Case Report: Androgenetic/biparental chimera with two biparental cell lines leading to placental mesenchymal dysplasia: a possible novel mechanism of formation.

Placental mesenchymal dysplasia (PMD) is a rare placental pathology that may be associated with Beckwith-Wiedemann features in the fetus and may be due to the presence of an androgenetic cell line. Many of the reported PMD cases describe the presence of a biparental and an isodisomic androgenetic cell line. The proposed mechanism of formation is by fertilization of a haploid ovum by a haploid sperm and duplication of the male pronucleus. We present a case with evidence of the participation of three different haploid gametes, one ovum and two spermatozoa, which led to an androgenetic/biparental chimera (ABC) in which three fetal cell lines were detected: two biparental, genetically different, cell lines but with the same maternal contribution, and one heterodisomic androgenetic cell line. To our knowledge, this is the first described case of ABC with two different biparental cell lines. We propose a novel mechanism based on the heterogoneic division of the tripronucleated zygote to explain the formation of this rare ABC.