ABSTRACT
The zona pellucida is a glycoprotein layer that surrounds oocytes and preembryos during the preimplantation period. A number of authors have demonstrated that the human zona is composed of three acidic glycoproteins called ZP1, ZP2 and ZP31-3. The zona has various possible roles in preimplantation development: prevention of polyspermy, protecting the preembryo from physical or immunological damage and preserving its integrity during cleavage. Although the zona is sometimes described as having roles in sperm binding, in induction of the acrosome reaction and as a matrix for sperm penetration, these last phenomena may be the simple consequence of its presence around the oocyte (personal communication, Michael Bedford). After compaction and formation of cell junctions, this protective coat is no longer necessary for the developing human preembryo, at least under in vitro culture conditions. Hatching is a process that involves blastocyst escape through the zona pellucida, a prerequisite for normal implantation. In the human, implantation is initiated by trophoblastic invasion of the uterine endometrium.
