Assisted hatching of embryos is a microscopic process where the outer layer of the embryos
The zona pellucida (ZP) is a non-cellular membrane that encloses an egg. It plays an important role in maintaining the integrity of the cleaving embryo during its development up to implantation. The ZP prevents the separation of the blastomeres, the individual cells of the cleaving embryo. The ZP may also be needed to prevent adherence of the embryo to the oviductal walls during its transit to the uterus. Finally, the ZP protects the embryo from hostile uterine factors, such as immune cell invasion or toxins released by noninvasive immune cells.
The embryo must escape (hatch) from the zona pellucida before implantation may occur.
By 5 or 6 days after fertilization, the embryo has developed into a blastocyst, a 32-64 cell embryo that has expanded forming a hollow sphere. The blastocyst accomplishes hatching by a combination of enzymatic thinning of the ZP and contraction and re-expansion of the blastocyst. This causes the ZP to become extremely attenuated and almost invisible, and results in the rupture of the ZP. The blastocyst escapes and implantation may occur.
Several factors may inhibit or delay hatching of the blastocyst from the ZP. These include an unusually thick or dense ZP, egg or embryo cryopreservation which causes the ZP to become brittle, poor embryo quality, and delayed growth. Any of these factors may be inherent in embryos from a select group of patients. These include older patients (38 years of age and older), patients with diminished ovarian function (DOR), or patients with repeated IVF cycle failures. Assisted hatching before embryo transfer may facilitate the natural hatching process in these patients.
Assisted hatching is performed on embryos prior to embryo transfer. A high-magnification microscope and delicate micromanipulation instruments are used to create an opening in the ZP. The blastocyst is able to use this opening to begin its escape from the ZP. This procedure has improved the outcome for IVF patients in this select group of patients.