Restoration of normal implantation rates in mouse embryos with a hatching impairment by use of a new method of assisted hatching

Abstract

To use a newly developed mouse model for embryos with a hatching defect to develop improved methods of assisted hatching. We had previously demonstrated that mouse embryos with reduced cell numbers, when resident within an intact zona, exhibit a hatching deficiency, and we used these "hatching-defective" embryos to demonstrate that hatching is mediated predominantly, if not entirely, by lysis of the zona pellucida. In the present study, the hatching-defective embryos were implanted into pseudo-pregnant females to determine if implantation rates were reduced. In addition, to compensate for reduced zona lysin production, the zonae of some embryos were circumferentially thinned with acid Tyrode's solution. The implantation rates and developmental potential of hatching-defective embryos subjected to zona thinning were compared with those of the hatching-defective embryos implanted without zona thinning. University-based basic research laboratory. When hatching-defective embryos, created by destruction of 1/4 of the blastomeres at the four-cell stage, were implanted into pseudopregnant female mice, no pregnancies were established, and no implantation sites could be identified after transfer of 41 embryos. However, when hatching-defective embryos were subjected to zona thinning before ET, a normal rate of implantation was observed (18 implantation events of 49 embryos transferred). Fetuses appeared morphologically normal. The difference in implantation rates between hatching-defective embryos subjected to zona thinning and those not treated by zona thinning was highly significant by chi 2 analysis. Failure of embryos with cell numbers reduced by micromanipulation to implant and restoration of normal implantation and development after circumferential zona thinning demonstrates that the developmental defect of the manipulated embryos is confined to hatching, and thus, that this procedure establishes a useful model for defective hatching. High rates of implantation after circumferential thinning of the zona shows that this procedure of assisted hatching is highly effective. These findings have important implications for use of assisted hatching in human IVF because the assisted hatching procedure is safe and effective and increases the efficiency of the normal hatching mechanism. Thus, the procedure may be useful in a wide variety of circumstances for which assisted hatching by zona opening has been found to be ineffective.