrel-shaped, anastral spindles as characteristic for the controls. Resolution of chiasmata without polar body extrusion during exposure to 50 and 125 micrograms/ml CH results in significant rises in 'diploid' metaphase II oocytes. Hyperploidy rates do not increase significantly at any concentration of CH, but hypoploidy levels are elevated at 125 micrograms/ml CH. CH induces lagging of chromosomes during telophase I, inhibits spindle elongation in anaphase B and causes chromosome displacement from the spindle equator in metaphase I and II. Oocytes also become irreversibly arrested in maturation when exposed to CH prior to resumption of maturation, or when CH is present during the first or second 8 h of maturation. Therefore, these data show unequivocally that CH is a potent aneugen in female germ cells, affecting spindle shape, cytokinesis and cell-cycle progression. However, a metaphase I checkpoint in mammalian oogenesis sensing disturbances in the spindle appears to prevent nondisjunction efficiently in most oocytes. In this in vitro model the aneugenic activity of drugs, critical periods in oocyte maturation, threshold concentrations, and targets of drug action can be directly assessed.