1. Crick Advanced Light Microscopy, Francis Crick Institute, London NW1 1ST, UK
2. Human Embryo and Stem Cell Laboratory, Francis Crick Institute, London NW1 1ST, UK
3. Max Planck Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108, 01307 Dresden, Germany
4. The Centre for Trophoblast Research, Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge CB2 3EG, UK