New evidence for monozygotic twins in the mouse: Twinning initiated in the late blastocyst can account for mirror image asymmetries

The 11.5‐day twin mice reported here support one of the classically described mechanisms for mammalian monozygotic twinning: subdivision of the blastocyst inner cell mass at the stage of proamnion cavitation. This particular method for monozygotic twinning has the attraction of providing a hypothesis for mirror image translocation of asymmetric traits. Monozygotic twins in laboratory rodents at or near parturition have not been identified and only two prior descriptions, for 7.5‐ and 9.5‐day mouse embryos, have been found in the literature. Experimentally induced twins having identical heredity, i.e., clones, would enable study of heritable and environmental regulation of the components of discordance. A set of twin mouse embryos at 11.5 days of gestation within a common yolk sac, and with shared vitelline and allantoic circulations, provides direct evidence that monozygotic twin mice can survive rotation within the common yolk sac and suggests a reasonable probability that they can survive to term.