Each cell in our body should have 46 chromosomes; 44+XX in the female and 44+XY in the male. Egg containes 46 chromosomes prior to ovulation and the sperm in the ejaculate has 23 chromosomes (22+X or 22+Y). In order to accomplish a healthy embryo which should have 46 chromosomes, the egg has to release half of its chromosomes to accommodate the other half from the sperm. In the process of ovulation and expulsion of the egg from the ovary, the egg releases half of its chromosomes (22+X) outside the egg which is called the first polar body. First polar body stays inside the shell of the egg called the zona pellucida and should be the mirror image of the egg chromosomally. Polar body can be removed adjacent to the egg and tested to get information about the chromosome make up of the egg. In cases of egg freezing, Dr. Bayrak recommends testing of the polar body to identify chromosomally normal eggs at the time of fertility preservation.
If the egg divides unevenly during ovulation, the final unfertilized egg is chromosomally abnormal (may have any number of chromosomes other than 23). A chromosomally abnormal egg never results in a healthy embryo, but may get fertilized and even implant, but most of the time results in a miscarriage. Rarely, nature will make a mistake and an abnormal egg may get fertilized, embryo may implant and result in a live birth such as Down’s syndrome (47 chromosomes).
Preimplantation genetic diagnosis (PGD) refers to the group of genetic tests that provide genetic and chromosomal information about eggs or embryos prior to embryo transfer with In vitro fertilization (IVF) treatment. PGD was recently renamed preimplantation genetic screening (PGS) due to its imperfect nature in identifying genetic and chromosomal abnormalities.
Historically, there were only two types of PGD tests available with IVF treatment. These included identification of chromosome pairs with a limited methodology called fluorescent in situ hybridization (FISH) and a more advanced method for specific gene testing called polymerase chain reaction (PCR). FISH is used to assess numerical abnormalities of chromosomes, whereas PCR allows for testing of specific gene disorders within a chromosome.
FISH test involves identification of a small part of the chromosome with a florescent dye and two separate signals connote the presence of the chromosome pair being tested. The limitation of test is the