who have PGD will undergo an in vitro fertilization (IVF)
cycle to create embryos. Genetic analysis will then be
performed on cells from each embryo prior to transfer into
the woman?s uterus. To analyze an embryo, we biopsy the
embryo around the third day of its development when the
embryo has approximately 6-8 cells. One or two cells are
taken from the embryo. The embryo is incubated until testing
The biopsied cells are analyzed using a technique called
fluorescence in-situ hybridization or FISH. This technique
uses probes, small pieces of DNA that are a match for the
chromosomes we want to analyze, to count the chromosomes
The biopsied blastomeres are first fixed to a microscope
slide and then the cellular material is digested away
leaving the nucleus, which contains the DNA, in a spread out
and decondensed form. These cells are then hybridised with
DNA probes, labeled with different fluorochromes. Each of
these probes are specific for part of a chromosome; they
will only attach to their exact DNA match on a particular
chromosome. Excess probe is washed off, and the cell is
examined under the fluorescent microscope. We then count the
number of chromosomes of each type (color) there are in that
cell. The geneticist therefore can distinguish normal cells
from cells with aneuploidy.
couples undergoing IVF and PGD for translocations, the
embryos will first be tested for unbalanced translocations.
If technically possible, we will also perform a second test
for chromosomes 13, 18, 21, X, and Y which are those
chromosomes most likely to result in a liveborn child with a
chromosome abnormality. The FISH analysis involves two
rounds of testing for each cell.
Testing of the cells destroys them because they must be
glued to a glass slide and repeatedly heated and cooled. As
such, one cannot use them for another purpose or return them
to the embryo. The slides are kept for future reference.
This analysis causes no extra inconvenience to the patient
as it is accomplished in one day.
The FISH technique is considered to have an error rate
between 5 and 10%. The main problem of the use of FISH to
study the chromosomal constitution of embryos is the
elevated mosaicism rate observed at the human
preimplantation stage. Sandalinas and collaborators found
that up to 70% of the embryos they studied by FISH were
mosaic for some kind of chromosomal abnormality (Sandalinas
et al., 2001).