Preimplantation genetic diagnosis
Preimplantation genetic diagnosis
Preimplantation genetic diagnosis (PGD) is a special method that allows us to detect certain genetic defects of future fetus by genetic testing of cells which were taken from fertilized eggs or embryos. It would be pointless to create an embryo with a genetic defect which would cause difficulties to the child, or could even lead to miscarriage. Therefore, it is necessary to control the genetic quality of sample, if any of assisted reproduction techniques is carried out. Assisted reproduction can not be performed without preimplantation diagnosis. Sperms and the eggs that will be used for future insemination are genetically tested. In vitro fertilization (IVF) is another method in which is preimplantation genetic diagnosis performed. Embryo is let to develop for some time before it is introduced into the uterus. When embryo consist of about 8 cells, thus it is around 30 hours old, it is genetically tested. Only one cell is taken from the embryo, i.e. cell from which is diagnosis carried out, by micropipette in special device. Cells removed at this stage will not impair the development of embryo, and so the embryo is able to develop without major difficulties. Even before there was a possibility of preimplantation genetic diagnosis, couples with genetic load underwent prenatal diagnosis. This process is carried out during the pregnancy with the fact that in case of proven harm to the fetus, the pregnancy must be artificially interrupted.
Each cell consists of a few basic parts. It has a membrane which forms its packaging, organelles and also the nucleus. However, it is important to add that not all cells of the human body contains the nucleus, for example, red blood cells and platelets. The nucleus is separated from the rest of the cell by membrane, so-called nuclear envelope. The nucleus is formed by a coiled double helix (DNA) in which is all the genetic information of an individual recorded. This DNA is organized into chromosomes. We can think of chromosomes as part of DNA clusters in the form of bars. It exists 46 of these bars, i.e. chromosomes and they form pairs. We have 22 pairs of the chromosomes which encode information for the whole body. Last, the 23rd pair, contains information about the gender of individual. It is customary to represent sex as the symbol XY for males and XX for females. However, each sex cell contains only a half of the genetic information. The egg, i.e. female sex cell, carries genetic information which contains 23 chromosomes. They need another 23 chromosomes in order to form pairs and create a full cell. These chromosomes are in sperms. Whether the sex of baby will be XX - girl or XY - boy depends on the last chromosome which is carried by sperm.
Preimplantation genetic diagnosis techniques
Preimplantation genetic diagnosis is performed with many techniques in which can be genetic information examined. These are cytogenetic examinations, i.e. FISH and PCR method. Cytogenetic examination is the basic examination during which are chromosomes stained with special dyes and afterwards, the resulting image is evaluated. This method can detect congenital defects and as well as gender of the fetus or embryo. Fluorescence in situ hybridization (FISH) is the method in which are special probes with glowing substance emitted into cell. These carriers can be bound only on certain parts of the gene, often to those which encode various congenital malformations. Therefore, we can accurately determine whether and which genetic defect fetus has. For example, Down syndrome can be detected by this technique. PCR is a very complex method in which are cells affected by enzymes that are similar to those involved in cell division in the human body. By a special mixture of substances , it is possible to achieve that the places responsible for coding a congenital defect in the genome, i.e. a group of chromosomes in the cell nucleus, are located and multiplied. There is so many of these places that they can be established by method such as electrophoresis. Electrophoresis makes it possible to divide substances according to their size and charge in electric field. If there are any defects in the genome, nothing is multiplied and the result of PCR is negative.
When we should undergo preimplantation genetic diagnosis
In preimplantation genetic diagnosis are all samples intended for donation tested, i.e. sperms and the eggs. Genetic tests are designed for couples in which are repeated miscarriages very common. This method is also recommended to those couples who have congenital defect or unsuccessful assisted reproduction in family. Furthermore, preimplantation genetic diagnosis is recommended to women with 35 years and more, if there is an increased risk of delivering the baby with a genetic defect. Further treatment possibilities are recommended to couples according to the established results, for example, the increased intake of donated gametes should be considered.