In just a few weeks a completely new organism develops from a fertilized egg cell. The real miracle is that a bunch of identical stem cells turns into completely different, specialized cell types. A team led by Christian Schröter, group leader at the Max Planck Institute of Molecular Physiology in Dortmund, has now been able to show that the specialisation of individual cells during embryonic development is not, as previously assumed, exclusively left to chance but is rather determined by cell communication.