B.M. and T.C. are supported by the Swiss SNF grant 320030-228142 and by the University Research Priority Program ‘Human Reproduction Reloaded’ of the University of Zurich. The authors thank C. Thomas (IBDM, Marseille, France), S. Makieva (USZ-UZH, Zurich, Switzerland) and the members of the Cavazza lab for useful feedback and discussion.