Preimplantation Genetic Diagnosis (PGD)PGD Improves Chances for Successful Pregnancy and Birth
PGD is performed as a part of an In Vitro Fertilization cycle where multiple eggs are produced, retrieved from the ovaries and fertilized with the husband's sperm in the Embryology Laboratory. IVF is necessary to give us access to the embryo in vitro. At their earliest stage of development , one or two cells are removed from each embryo through a procedure called embryo biopsy. These cells are analyzed in the laboratory to determine which embryos are free of genetic abnormalities. In most cases, results are available within 24 hours. This sophisticated and technologically advanced testing identifies which embryos are free of abnormalities and better able to achieve the patient's goal of a healthy baby.
Preimplantation Genetic Diagnosis Procedure
PGD can only be performed on embryos in vitro (in a laboratory). That means this test is always performed in conjunction with an in vitro fertilization cycle.
To enable screening of a human embryo before transfer to the uterus, it is possible to remove one or two cells from the early multi-celled embryo so that the genetic material in these cells can be analyzed. It must be noted that in routine genetic analysis there are usually hundreds of cells available for processing; however, with embryo biopsy only one or two cells are commonly available, and they must contain a nucleus with chromosomes present to determine the genetic status of the rest of the embryo.
The biopsy method is relatively straightforward, but this does not mean that it is an easy procedure to undertake. The embryos are typically biopsied at the pre-implantation stage on day three of development.
At this point, the embryo will be composed of between four and 12 cells that are still distinct from much other. By the third day of development, or at the latest on day four, the embryo begins to compact, a process whereby the individual cells lose their clear outline and they seem to fuse together with the other cells to form the morula stage embryo. On the third day, however, single cells can be individually removed without disruturbing the adjacent cells in the embryo.
At this time the embryo is still surrounded by a glygoprotein 'Jelly coat" of the zona pellucida, so to remove any cells this jelly coat must first be cut open. This can be done either using acidified culture medium that "dissolves" the zona pellucida, or mone conveniently can be done with a simple laser to drill the eract size hole to allow a glass micro-tool to be pushed through the hole and extract a cell. The hole that is drilled is usually made a little smaller than the cell itself, and this helps maintain the integrity of the embryo within the jelly coat during further development in the IVF lab.
The embryo during manipulation on the inverted microscope is held in a warm culture medium that helps to allow the cells to be removed with a minimum of trauma to the overall embryo. The removal of up to a quater of an embryo is not known to be deleterious to the further development of that embryo, as the embryo at this mrly stage of development ctrn compensate for this loss of material. All cells at this stage are still totipotent, meaning, all are fully capable of directing further embryonic development.
Once a single cell (a blastomere) is removed, the cell is either fixed on a glass slide for chromosomal analysis, or placed in a small tube of chemical buffer for single gene diagnosis. The cells are then analyzed using techniques called fluorescence in situ hybridization (FISH) or DNA analysis. During the genetic analysis, the embryos are usually grown to the fifth day of development at which time they may either be at the morula or blastocyst stage. Those embryos found to be free of genetic abnormalities are then placed into the uterine cavity.
Are there risks associated with PGD?
The micromanipulation techniques used for blastomere biopsy are safe with little risk to the embryo. The risk of accidental damage to the embryo during biopsy is less than 10%. There is no risk to the embryo following chromosomal or single gene defect analysis because the analyzed cells are not put back into the embryo. There may be a slightly lower likelihood of implantation after embryo biopsy compared to an embryo not having been biopsied. Other risk may become apparent over time, but to date appear quite limited and need to be weighed against the potential benefits for each couple.
Infertility & Recurrent Miscarriage
PGD is recommended most frequently for patient with unexplained infertility, recurrent miscarriages, unsuccessful IVF cycles, advanced maternal age, or male factor infertility. The most likely cause is a chromosome abnormality. Chromosome abnormalities include aneuploidy and structural abnormalities. Aneuploidy is the most,common chromosomal abnormality.
Aneuploidy can occur in both eggs and sperm. Structural abnormalities include translocations, inversions, and deletions. Structural chromosome abnormalities can also be present in eggs and sperm. The transmission of a chromosome abnormality to an embryo can result in a low implantation rate, miscarriage or the birth of a baby with a genetic disorder. Using Fluorescence In Situ Hybridization (FISH), the scientists in the PGD laboratory can identify the absence of these specific genetic disorders in each normal developing embryo. As a result, only those embryos free of genetic disease will be transferred to the patients uterus so as to increase the chance of coneption and ultimately a healthy baby.
Fertile couples with repeated miscarriages should be evaluated for the presence of a chromosomal abnormality. The female or male partner may be a carrier of a balanced translocation or be an aneuploid mosaic.
Unsuccessful IVF cycles
Couples with repeated unsuccessful IVF rycles should be evaluated for the presence of a chromosome abnormality.The female or male partner may be a canier of a balanced translocation or be an aneuploid mosaic.
The most probable cause of unexplained infertility or history of habitual miscarriage is a chromosome abnormality. The male or female partner may be a carrier of a translocation or be an aneuploid mosaic.
Preimplantation Genetic Diagnosis technology improves the likelihood of a successful pregnancy and birth for two distinctly different groups of patient. Couples with infertility related to recurrent miscarriage or unsuccessful IVF cycles and couples who are at risk for passing on inherited genetic disease to their offspring.
PGD Can Be Tested For
- Family Balancing
- Huntington's Disease
- Sickle-Cell Anaemia
- Chromosome Translocation
- Irreversible Vasectomy
- Down's Syndrome
- Breast and Ovarian Cancers