Hatching is the process by which the expanded blastocyst breaks through and escapes from the zona pellucida. It must occur before implantation into the endometrium is possible. Hatching of healthy blastocysts in vitro usually occurs between post insemination days 7 and 8.
Hatching begins with the gradual accumulation of fluid in the blastocystic cavity and expansion of the blastocyst. The fluid accumulation causes increased pressure on both the trophoblast and the zona pellucida. Simultaneously, trophoblast cells proliferate to form a cohesive, single layer. The blastocyst contracts and expands intermittently. Proper expansion of the blastocyst by intake of fluid into the blastocystic cavity causes an increase in internal hydrostatic pressure that stretches the trophoblast epithelium. The blastocyst enlarges and its volume increases two- to three-fold causing the zona pellucida to thin. The blastocyst usually hatches out at the pole opposite the inner cell mass. The hatching locus is indicated by the appearance of a group of large, rounded, plump trophoblast cells called zona-breaker cells.
Usually the trophoblast cells begin to herniate through the zona pellucida first just prior to hatching. Once an opening occurs, the blastocyst begins to protrude through. After about 50% of the blastocyst has hatched out, the remaining process is completed quickly. After the zona pellucida ruptures the eventual size of the opening produced is about one-quarter to one-third of the circumference of the capsule so that blastocyst escape is easy and rapid. During hatching and immediately afterwards, collapse and expansion of the blastocyst is a common phenomenon.
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