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Assisted Reproductive Technology

Selective Assisted Hatching

Robert B. Shabanowitz, PhD
The human egg is surrounded by a non-cellular layer called the zona pellucida (ZP). The zona pellucida remains structurally intact during ovulation, fertilization and early preimplantation embryonic development, and therefore serves both as a barrier for the freshly ovulated ovum, and after fertilization, as a protective structure for the developing embryo.

Fertilization of the ovum requires attachment to, and penetration of the ZP by the spermatozoon. Normal sperm cells contain enzymes that digest the ZP as the sperm penetrates this structure to reach the egg. Once the egg is fertilized, in the normally dividing and growing embryo the ZP eventually becomes very thin. This may not only be due to a stretching and thinning of the ZP as the embryo increases in size, but also may be due to thinning of the ZP by the action of embryonic enzymes that digest the ZP.

Implantation of the growing embryo into the uterine wall normally takes place several days after a Day 2 embryo transfer (two days after egg retrieval). However, in order for the embryo to implant, it must first "hatch" from the ZP.

There is current evidence that in some cases impaired or failed implantation is directly related to embryos that fail to "hatch" from the ZP, or when the process of "hatching" is delayed. In these studies, implantation was improved by dissolving a small hole in the ZP prior to embryo transfer. This procedure, called assisted hatching, allows the growing embryo to eventually "hatch" from the region where the hole in the ZP was artificially formed.

However, assisted hatching should not be performed on every embryo. In fact, studies have shown that when assisted hatching is performed on healthy embryos, it can actually reduce the embryos ability to implant. Therefore, selective assisted hatching is the preferred method. In selective assisted hatching, only embryos pre-selected according to specific embryonic criteria are chosen for micromanipulation. This procedure is also limited to a specific group of infertility patients. 

Patient criteria include:

Women 38 years of age or older
Previous failed implantation
Day three FSH >15

Embryonic criteria include:

ZP >17µm in thickness
Low developmental rate (<5 cells on Day 3)
Excessive fragmentation (>20%)

When selective assisted hatching is a consideration, embryo transfer may be performed on Day 2 (Day 0 is day of egg retrieval) if the embryo quality is good and none of the above criteria are met, or Day 3 if some embryos meet criteria. Not every embryo will be micromanipulated, only those that meet the above criteria. Accordingly, if none of the embryos meet these criteria, no assisted hatching will be performed.

Below is a diagram to demonstrate the process of assisted hatching. Only those embryos having an abnormally thickened ZP, low embryonic cell numbers or >20% fragmentation will be chosen for assisted hatching. These embryos will be micromanipulated under the microscope, and using a very small glass pipet, Tyrode's solution will be used to slowly dissolve an opening in the ZP as depicted in steps A-D.

fertility diagram