The chromosome complement of first-cleavage mouse embryos after in vitro fertilization

A comparison of the first cleavage-stage chromosome complements of 1022 in vivo fertilized mouse embryos and 1033 in vitro fertilized mouse embryos is reported. The chromosome analysis of first-cleavage embryos allows us to study directly the chromosome complement of the sperm and oocyte that contribute to the embryo, since both chromosome clusters remain separate when an antimitotic agent is used to prevent syngamy. In this paper we show that the sex ratio and the incidence of aneuploidy are similar, irrespective of the fertilization system used. Male and female gametes have the same levels of aneuploidy. Triploidy is more frequent in the in vitro fertilized embryos and the difference can be ascribed to a higher incidence of polyspermy and diploid spermatozoa.