A definition of PGD
PGD also know as embryo screening is an in vitro procedure which is performed on a fertilized embryo prior to implanting in the mother's womb. This entails removing cells from the egg or fertilized embryo to perform "preimplantation genetic screening". This kind of genetic testing can be used to find a multitude of genetic mutations both good and bad.
PGD Biopsy Procedures
PGD can be performed on cells and embryos at different developmental stages. Although it can be performed at any time preimplantation into the womb, only three have been adopted. All biopsy procedures involve making an incision into the zona pellucida, which is a hard protein shell that surrounds embryo. Then removing the cells for testing. The three most common procedures are Polar body biopsy, Cleavage-stage biopsy (Blastomere biopsy) and Blastocyst biopsy
(PB) Polar body biopsy
This procedure removes the first and second polar body "a cell structure found inside an ovum" at the end of meiotic division. The first polar body is removed from the unfertilised oocyte, and the second polar body from the zygote, shortly after fertilization. These particular polar bodies are not required for successful fertilization or continued devleopment of the fetus. This procedure can only provide information about the mothers genetic contribution to the developing embryo. This procedure is consider the least intrusive and generally considered the most acceptable. Even in Germany where laws are very strict when dealing with preimplentation procedures, polar body biopsy is accepted.
Cleavage stage biopsy (Blastomere biopsy)
This type of biopsy is done when the embryo has split into eight cells, generally day 3 of fertilization. This type of biopsy is usually considered superrior for one main reason and that is because the fathers genetic input can be analyzed as well as the mothers. Similar to polar body biopsy, and incision is made in the zona pellucida. At this point two blastomere's containing nuclie are removed and analyzed. There is debate as to whether this procedure provides enough evidence of future diseases as not all chromosones are present in every cell. We are dealing with very early embryo development.
Blastocyst biopsy
The technique to remove the genetic information is bascially the same as the other two techniques. More cells are extruded with this procedure than any other. Although accuracy of diagnosis tends to be higher with this method. It still suffers from the same chromosonal mosaicism as "cleavage stage biopsy". The other major problem is by waiting so long to do the biopsy limits the time available to do the actual testing of the embryo.
Fluorescent in situ hybridization (FISH)
FISH is the first of two of the most common testing procedures for 'Preimplantation Genetic Diagnosis (PGD)" and is the most commonly used method. It can be used on PB's, blastomoeres and TE samples.
Polymerase chain reaction (PCR)
PCR was one of the first testing methods developed back in 1983. It involves amplifying a single piece of DNA and generating multiple copies. It is also used in Cloning and DNA sequencing.
Disorders that can be detected
Almost any genetic disorder can be detected using preimplantation genetic screening, however the most comon ones that are screened for are "cystic fibrosis", "spinal muscular atrophy", "haemophilia A", "sickle cell disease/sickle cell anaemia" and "huntingtons disease". All of which can cause life threatening symptoms in people afflicted with them.
Determining Gender of Embryo
The procedures can be used to determine the sex of an embryo to a high level of accuracy and has become one the biggest ethical issues in IVF today. PGD can determine the difference bewteen male and female chromosomes and can influence the gender selection of the embryo. When presented with two different sets of cells parents can choose which they would prefer to have implanted.