Two examples of monoamniotic monozygotic twinning in diploid parthenogenetic mouse embryos.

In experiments in which diploid parthenogenetic mouse embryos cultured entirely in vitro from the one-cell stage to the blastocyst were transferred to suitable recipients and maintained in "delay" for about 3 days before implanting, about 25-40% subsequently developed to somite-containing stages. In all, over 30 such embryos were examined. Most were morphologically normal, and equivalent to fertilized development observed on the 9th-11th days of pregnancy. A few embryos, however, had neural tube abnormalities, but of greatest interest were two sets of monoamniotic monozygotic twins. It is unclear whether the twins in some way resulted from the parthenogenetic condition, or from the "delayed" state. The present examples are discussed in the context of previous observations of monozygotic twinning in the mouse and man.