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How PGD is performed
Polar body removal and/or blastomere biopsy
Blastomere biopsy
Blastomere biopsy (also known as embryo biopsy) is a
technique that is performed by removal of one or two cells (blastomeres)
from the 4 to 8 cell pre-embryo stage for the purpose of
preimplantation analysis. The egg will typically be
fertilized by using intracytoplasmic sperm injection or ICSI.
Following fertilization, the zygote begins to divide.
On the third day following the egg retrieval, the embryo is
at the blastomere stage, and a cell may be carefully removed
for genetic analysis. With the embryo maintained in position
by a pipette with rounded margins, an opening is performed
in the embryo by using a laser device or thyroid acid: Once
realized the hole, a new micropipette having a greater
diameter than the first is positioned: this will consent, by
means of aspiration, the removal of a cell(s) that will be
then be released by applying negative pressure. At this
early point of embryo development, all of the cells are
equivalent and thus, removal of a cell from the embryo at
this stage does not remove anything critical for normal
development. The embryo compensates for the removed cell and
should continue to divide following blastomere biopsy.
Therefore, if the technique is carried out correctly, there
are no risks for the embryo.
After removal of the cell(s) from the blastomere, the
developing embryo is placed back into the culture dish and
the removed cell(s) is inserted into a test tube for
subsequent genetic analysis.
Polar Body Removal
As indicated above, the first polar body is produced from
the division of the egg and can be removed and tested for
its chromosome complement or to identify whether it contains
the abnormal gene of concern.
Upon penetration of the egg by the sperm (fertilization),
but prior to the joining of the sperm’s genetic material
with the egg’s genetic material, the egg undergoes another
cell division, producing two unequally sized cells. The
larger cell will join with the sperm’s genetic material to
create the pre-embryo, and the smaller cell is called the
second polar body. The polar bodies have no known function
except to assist in cell division. They are simply
“by-products” of the egg’s division. Once implantation
occurs, the polar bodies disintegrate and are not part of
the developing fetus.
By testing the first and second polar bodies, the genetic
make-up of the egg, and maternal genetic contribution in the
resultant embryo, can be determined. Removal and genetic
analysis of the polar bodies occurs on the first and second
day after aspiration. In some instances, it is necessary to
confirm a diagnosis made on polar body analysis by
performing blastomere biopsy. It is also possible that one
or more polar bodies fail to provide a conclusive result. In
these situations, it may be possible to perform blastomere
biopsy (embryo biopsy) for further genetic analysis.
Next :
Preimplantation Genetic - Diagnosis in blastomeres
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