The human
egg/ovum is surrounded by a non-cellular layer called zona pellucida
(ZP). It remains intact during ovulation, fertilization and early
preimplantation embryonic development.
The fertilization
process involves attachment of the sperm to the egg, and penetration
of the ZP. The normal human sperm contains enzymes
that digest the ZP as the sperm penetrates through ZP to reach
the egg. After fertilization ZP closes the gate, so that a second
sperm cannot enter into the egg.
The fertilized
egg i.e. the Zygote undergoes divisions or cleavage to ultimately
form a structure called the blastocyst. The blastocyst then
breaks through the ZP or hatches. After this it embeds or implants
in the endometrium (uterine lining). An inability
of the blastocyst to hatch from the zona pellucida might be one
of the factors involved in the high implantation failure rate
in IVF cycles. Hence the process of hatching
is carried out in selected cases. Assisted hatching has been performed
since early 1990s Selection
of casesWomen > 38 years of age
- Previous
failed IVF cycles
- Basal
level FSH more than 15 IU/ml.
- When
ZP thickness is > 17um.
- Certain
features as shown in the zygote during daily evaluation after
oocyte retrieval in IVF (link).
Methods of
assisted hatching
- Mechanical
method
- Chemical
method
- Laser
method - most commonly used.
Laser
Assisted Hatching
A diode laser is used. The use of laser technique allows good
control and precision during assisted hatching. This procedure
has been shown to be simple, rapid, safe and accurate.
The procedure is performed, using a special microscope,
attached with a micromanipulator and laser system.
Using a laser
beam it is possible to make a standardized hole in the ZP through
which the embryo can escape safely. This is termed zona drilling.
The embryo is then transferred into the uterus as per routine
IVF procedure. The other method of laser hatching is termed zona
thinning. Here no holes are made, instead the thickness of the
zona pellucida is reduced using the laser beam.
Laser thinning
of the zona pellucida has been proposed as being more natural
than laser drilling because it allows blastocyst expansion and
hatching that is more in tune with what happens naturally.
In either
method there is no damage to the embryo/fetus as a result of the
procedure. The incidence of twinning may be slightly higher
in pregnancies resulting from laser assisted hatching
