Figure 2.

Formation of dichorionic monozygotic (MZ) twins. (A) The classic model of dichorionic MZ twin formation proposed that it occurs when blastomeres split before the morula stage. (B) The divided blastomeres of zona pellucida (ZP)-free mouse embryos at the two-cell stage can be separately cultured in U-shaped or V-shaped bottom wells, eventually developing into small blastocysts. (C) In an ART case, one of the blastomere emerged from the ZP through a breach at the two-cell stage, with each blastomere forming an individual blastocyst. (D) Dichorionic MZ twins can result from the separation of the ICM and trophectoderm during atypical 8-shaped hatching, forming two individual small blastocysts. (E) In certain species such as sheep, cattle, goat, and pig, embryos can be replicated by splitting a blastocyst into two halves using a sharp needle, each containing a similar number of ICM and TE cells. (F) Blastocyst separation was observed in an ART case in a vitrified-warmed cycle, leading to a dichorionic MZ twin pregnancy.