Incidence of chromosome anomalies in first-cleavage mouse embryos obtained from frozen-thawed oocytes fertilized in vitro.

To assess the effect of low temperature storage on mouse oocytes we (1) examined the capacity for normal development of embryos derived from frozen oocytes fertilized in vitro after transfer to pseudopregnant foster mothers and (2) analyzed the chromosome complement at the first cleavage division. Fewer frozen than control oocytes were fertilized (36% vs 66%), but after embryo transfer the proportion of fertilized eggs that implanted (67-68%) and formed normal foetuses (50-53%) was similar in the two groups. Freezing did not affect the observed incidence of aneuploidy (1.5-3.3%). The frequency of polyploid embryos derived from frozen oocytes was almost double that of controls (15.8% vs 8.5%), but it is unclear whether this is a real effect of freezing or is an artifact produced by the chromosome preparation technique.