A mammalian embryo cannot develop on its own. It has to implant in the uterus, where it is supplied with everything it needs to survive. Until this happens, the egg cell nourishes the early embryo. Among other things, it provides essential proteins. Researchers led by Melina Schuh at the Max Planck Institute (MPI) for Multidisciplinary Sciences, together with colleagues in Göttingen, have now elucidated how egg cells store proteins. Their experiments also provide important insights into how errors in protein storage can lead to infertility. Structures of the egg cell that have puzzled scientists for over 60 years play a crucial role in this.