The ZP of oocytes and eggs is a viscoelastic ECM whose viscosity increases several fold following fertilization ( Kim and Kim, 2013). For example, free-swimming sperm are unable to bind to the ZP of fertilized eggs or cleavage-stage embryos. Shortly after an egg is fertilized, the ZP undergoes changes that affect its physical and biological properties that are important for preventing polyspermy and protecting preimplantation embryos in the female reproductive tract. Scale bar (≃1 cm) = ≃14 µm.ĭuring fertilization, free-swimming sperm recognize and bind to the ZP of unfertilized eggs ( Figure 1) in a species-restricted manner, penetrate through the ZP, and fuse with egg plasma membrane to form a zygote. Sperm are shown bound to the zona pellucida. A ZP consists of long interconnected fibrils laid down by a growing oocyte during oogenesis as ovarian follicles mature and prepare to ovulate an unfertilized egg. Messenger RNA (mRNA) encoding ZP proteins is undetectable in nongrowing oocytes, but as soon as oocytes begin to grow ZP mRNA can be detected and increases to hundreds-of-thousands of copies in fully grown oocytes. As oocytes begin to grow in the ovary the ZP first appears as diffuse fibrils in localized pockets around the surface of growing oocytes and shortly thereafter the fibrils coalesce to form a uniform ZP that continues to thicken as oocytes grow. Fully grown oocytes become unfertilized eggs when released or ovulated from the ovary into the fallopian tube where they may be fertilized by sperm. During oogenesis, small nongrowing oocytes that lack a ZP undergo tremendous growth to give rise to fully grown oocytes that have a ZP ( Figure 1). The zona pellucida (ZP) is a relatively thick extracellular matrix (ECM) that surrounds all mammalian oocytes, eggs, and preimplantation embryos, from monotremes that lay eggs to placental mammals ( Wassarman and Litscher, 2016 Wassarman and Litscher, 2021).

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