How is Preimplantation Genetic Diagnosis (PGD) Performed?
- Posted on: Oct 27 2013
PGD, or pre-implantation genetic diagnosis, is used to check an embryo prior to use within a Denver IVF cycle for any potential genetic defects present. There are three separate stages of embryo progression in which the testing is done, each with a unique purpose for testing at that stage.
The polar bodies of an embryo are the products of egg division. These cells serve no actual purpose in either the egg or the embryo, and if left alone will degrade naturally over time. They will provide the current genetic status of the egg if they are tested.
The upside of using the polar bodies is that when tested in tandem, they provide a more accurate representation of the chromosomal composition of the embryo when compared to blastomere biopic testing. This testing occurs at either day 0 or day 1 of growth and both polar bodies are removed for testing.
One negative about performing the test this early is that at this stage in development, you will only be able to test for maternal genetic contributions. For those couples who do not have the option of discarding a fertilized embryo, it is possible to place this cell cluster into cryopreservation prior to the fusion of egg and sperm.
Cells for this testing are derived from a day 3 embryo that is in the cleavage division stage. Typically at this point, there are between 6 and 8 cells available for testing. By this time, these cells have both maternal and paternal genetic material within them. This is the most common method of testing used, but is not always the most accurate representation of the current genetic content contained within the embryo. This is because the cells potentially have differing numbers of chromosomes at this point in time, as chromosomal mosaicism is at its highest in day 3.
Blastocyst (trophectoderm) biopsy
This is testing performed on the fifth or sixth day of development after the embryo has reached a specific stage. There are two cell types at this age: an inner cell mass that develops in the fetal tissues, and the outer lining of cells encompassing this mass that eventually grow into the placenta. A small number of these outer lining cells are removed for testing and provide a significantly reduced chance of chromosomal mosaicism, as well as providing DNA amplification to provide for accurate testing.
Posted in: Infertility Treatment