But they have now been able to overcome that obstacle and film the implantation of mouse embryos:
To be able to support development, they created a system comprising a gel and medium that, as well as having the right chemical and biological properties, was of similar elasticity to uterine tissue. Crucially, this gel was transparent to optical light, allowing then to film the embryo during implantation.
This new method revealed that on its way from ball to cup, the blastocyst becomes a 'rosette' of wedge-shaped cells, a structure never before seen by scientists.
"It's a beautiful structure. This rosette is what a mouse looks like on the 4th day of its life, and most likely what we look like on the 7th day of ours, and it's fascinating how beautiful we are then, and how these small cells organise so perfectly to allow us to develop."