Analysis of aneuploidy in first-cleavage mouse embryos fertilized in vitro and in vivo.

First-cleavage mouse embryos, fertilized in vitro and in vivo, provide ideal material for chromosomal analysis. With the appropriate incubation in a mitotic inhibitor, syngamy is prevented and the sperm- and egg-derived chromosomes remain as separate clusters. Because the latter chromosomes undergo condensation sooner than those from the spermatozoon, the parental source of chromosome sets can be identified even without a marker chromosome. Thus these embryos can be analyzed both for the primary incidence and the parental source of a number of chromosomal anomalies, including aneuploidy. By using fertilization in vitro to obtain the embryos, the synchrony of fertilization and nuclear development is such that 80% or more of the chromosomal preparations are suitable for analysis, compared with about 50% for embryos fertilized in vivo.