Hatching of the Human Blastocyst
The human zona pellucida (ZP) is a gelatinous, extracellular matrix that surrounds oocytes and embryos until hatching and implantation. It is 15 to 20 μm thick and is formed in the early stages of ovarian follicular development by secretions from the oocyte and follicle granulosa cells.1 It is a multilaminar structure composed primarily of four glycoproteins, designated ZP1, ZP2, ZP3, and ZP4, that are synthesized in primordial oocytes with each having specific roles in fertilization.2 As well as regulating spermatozoa-egg interaction, the ZP has a critical role in gamete recognition and prevention of polyspermy. It also protects the embryo from immune cells and, acting as a physical retainer, prevents blastomeres dispersing during preimplantation development.3 During formation of the blastocyst the embryo remains enclosed in the ZP. Prior to implantation, however, the expanded blastocyst must emerge out of the ZP to attach, adhere, and invade the receptive female uterine endometrium.4 The emergence of the blastocyst from the ZP is referred to as hatching. Video 11.1 shows a blastocyst expanding and then hatching through the zona pellucida. The video can be viewed online: http://goo.gl/y0zYFA.