A new mouse model for embryos with a hatching deficiency and its use to elucidate the mechanism of blastocyst...

To develop an experimental model for embryos with a defect specific to hatching, with the purpose of clarifying the mammalian embryo hatching mechanism.

A microneedle was inserted under the zona pellucida (ZP) of mouse embryos, and either 1/2 or 1/4 of the blastomeres were mechanically destroyed. The remainder of embryos that developed into blastocysts were compared for hatching to controls wherein a microneedle was inserted and withdrawn without harming the embryo. Experiments were done to increase pressure within the perivitelline space to decrease the amount of ZP material to study the hatching mechanism.

University-based basic research laboratory.

When 1/2 or 1/4 of the embryo was destroyed and the remaining cells developed into healthy blastocysts within the intact zona, hatching was significantly impaired, and zona thickness was markedly increased relative to controls. When a mineral oil droplet was inserted under the zona to enhance a possible mechanical component of hatching, manipulated embryos were not restored to normal hatching, and hatching of unmanipulated blastocysts was not improved. However, when the zona was circumferentially thinned by application of acid Tyrode's solution, the hatching defect in manipulated "3/4 embryos" was corrected.

Because it is known that embryos with cells reduced by 1/2 or 1/4 have normal developmental potential, we conclude that the inability of such embryos to hatch from an intact zona constitutes a mouse model for a defect specific to hatching. Moreover, results from mineral oil droplet insertion and circumferential thinning of the zona indicate that normal hatching is accomplished predominantly, if not entirely, by zona lysis, not by pressure exerted against the zona by the expanding blastocyst.