The first embryonic replication critically affects the outcome of pronuclear transfer

Pronuclear (PN) transfer has been successfully used in human assisted reproduction to suppress the negative effects of a defective egg cytoplasm or to bypass an idiopathic developmental arrest. However, the effects of a pronucleus formation and initial parental genome remodelling in a defective cytoplasm on subsequent development have not been systematically studied. Using the PN transfer in the pre-replication and post-replication mouse embryos, we show that the first embryonic S-phase plays a critical role. Although the developmental rates of pre-and post-replication PN transfer embryos were comparable and apparently normal looking blastocysts were formed in both groups, the post-replication PN transfer led to changes in the embryonic gene expression. Furthermore, differences in the blastocyst quality in the post-replication PN transfer embryos were detected. Therefore, although the PN transfer is the method of choice to mitigate the impact of a faulty egg cytoplasm on early development, our results show that the timing of this intervention should be restricted to the pre-replication phase.