The zona pellucida (ZP) of human eggs and embryos is an outer acellular layer with different roles during fertilization and embryo development. The main function of the ZP after fertilization is the protection of the embryo against hostile uterine factors and the maintenance of its integrity during the migration through the reproductive tract. However, once the embryo reaches the uterus it must get out of the ZP (hatching) in order to attach to the womb. In some cases, after fertilization zona pellucida becomes unusually hard, thus inhibiting embryo hatching and reducing the chances of implantation.
Assisted hatching is a relatively new technique used during certain IVF procedures. The ability of the embryo to ‘hatch’ from its protective shell is a crucial step in enabling it to implant in the uterus. It is performed in order to help an embryo hatch out of its protective layering and implant into the uterus.
Sometimes, embryos have difficulty in hatching out of their protective layer. This can occur if the zona pellicuda is too thick or if the embryo does not have enough pressure from the cells to break through the layer. Assisted hatching is attempted to help these embryos break out away from the zona pellicuda by creating a small hole in this outer lining.
There are many known methods used for assisted hatching. One procedure entails the opening of a small hole to the outer layer of the embryo, using laser technology. It is important that the hole created in the ZP is large enough to avoid trapping of the embryo during hatching, but not so large that it allows blastomere loss. Another method involves the thinning of the zona, using special enzymes, without complete lysis and perforation. Both procedures are performed in the lab by the embryologists using special equipment, for embryo manipulation, under the control of high-powered microscopes. Assisted hatching is performed separately for each embryo.
Because the shell is not a living part of the embryo, penetrating this outer structure poses virtually no threat. Studies indicate that assisted hatching using laser technology is superior to chemical assisted hatching, in which the egg is penetrated with an acidic solution. Laser technology enables far more precision and is also done more quickly, thus reducing the length of time that embryos are handled before transfer. The laser also enables embryologists to prevent excessive heat exposure, thus eliminating virtually all risk to the embryo.
Assisted hatching techniques are required for some couples. Instead, the procedure is typically recommended for : -
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