Introduction

Several factors affect the chances of conception following treatment with conventional in vitro fertilization (IVF) or any of its variants. However, while the negative influence of factors such as increasing age of the female is known, many others still remain unknown. The normal process of human embryo hatching seems to be another one of those factors. The embryo must ‘hatch’ through the zona pellucida before it can implant. Although the exact mechanisms are not fully understood, this may involve the production of compounds, called lysins, by the embryo and/or endometrium that dissolve the zona pellucida thereby reducing its thickness. The blastocyst then expands and contracts repeatedly leading to further thinning and eventual rupture of the zona pellucida. It is possible that a defective hatching process is a cause of failed implantation in some women who have IVF treatment. The most common hypothesis to explain this phenomenon is that of abnormal zona hardening. Following fusion of the sperm with the oolemma of the oocyte the chemical constitution of the zona pellucida changes somewhat and in so doing prevents penetration by more spermatozoa. The process of excess zona hardening can occur spontaneously and to a greater degree following gonadotrophin therapy and in vitro culture. Another reason for non-hatching of the cleaving embryo can be the presence of an unduly thick zona pellucida.