PGD (preimplantation genetic diagnosis) enables embryos to be genetically evaluated before they are implanted into the womb. It is applied to day 3 embryos which display 4 to 10 cells and a single nucleus. One or two cells are removed from embryos with suitable morphology with the aid of a biopsy pipette and the appropriate genetic examination is then performed. One may wonder whether this procedure is harmful to the embryo. It has been reported that it does not compromise subsequent embryo development if the procedure is properly and gently performed. According to the result of PGD, embryos with normal genetic content are transferred to the patient.
PGD is applied under these circumstances:
-after a minimum of two to three failed attempts
-hereditary disease in the family history
-advanced maternal age
-diseases that are transmitted with sex chromosomes
-HLA (human leukocyte antigen) typing
-Sex selection (family balancing)
The first step is the biopsy procedure. It is performed in the IVF laboratory by an experienced embryologist. Biopsy can be taken from the polar body of the egg before and/or after fertilization or from the cells of the embryo. The latter is more extensively used. One or two cells are removed from the embryos with the aid of a pipette.
The second step is the fixation procedure. Cells removed from the embryos are fixed onto glass slides. Their genetic content are exposed and sent to the genetic laboratory for evaluation.
At the last step the genetic laboratory examines the material. There are two basic examination techniques; in the FISH (fluorescent in situ hybridization) technique the fixed cells are stained with special probes which detect the abnormality to be examined. As these probes are attached to fluorescent substances they transmit signals under ultraviolet light. Nowadays probes with various colors for every examined chromosome are available, two to three regions of the same preparation can be examined with this technique. The number of diseases which can be diagnosed with this technology are increasing day by day.
Another genetic examination technique is PCR (polymerase chain reaction) which is used for detecting single gene disorders. It is possible that PGD could improve pregnancy rates because it selects embryos that are genetically normal for transfer. However, PGD, is not a therapeutical approach but is a diagnostic tool.
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