Mechanisms that position the embryonic axis and the causes of identical twins

In humans, identical (as well as conjoined) twins can arise by spontaneous splitting of the embryo relatively late in development, right up to the time when the primitive streak forms, when the embryo already contains many cells. This suggests that the mechanisms that determine the position of the embryonic axis remain plastic for a long time, and that in normal development something must inhibit formation of multiple embryos. Almost nothing is known about the mechanisms involved in this embryonic regulation, which is likely to be responsible for the generation of monozygotic and conjoined twins in humans and other amniotes. Professor Stern plans to take a multidisciplinary approach, using chick embryos and the genetics of human populations systematically to identify inducers and inhibitors of axis formation and generate a comprehensive model of the cell and gene interactions controlling polarity, regulation and twinning.