Amy E. Jones

Assisted Hatching in Clinical IVF

One of the persistent mysteries in clinical in vitro fertilization (IVF) is why embryos that appear to be developing normally throughout the preimplantation stages fail to implant after uterine transfer. While it has been proposed that a certain proportion of failures may have a genetic or chromosomal basis, it has also been suggested that the inability of the embryo to emerge from the confines of the zona pellucida at the expanded blastocyst stage by a process commonly termed ‘hatching’ may render an otherwise normal embryo incompetent to establish a pregnancy owing to the persistence of a physical barrier between embryo and a receptive endometrial epithelium. The notion that the zona pellucida may be refractory to hatching is derived from observations of human embryos cultured in vitro through the blastocyst stages where some proportion of embryos within cohorts cultured in the same dish remain entrapped within the sibling blastocysts hatch and emerge completely. Although reasons why differential resistance to hatching may exist between embryos in the same and different cohorts is unknown, physical and biochemical differences in zona structure, zona hardening in response to culture conditions that are presumed to be suboptimal, and embryo defects which prevent an active participation in emergence have been suggested.