Zona hardening, zona drilling and assisted hatching: New achievements in assisted reproduction

Prior to fertilization, the zona pellucida surrounding the mammalian oocyte acts as a species-specific sperm barrier and is involved in sperm binding. After fertilization, the zona plays a role in blocking polyspermic fertili zation, it protects the integrity of the preimplantation embryo during earl y embryonic development, and also helps its oviductal transport. Zona harde ning occurs naturally after fertilization in order to ensure this threefold function. A combination of lysins produced by the cleaving embryo or the u terus and physical expansion then reduces the zona thickness in preparation for hatching. Zona hardening, although not readily quantifiable, may also be induced by in vitro culture and by in vivo aging. Indeed, prolonged expo sure of human oocytes and embryos to artificial culture conditions seems to impair their ability to implant. Implantation rates are also inversely cor related with advanced female age. Recently, failure of the embryonic zona p ellucida to rupture following blastocyst expansion has been put forward as a possible contributing factor in implantation failure, in order to help em bryos escape from their zonae during blastocyst expansion, different types of assisted hatching have been developed. Zona drilling involves the creati on of an opening in the zona with acidified medium, whereas zona slitting i s carried out in the same manner as partial zona dissection. In zona thinni ng, the zona is just made thinner over a certain area without a hole or a s lit being created. More recently, laser-assisted hatching has been introduc ed. In vitro studies with both mouse and human embryos have indicated that an artificial gap in the zona pellucida significantly improves the hatching ability of blastocysts grown in vitro as compared to non-micro-manipulated embryos. However, the clinical relevance of assisted hatching within an as sisted reproduction program remains controversial and elusive. Very few ran domized studies are available. Most reports are of retrospective analyses w hich report either no differences in implantation and pregnancy rates betwe en assisted hatching and control embryos or better results after assisted h atching. Five randomized controlled studies suggest that assisted hatching - of no benefit to the overall patient population - might be of value in in creasing embryo implantation rates only in selected cases. No further evide nce exists for an age-related benefit from assisted hatching in patients wi th advanced maternal age. Copyright (C) 2000 S. Karger AG. Basel.