Nolte Thomas, Halabian Reza, Israel Steffen, Suzuki Yutaka, Avelar Roberto A, Palmer Daniel, Fuellen Georg, Makalowski Wojciech, Boiani Michele
Department of Cell and Tissue Dynamics, Max Planck Institute for Molecular Biomedicine, Münster, Germany.
Institute of Bioinformatics, Faculty of Medicine, University of Münster, Münster, Germany.
Mol Hum Reprod. 2025 Jan 17;31(1). doi: 10.1093/molehr/gaae045.
Recent advances in embryology have shown that the sister blastomeres of two-cell mouse and human embryos differ reciprocally in potency. An open question is whether the blastomeres became different as opposed to originating as different. Here we wanted to test two relevant but conflicting models: one proposing that each blastomere contains both animal and vegetal materials in balanced proportions because the plane of first cleavage runs close to the animal-vegetal axis of the fertilized oocyte (meridional cleavage); and the other model proposing that each blastomere contains variable proportions of animal and vegetal materials because the plane of the first cleavage can vary - up to an equatorial orientation - depending on the topology of fertilization. Therefore, we imposed the fertilization site in three distinct regions of mouse oocytes (animal pole, vegetal pole, equator) via ICSI. After the first zygotic cleavage, the sister blastomeres were dissociated and subjected to single-cell transcriptome analysis, keeping track of the original pair associations. Non-supervised hierarchical clustering revealed that the frequency of correct pair matches varied with the fertilization site (vegetal pole > animal pole > equator), thereby, challenging the first model of balanced partitioning. However, the inter-blastomere differences had similar signatures of gene ontology across the three groups, thereby, also challenging the competing model of variable partitioning. These conflicting observations could be reconciled if animal and vegetal materials were partitioned at the first cleavage: an event considered improbable and possibly deleterious in mammals. We tested this occurrence by keeping the fertilized oocytes immobilized from the time of ICSI until the first cleavage. Image analysis revealed that cleavage took place preferentially along the short (i.e. equatorial) diameter of the oocyte, thereby partitioning the animal and vegetal materials into the two-cell blastomeres. Our results point to a simple mechanism by which the two sister blastomeres start out as different, rather than becoming different.
胚胎学的最新进展表明，二细胞期小鼠和人类胚胎的姐妹卵裂球在发育潜能上呈相互差异。一个悬而未决的问题是，卵裂球是一开始就不同，还是后来变得不同。在这里，我们想要测试两个相关但相互矛盾的模型：一个模型提出，每个卵裂球都含有比例均衡的动物极和植物极物质，因为第一次卵裂平面靠近受精卵母细胞的动物极 - 植物极轴（经线裂）；另一个模型提出，每个卵裂球含有比例可变的动物极和植物极物质，因为第一次卵裂平面会因受精拓扑结构而变化，直至达到赤道方向。因此，我们通过胞质内单精子注射（ICSI）将受精位点设定在小鼠卵母细胞的三个不同区域（动物极、植物极、赤道）。在第一次合子分裂后，将姐妹卵裂球分离并进行单细胞转录组分析，同时记录原始的配对关系。非监督层次聚类分析显示，正确配对的频率随受精位点而变化（植物极>动物极>赤道），从而对平衡分配的第一个模型提出了挑战。然而，三组中卵裂球间的差异在基因本体论上具有相似的特征，因此也对可变分配的竞争模型提出了挑战。如果动物极和植物极物质在第一次卵裂时就被分配，那么这些相互矛盾的观察结果就可以得到调和：这一事件在哺乳动物中被认为不太可能发生，而且可能有害。我们通过从ICSI时起直至第一次卵裂将受精卵母细胞固定不动来测试这种情况的发生。图像分析显示，卵裂优先沿着卵母细胞的短（即赤道）直径进行，从而将动物极和植物极物质分配到二细胞期的卵裂球中。我们的结果指出了一种简单的机制，即两个姐妹卵裂球一开始就不同，而不是后来变得不同。
