In-vitro fertilization is among the most widely used techniques to deal with infertility and to help couples become pregnant. Although it has many success stories to its credit, IVF is sometimes inefficient, and this is usually due to implantation failure. Failure to implant occurs because of several factors, including genetic abnormalities within the embryos, less-than-optimal culture conditions, and anomalies in the zona pellucida.
The zona pellucida, or egg coat, is the glycoprotein membrane that surrounds the plasma membrane of an oocyte. It is involved in the acrosome reaction, during which the sperm is bound to the oocyte, and subsequent fertilization.
Under normal conditions, upon fertilization, the zona pellucida transforms into a hard coat, preventing fertilization by another sperm cell. This hard coat protects the developing embryo, maintaining its integrity. Upon entering the uterus, the embryo, which by now has divided several times to form a blastocyst, must implant on the uterine walls. The first step of this process is hatching, where the blastocyst exits the zona pellucida to interact with the trophectoderm and endometrial layer of cells.
Assisted hatching is a technique used to overcome cases of impaired hatching, which may be caused by sub-optimal in vitro conditions. Assisted hatching is the creation of a gap in the zona pellucida, about 30-40µm in size. It is indicated in patients wih elevated FSH at day 3, advanced maternal age, embryos with a thick zona pellucida, and embryos that are highly fragmented or slow to divide. This technique is not without pitfalls however, thus the advent of several modifications, such as acid Tyrode hatching, laser hatching, atraumatic assisted hatching via pronase thinning of the zona pellucida.
Atraumatic hatching and partial zona dissection is the process of thinning the zona pellucida without resulting in lysis or perforation. This reduces the risk of blastomere loss as well as infection to the embryo.
The steps to this process are as follows: First, a solution of pronase is used. The IVF specialist transfers the embryo to the pronase solution for a brief period of around 60 seconds. This aims to soften and stretch the zona pellucida. Then the embryo is examined for any expansion of the zona as well as enlargement of he perivitelline space. This process is repeated if necessary.
Once the embryos meet the proper criteria for thinning, they are transferred to fresh G2 medium and prepared for incubation. Hereafter, incubation takes place until the embryos are ready to be transferred.