Preimplantation Genetic Diagnosis screens an embryo’s quality at the chromosomal and genetic level. It can be performed for a patient at high risk of carrying an inheritable genetic disease. Using this technique, embryos that do not carry the disease can be selected for transfer.
A single cell from an 8-cell stage embryo is removed and sent off for testing. DNA analysis identifies embryos that are free of the disease, making them good candidates for transfer.
Why is Embryonic selection beneficial?
For patients with genetic diseases, selection of embryos most likely to implant and develop into a healthy child becomes extremely important. Historically, the microscopic appearance of the embryo was the only criteria available to the physician and patient for selection of “healthy” embryos for transfer. Unfortunately, there is no correlation between embryonic appearance and genetic health. Perfect-appearing embryos can carry chromosomal abnormalities and “unattractive” embryos can be genetically normal. The advent of PGD gave physicians and their patients with genetic disease an effective
Clinical indications for which PGD can be used include, but are not limited to:
- Aneuploidy testing for advanced maternal age and recurrent pregnancy losses (duplicated or missing chromosomes
Embryo Biopsy for PGD and PGS
The process of IVF PGD as well as IVF PGS can be performed on the embryos that are in different developmental stages, therefore the biopsy procedure varies accordingly. In clinical practice, only two types of biopsy are used:
- Blastomere biopsy – (on day three at cleavage stage embryos) – 1 or 2 cells are removed from 8-cell embryo for genetic analysis
- Trophoectoderm biopsy – (on day five at blastocyst stage embryos) – 4 or 5 cells are taken from the outer trophectoderm layer without affecting the inner cell mass from which the fetus later develops.
The Biopsy procedure always involves two steps. The first is the opening of the zona pellucida and the second being removal of the cell(s). Both the steps have different approaches, including mechanical as well as chemical procedures, but most advanced laser technology is used for breaching the zona pellucida
Using the advanced technology, the zona thinning is possible to create a point of herniation already at the time of ICSI procedure, that is less disturbing for embryo than traditional zona-opening. During trophoectoderm (TE) biopsy we operate with accurately controlled laser beam to release the outer cells of the blastocyst.
The retrieved embryonic cells are then sent for genetic analysis by the use of microchip method. In this a larger number of cells allows a more accurate genetic assessment of the embryo without affecting its further development. Once the cells have been extracted from a blastocyst, the embryos are then frozen and stored for a few days until the genetic results are available. The main limitation of the process of blastocyst biopsy is that usually only a limited number of embryos will reach the blastocyst stage. Therefore, the final decision on biopsy approach should be left to our experienced specialists.