Anna Ferrer Vaquer
Additional Author(s): Anna-Katerina Hadjantonakis
Early Embryogenesis: Cleavage; Blastula
Extraembryonic: Extraembryonic Tissues
Stage of Development: Embryo
Twins originating from two zygotes are referred to as dizygotic or fraternal. Twins that arise from the splitting of a single zygote are referred to as monozygotic or identical. Dizygotic twins implant separately and develop separate fetal membranes, whereas monozygotic twins are genetically identical and may share one, some or all their fetal membranes. Placentation of monozygotic twins depends on the time of embryo spliting. Division of the morula or blastocyst before the inner cell mass (ICM) forms generates two genetically identical embryos that would implant separately. In such cases placentation will be dichorionic diamniotic (a). This class accounts for up to a third of monozygotic twins. If the inner cell mass within the blastocyst divides between the third and eighth day, the shared trophoblast will give rise to a single placenta and each ICM will generate an amnion of its own. Placentation will be monochorionic, diamniotic and represents up to two thirds of all monozygotic twins (b). If division occurs later than the eighth day, when the bilaminar disc and amnion have formed, twins will share a common amniotic sac in a monochorionic, monoamniotic placentation (c). This class represents only a 2%-4% of monozygotic pregnancies. Splitting occurring beyond the 13th day of development results in incomplete separation of the embryo forming conjoined twins.
Ferrer-Vaquer, A., Hadjantonakis, A.K. Birth defects associated with perturbations in preimplantation, gastrulation, and axis extension: from conjoined twinning to caudal dysgenesis. WIREs Dev Biol, 2012, Published Online: Nov 26 2012.