Fertilization by more than one sperm causes polyploidy, a condition that is generally lethal to the embryo in the majority of animal species. To prevent this occurrence, eggs have developed a series of mechanisms that block polyspermy at the level of the plasma membrane or their extracellular coat. In this review, we first introduce the mammalian egg coat, the zona pellucida (ZP), and summarize what is currently known about its composition, structure, and biological functions. We then describe how this specialized extracellular matrix is modified by the contents of cortical granules (CG), secretory organelles that are exocytosed by the egg after gamete fusion. This process releases proteases, glycosidases, lectins and zinc onto the ZP, resulting in a series of changes in the properties of the egg coat that are collectively referred to as hardening. By drawing parallels with comparable modifications of the vitelline envelope of nonmammalian eggs, we discuss how CG-dependent modifications of the ZP are thought to contribute to the block to polyspermy. Moreover, we argue for the importance of obtaining more information on the architecture of the ZP, as well as systematically investigating the many facets of ZP hardening.

中文翻译：

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哺乳动物的蛋皮被修饰并阻碍了多精子。

一个以上的精子受精会导致多倍性，这种情况通常对大多数动物物种的胚胎都是致命的。为了防止这种情况的发生，卵子已经形成了一系列在质膜或其细胞外被膜水平上阻断多精子的机制。在这篇评论中，我们首先介绍哺乳动物的卵皮，透明带（Zona pellucida，ZP），并总结了目前对其组成，结构和生物学功能的了解。然后，我们描述了这种专门的细胞外基质是如何通过配子融合后卵子胞吐的皮质颗粒（CG），分泌型细胞器的含量来修饰的。此过程将蛋白酶，糖苷酶，凝集素和锌释放到ZP上，导致蛋壳特性发生一系列变化，这些变化统称为硬化。通过绘制与非哺乳动物卵卵黄膜包膜的可比修饰的相似之处，我们讨论了ZP的CG依赖性修饰如何被认为对多精子的形成有贡献。此外，我们主张获得有关ZP架构的更多信息以及系统研究ZP强化的许多方面的重要性。