Female fertility relies on successful egg development. Besides chromosome segregation, complex structural and biochemical changes in the cytoplasmic compartment are necessary to confer the female gamete the capacity to undergo normal fertilization and sustain embryonic development. Despite the profound impact on egg quality, morphological bases of cytoplasmic maturation remain largely unknown. Here, we report our findings from the ultrastructural analysis of 69 unfertilized human oocytes from 34 young and healthy egg donors. By comparison of samples fixed at three consecutive developmental stages, we explored how ooplasmic architecture changes during meiotic maturation in vitro. The morphometric image analysis supported observation that the major reorganization of cytoplasm occurs before polar body extrusion. The organelles initially concentrated around prophase nucleus were repositioned toward the periphery and evenly distributed throughout the ooplasm. As maturation progressed, distinct secretory apparatus appeared to transform into cortical granules that clustered underneath the oocyte's surface. The most prominent feature was the gradual formation of heterologous complexes composed of variable elements of endoplasmic reticulum and multiple mitochondria with primitive morphology. Based on the generated image dataset, we proposed a morphological map of cytoplasmic maturation, which may serve as a reference for future comparative studies. In conclusion, this work improves our understanding of human oocyte morphology, cytoplasmic maturation, and intracellular factors defining human egg quality. Although this analysis involved spare oocytes completing development in vitro, it provides essential insight into the enigmatic process by which human egg progenitors prepare for fertilization.

中文翻译：

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人卵母细胞的细胞质成熟：超微结构研究†

女性的生育能力依赖于成功的卵子发育。除了染色体分离之外，细胞质区室中复杂的结构和生化变化对于赋予雌配子进行正常受精和维持胚胎发育的能力是必要的。尽管对卵子质量有深远的影响，但细胞质成熟的形态学基础仍然很大程度上未知。在这里，我们报告了我们对来自 34 名年轻和健康卵子捐赠者的 69 个未受精人类卵母细胞的超微结构分析结果。通过比较固定在三个连续发育阶段的样本，我们探讨了体外减数分裂成熟过程中卵质结构的变化。形态测量图像分析支持观察到细胞质的主要重组发生在极体挤出之前。最初集中在前期细胞核周围的细胞器被重新定位到外围并均匀分布在整个卵质中。随着成熟的进行，不同的分泌装置似乎转变为聚集在卵母细胞表面下方的皮质颗粒。最显着的特点是逐渐形成由内质网可变元件和形态原始的多个线粒体组成的异源复合物。基于生成的图像数据集，我们提出了细胞质成熟的形态学图，可作为未来比较研究的参考。总之，这项工作提高了我们对人类卵母细胞形态、细胞质成熟和决定人类卵子质量的细胞内因素的理解。