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Submitted by Dr. Hesham Al-Inany, M.D. Lecturer, Gynaecology
& Obstetrics dept. Kasr El-Aini hospital, Cairo University, Egypt.
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During Gamete Micromanipulation sperms are injected into an ovum to assist in
union of the gametes.
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Assisted hatching
The human zona pellucida has many biologic roles. Either these are
temporarily brief, such as the species-specific binding of sperm to the
oocyte and the subsequent block to polyspermy, or they exist over longer
periods, principally as a mechanical barrier to prevent disruption of
the oocyte or embryo within it from immune cell attack, physical squeezing,
or temporary biochemical toxicity ( Tucker et al, 1991).
Recently, certain techniques of ART have highlighted the major functional
aspects of the ZP. The rate of successful implantation after replacement
of frozen-thawed embryos in IVF is commonly only 5% to 10% per embryo.
A limiting factor may be the inability of viable embryos to be released
from the intact zonae pellucidae. Culture condition and/or cryopreservation
may affect the zona and impair blastocyst hatching (Tucker et al, 1991).
Cohen et al (1990), have proposed that opening of the zona by partial
slicing by means of micromanipulation before replacement of early cleaved
embryos may improve chances of eventual hatching.
The risks of damage from the micromanipulation procedure are minimal,
for >100 embryos were micromanipulated by Cohen et al 1990, none of the
blastomeres were damaged. The incidence of cell damage is 4% following
PZD in oocytes, due to the presence of corona cells. In addition, the
embryonic zona is easier to pierce and manipulate than the unfertilized
zona.
A possible negative side-effect of assisted hatching may be the increased
risk of triplets or higher order pregnancy. The embryos obviously follow
the route of the least resistance and expel through the artificial gap.
The ZP does not thin , expansion of the blastocyst does not occur. This
result in constriction of the trophoblast and inner cell mass . This mechanism
also explains the increased frequency of dizygotic twins.
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Technique
Initially, the technique was identical to that described for partial
zona dissection of unfertilized oocytes except there is no need to remove
cumulus cells with hyaluronidase, and the embryo is not exposed to sucrose
to shrink it and so increase the perivitelline space.
This method was applied prospectively in 99 consenting patients in randomized
fashion (Cohen et al, 1990). Though the pregnancy rate increased by 20%
and the embryonic implantation nearly doubled, partial zona dissection
is no longer the method utilized for assisted hatching .
The holes created with this technique may be too small to allow normal
completion of hatching. Furthermore, results of some clinical follow-up
studies have been disappointing. In mouse studies, it was shown that one
or several large holes introduced by zona drilling was more favorable
to the integrity of the embryo, than the introduction of a small slit
in the zona formed by partial zona dissection ( Cohen et al, 1991 ) .
High rates of implantation following biopsy of eight-cell human embryos
using acidic Tyrode's have been reported (Handyside et al, 1990). Biopsied
embryos were replaced at the time of initial compaction.
Cohen et al, 1991 suggested that it may be advantageous to transfer embryos
with substantially large holes in their zonae after increased blastomere
adherence has been initiated.
This technique has subsequently been used with cryopreserved embryo after
thawing to determine if chances of implantation can be improved in such
cases.
In 65 thawed embryo replacement cycles methyl-prednisolone and antibiotics
were given for 4 days midcycle. Assisted hatching was performed in 32
cycles left as controls. Patient's age, infertility, cycle supplementation
and number of thawed and replaced embryos did not differ significantly
between the two groups.
Rates of viable embryonic implantation were 16% (10/63) and 9% (6/64)
in the assisted hatching and control groups, respectively ( Tucker et
al, 1991).
The use of zona drilling on day 3 of embryonic development was investigated
in three fully randomized prospective trials in 330 consenting patients.
The incidence of clinical pregnancy ( fetal heart beat per patient) increased
significantly from 37% (62/166) in the control group to 52% (85/164) following
zona drilling.
Of the zona drilled embryos, 27% implanted (147/555) and showed fetal
heart activity on ultrasound. This was compared favorably with the control
group in which 19% (104/555) of the embryos implanted.
Selective assisted hatching
In a retrospective study of embryos replaced by Cohen et al, (1991)
has shown that the ZP thickness largely determined the outcome of the
procedure. Control embryos with zonae thicker than > 15 um rarely implanted,
whereas zona-drilled embryos with similar zona characteristics frequently
implanted. The findings also suggested that zona drilling was detrimental
in embryos with thin zonae (<12um). However, the latter evaluation was
not statistically significant.
These retrospective findings were prospectively tested during a recent
study involving 163 patients. Zonae from embryos of patients allocated
to the zona drilling group were measured prior to micromanipulation .
Embryos with thick zonae ( >15 um) were micromanipulated and those with
thin zonae ( <12 um ) were left intact.
The result of this group of patients ( two- thirds of whose embryos were
micromanipulated ) was compared with a control group in which embryos
were never micromanipulated. This process has been called "selected assisted
hatching".
Likewise patients aged 38 years and those with elevated basal FSH levels
(15 IU/L) showed significantly more pregnancies when assisted hatching
was performed in comparison to the controls (15% compared with 5% and
26% compared with 10% respectively). These results may indicate a possible
correlation between age, basal FSH levels and physical or chemical changes
in the zona pellucida (Cohen et al, 1992).
Conclusion
A relatively small incision in the ZP is not incompatible
with development and implantation of human embryo. In fact, the results
even suggest that such procedure may improve chances of implantation through
enchanced chances of hatching of the blastocyst.
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