The effects of assisted hatching on the hatching process and implantation

The rationale for assisted hatching Among the numerous reasons that could explain the low implantation rate of embryos used for in-vitro fertilization (IVF), an impairment of in-vivo hatching has been proposed as a possible cause of implantation failure by Cohen et aI. (1990a). Indeed, this group had shown that implantation rates of human embryos were correlated to the variability of zona pellucida thickness, ranging from 10% for embryos with uniform zonae to 29% for embryos with a thin or irregular zona pellucida (Cohen et aI., 1989). They also reported that micromanipulated embryos taken from couples with male problems implanted at higher rates than intact embryos, and that 25% of human embryos displayed zona pellucida thinning when proceeding from the zygote to the 4-cell stage (Wright et aI., 1990). In vitro, any effect of uterine fluid in digesting the zona pellucida is lacking, and only 25% of human blastocysts achieve hatching in culture (Lindenbergh et aI., 1989; Dokras et at., 1991), even when they look normal and secrete adequate amounts of human chorionic gonadotrophin (HCG) (Woodward, 1993). In vivo, however, how can hatching be impaired? Several hypotheses have been proposed: these include extrinsic factors such as zona pellucida hardening in culture, as described by De Felici and Siracusa (1982) in mice, or intrinsic factors of embryonic or uterine origin. The lytic action of uterine fluid may be impaired in some patients or in some cycles. Likewise, some embryos may be unable to escape from their zonae. Moreover, a thick or hard zona pellucida may hinder the hatching process. Cohen et at. (1990a) reported that 15% of IVF embryos displayed a thick zona pellucida, on average> 15 /-lm. It is more difficult to assess zona pellucida hardness, as did Longo (1981) in mice, by calculating the time of exposure to trypsin necessary to digest the zona pellucida and so detected an increased hardness in ageing eggs. In humans, too, it is apparent that spontaneous zona hardening can occur within 24 h of in-vitro culture, and involves similar changes to the zona as after fertilization. However, this zona hardening does not increase further whereas there is no correlation between zona hardness and zona thickness or patient age (Schiewe et at., 1995a).