The selected Spermatozoa when it reaches the ovulated Cumulus Oocyte
Complex after ovulation, dispersion of the granulosa cells and corona radiata cells
occur. The Spermatozoa then must cross the Zona Pellucida (ZP), fuse with the
oolemma, and then subsequently fertilize the oocyte. Embryologist Karl Ernst von Baer
coined the term ‘Zona Pellucida’ from Greek work Zone which means belt or girdle
and Latin work Pellucida which means transparent or shining. This extracellular matrix
is about 13-15 um thick and surrounds all the mammalian eggs and pre-implantation
embryos. Zona Pellucida structure is made up of carbohydrates, specific proteins,
glycoproteins, hyaluronic acid, heparin, collagen, and fibrous proteins. Human Zona
Pellucida contains 4 glycosylated proteins namely ZP1, ZP2, ZP3, and ZP4. ZP plays
an important role in helping oocytes to transport essential nutrients and helps in
avoiding polyspermy by hardening after fertilization. The embryos must break open the
protective ZP layer to the implant, the process is called hatching. It is said that in
Assisted reproductive treatment (ART) factors such as the non-availability of enzymes
from the endometrium which helps in hatching, extended culture, vitrification may lead
to failure in the hatching of embryos from ZP. It was postulated that micromanipulation
of ZP to create an opening will help the embryos to hatch and thus implant and will
lead to an increase in Implantation rates (IR). This process was later called Assisted
Hatching (AH). Various methods were discovered for Assisted hatching such as
mechanical ZP AH, zona digestion using enzymes, and laser-Assisted hatching. This
chapter will focus on the advantages and disadvantages of each method of AH and their
applications in ART along with the impact of AH on clinical outcomes. The use of any
method of AH should be chosen carefully to avoid damage to the embryo which will
defy the whole purpose of application of AH. In any case, laser-assisted hatching is
widely used for Pre- Implantation Genetic Testing (PGT) of the embryos as it is very
safe if applied properly, convenient, easy to use, and faster compared to other methods
of AH. Each laboratory should identify the correct time and stage at which application
of AH is considered based on whether it is helping to improve clinical rates or not.
Keywords: Assisted hatching, Chemical hatching, Laser-assisted hatching, Mechanical hatching, Zona Drilling, Zona Pellucida.
