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How is PGD done? After IVF, on the third day, the eight-cell embryo is biopsed to obtain blastomeres (single cells) for molecular diagnosis. An embryo biopsy is done using micromanipulators under the visual control provided by an inverted tissue culture microscope. The embryo is held in position using a holding pipette, while a fine needle is used to drill a hole through the zona pellucida (the shell or the outer layer of the embryo) using acid Tyrode's or with a laser. A single cell is then removed by gentle suction. The cell (called a blastomere) is then available for genetic diagnosis.
Fig 1. Embryo biopsy, with a single blastomere being sucked out from the 8-cell embryo. This will be sent for analysis.
Analysis of genetic material (DNA) from a single cell is performed either using a technique called FISH (fluorescent in situ hybridization) or PCR (polymerase chain reaction). FISH utilizes fluorescent probes, which are specific for a given chromosome, and therefore allows one to screen embryos for chromosomal normality. PCR on the other hand allows one to amplify (multiply) a selected DNA sequence of interest, so that it can be analyzed. While the genetic analysis on the single cell, is being performed which can take 4 - 24 hours, the embryos are kept in culture and allowed to further divide. Once the appropriate molecular diagnosis is made, unaffected embryos can be transferred back into the uterus in the IVF cycle.
PGD is now also being used in order to increase pregnancy rates for older infertile women. One of the reasons older women have a poorer pregnancy rate is because their embryos are often chromosomally abnormal, because of the fact they have older eggs (which may have genetic defects). PGD allows the doctor to select only the chromosomally normal embryos, so that only these can be transferred back into the uterus, resulting in a higher pregnancy rate.
© Dr. Aniruddha Malpani and Dr. Anjali Malpani www.drmalpani.com
Credits: How to Have a Baby: Overcoming Infertility
