Origin and ploidy of multipronuclear zygotes

Recently, several authors have proposed strategies for correction of triploidy based on the removal of the extra pronucleus at the zygote stage. In the present bioassay, the following were analysed: (1) the different factors that can induce the formation of multipronuclear zygotes in mammals; (2) the different morphological patterns established according to the number of pronuclei and polar bodies that can be observed at the zygote stage and used to distinguish the origin of multipronuclear zygotes; and (3) the pattern of chromosomal segregation during the first mitotic division and ploidy status of the resulting preimplantation embryos. Such an analysis shows that the morphological criterion of counting the number of pronuclei and polar bodies can be misleading and should not be used for ascertaining the origin of tripronuclear zygotes. In addition, although monospermic digynic zygotes have a single sperm centromere, which likely organizes a single bipolar spindle during the first mitotic division, more data on chromosomal distribution and segregation of digynic tripronuclear zygotes should be collected before strategies for correction of triploidy are implemented in humans.