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PGD is the
analysis of a
single cell
removed from an
embryo. The
purpose of PGD
is to identify
and select
embryos with the
greatest
potential for
the
establishment of
a healthy
pregnancy.
Reproductive
Science
Institute
performs all of
the IVF services
and embryo
biopsy involved
in PGD. The
genetic testing
is performed by
two
collaborating
laboratories
recognized
worldwide as
experts in the
field of genetic
diagnosis.
To enable the
analysis a
single cell from
an embryo, a
patient first
undergoes an IVF
cycle to produce
multiple eggs
which are
fertilized in
the laboratory.
Approximately
two days after
fertilization, a
biopsy of the
embryo is
performed by an
embryologist at
RSI. At this
early stage of
embryo
development, the
embryo is still
undifferentiated.
This means that
all the cells in
the embryo are
identical and
each has the
potential to
develop into any
type of tissue.
The biopsy
involves the
removal of one
or two cells (blastomeres)
from the 6-8
cell embryo
through
microsurgery.
Following the
biopsy, the
embryo is
returned to
culture for
several days
while the
biopsied cell is
analyzed.
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Diagnostic analysis of
the blastomeres can provide
information about the
chromosomes contained in the
cells or about a specific gene
of interest. Whether the
chromosomes or a gene are the
focus determines what kind of
testing is performed on the
blastomeres. FISH
(florescent in-situ
hybridization) is performed to
provide information about
chromosomes. PCR
(polymerase chain reaction) is
performed to provide information
about a specific gene of
interest. These two types of
testing are discussed below.
FISH (Fluorescent
In-Situ
Hybridization)
Human cells contain 46
chromosomes: 22 matched pairs of
chromosomes plus 2 sex
chromosomes, 2 X chromosomes in
females, or an X and a Y in
males. This normal chromosomal
complement is termed “euploid”.
Any other chromosome number is
considered abnormal or “aneuploid”.
Occasionally an error occurs
in chromosome number when the
egg, sperm, or embryo is formed.
This results in too many or too
few copies of a chromosome or
chromosomes in an embryo, or
aneuploidy. Most aneuploid
embryos fail to make a pregnancy
or result in a miscarriage. Some
however can result in the birth
of a child with birth or
developmental defects. While
there are many errors in
chromosome number, the most
common occur in chromosomes 13,
16, 18, 21, X and Y.
To analyze the chromosomes in
an embryo, the biopsied cells
are fixed onto a glass
microscope slide. The slides are
sent to RMA Genetics where the
FISH is performed. RMA stains
the nucleus of the cell with
fluorescent stains that bond to
sites on specific chromosomes.
The nucleus can then be analyzed
using a fluorescent microscope
to determine the number of each
chromosome present. Not all 23
pairs of chromosomes are
analyzed, only a select panel of
at least 5.
Once the FISH analysis is
complete, RMA advises the IVF
team of the results. Only the
embryos which have been
identified as “euploid” or
normal for the chromosomes
analyzed are considered for
transfer to the uterus or for
cryopreservation.
PCR (Polymerase
Chain
Reaction)
PCR is performed on the
biopsied blastomere(s) to
determine the presence of a
single gene. This is done when a
couple has a significantly
increased risk of conceiving a
child with a severe genetic
disorder. When PCR is to be
performed, the blastomere
obtained at biopsy is loaded
into a tiny tube of medium and
sent to Genesis Genetics
Institute. Genesis Genetics
amplifies the area of DNA of
interest and makes many, many
copies of the DNA through
repeated cycles of DNA strand
separation and replication. The
sample can then be analyzed for
the presence of a specific
sequence
of DNA or gene.
Once the PCR analysis is
complete, Genesis Genetics
advises the IVF team of the
results. Only the embryos which
do not carry the disease gene(s)
are considered for transfer to
the uterus or cryopreservation.
When is Embryo Biopsy and
PGD an Option?
FISH, or aneuploidy testing,
may be appropriate for some
patients. Repeated failure to
achieve a pregnancy after IVF,
repeated pregnancy loss,
advanced maternal age, or
previous conceptions with
chromosomal anomalies may make
aneuploidy testing a useful
adjunct to your therapy. In
addition RSI performs chromosome
testing to determine the gender
of an embryo under certain
circumstances.
PCR is used when a couple has
an increased risk of conceiving
a child with a genetic disease.
When one or both partners carry
the gene for a hereditary
disease, such as Sickle Cell
Disease, Tay Sachs, Cystic
Fibrosis, Huntington Disease,
and many others, PCR can help
the IVF team select the embryos
that are disease-free for
transfer.
Embryo biopsy and PGD may
enable a couple to conceive a
pregnancy with significantly
decreased likelihood of
chromosomal or genetic
abnormalities.