Mechanisms giving rise to triploid zygotes during assisted reproduction

To review information on the origin of triploid zygotes as gathered from assisted reproduction techniques.

Identification of relevant literature by a MEDLINE search and own experience on the basis of cytogenetic studies of abnormally fertilized oocytes.

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Penetration of two haploid spermatozoa or of a single diploid spermatozoon into the oocyte causes diandric triploidy. The first case can be discerned by formation of a total of three pronuclei, whereas the second process will remain undetected, because it involves a female and a single but diploid male pronucleus. Digynic triploidy after intracytoplasmic sperm injection is characterized by nonextrusion of the second polar body and formation of three pronuclei. Digyny can also result from the fertilization of diploid giant oocytes. Depending on how maturation of these gametes proceeds, three or only two pronuclei will be observed. Thus, the size of the pronuclear stage must be considered for a successful identification of the abnormality. Endoreduplication within the female pronucleus is not detectable and may represent another, albeit rare, origin of digynic triploidy.

Routine inspection of the number of pronuclei is not an absolutely reliable tool for excluding the development of triploid embryos. Observations during assisted reproduction may yield valuable information on the origin of triploidy.