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. Overall design: Three embryos from each experimental condition collected at 34h post fertilization were pooled and used for library preparation, followed by RNA-sequencing analysis. SPN: sham pronuclear transfer EPN: early pronuclear transfer LPN: later pronuclear transfer SPN is regarded as control, EPN and LPN refer two different timing of the PN procedures.

原核（Pronuclear, PN）移植已成功应用于人类辅助生殖领域，用于规避缺陷卵细胞质带来的不良影响，或是绕过特发性发育停滞。然而，缺陷细胞质中发生的原核形成与初始亲本基因组重塑对后续发育的影响，尚未得到系统性研究。本研究针对复制前及复制后小鼠胚胎开展原核移植实验，结果证实胚胎首个S期发挥关键调控作用。尽管复制前与复制后原核移植胚胎的发育率相近，且两组均能形成外观正常的囊胚，但复制后原核移植会改变胚胎的基因表达谱。进一步分析发现，复制后原核移植胚胎的囊胚质量存在显著差异。因此，尽管原核移植是减轻缺陷卵细胞质对早期发育负面影响的优选方案，但本研究结果表明，该干预手段的实施时机应严格限制在复制前阶段。整体实验设计：将每个实验条件下受精后34小时采集的3枚胚胎混合，用于文库构建，随后进行RNA测序分析。SPN：假手术原核移植（sham pronuclear transfer）；EPN：早期原核移植（early pronuclear transfer）；LPN：晚期原核移植（later pronuclear transfer）。其中SPN作为对照组，EPN与LPN分别代表两种不同时机的原核移植操作。