Pre-implantation genetic Diagnosis and Screening (PGD and PGS) using Trophectoderm (TE) Biopsy.
With advances in assisted reproductive technologies, physicians are now able to offer couples genetic screening of embryos prior to placing them into the uterus during an in vitro fertilization (IVF) cycle. This method of embryo screening is now commonly referred to as pre-implantation genetic screening or pre-implantation genetic .
What is PGD vs. PGS?
Preimplantation genetic diagnosis (PGD), involves removing a cell from an embryo to test it for a specific genetic condition (sickle cell anemia, cystic fibrosis, for example) before transferring the embryo to the uterus. It enables people with an inheritable condition in their family to avoid passing it on to their children.
Preimplantation genetic screening (PGS), is the proper term for testing overall chromosomal normalcy in embryos. PGS is not looking for a specific disease diagnosis – it is screening the embryo for normal chromosome copy number.
There are various techniques that can be used for getting genetic material from an embryo, and recently, trophectoderm biopsy is gaining popularity as the method of choice for embryo biopsy.
Trophectoderm Biopsy
Trophectoderm biopsy involves removing some cells from the trophectoderm component of an IVF blastocyst embryo at day 5/6. Trophectoderm cells are extra-embryonic tissue; they do not become part of the fetus but do become part of supporting structures, such as the placenta and membranes.
Trophectoderm biopsy is carried out on day 5 or 6 after fertilization. At this point, the embryo is at the expanded blastocyst stage or beyond.
Blastomere Biopsy
Blastomere biopsy, involves the removal of one or two cells when the embryo reaches the eight-cell stage, typically at the third day of development before a blastocyst is formed.
How is Trophectoderm Biopsy Performed?
• A laser is used to create a small opening in the zona pellucida on day 3 of development. This allows the embryo to begin the hatching process early on day 5 of development.
• With hatching, a portion of the trophectoderm begins to herniate through the zona pellucida.
• Using micro tools (and the help of a laser), the embryo is held and a few cells of the herniating portion of the trophectoderm is removed.
• The cells that will become the fetus (inner cell mass) are not disturbed.
• After trophectoderm removal, all biopsied embryos are frozen prior to transfer.
Advantages of trophectoderm biopsy over blastomere biopsy.
• Biopsy of a day 3 embryo involves removal of 1 or 2 blastomeres from an eight cell embryo which require a large hole in the shell and the embryo losing a significant part of its total mass.
• Day 5 or 6 trophectoderm biopsy involves removal of a few cells from the placental portion of the embryo with little or no impact on its potential.
• The implantation rate in embryos after blastomere biopsy is less than those after trophectoderm biopsy by 20%.
• Trophectoderm biopsy has been shown to be more accurate than cleavage stage biopsy because there is more DNA for genetic analysis.
• Trophectoderm biopsy increases PGD success rate because the embryos are much less traumatized from this procedure.
Disadvantages of trophectoderm biopsy
• In trophectoderm biopsy, results mostly are not gotten quickly enough to perform a fresh transfer. Frozen embryo transfer has to be done.
• In some couples with poor quality embryos, there is a chance that no embryo develops to blastocyst stage.
• There is additional cost from adding a frozen embryo transfer cycle.
How PGD, PGS is done with blastocyst IVF embryos.
• The trophectoderm cells are removed from the blastocyst as described above.
• The biopsied sample is sent for chromosomal (and/or genetic) analysis and unaffected embryos for transfer are identified.
• The test result is received within days.
• The appropriate embryo/s are thawed and transferred back to the uterus.
How PGD, PGS is done with blastocyst IVF embryos.
• The trophectoderm cells are removed from the blastocyst
• The biopsied sample is sent for chromosomal (and/or genetic) analysis and unaffected embryos for transfer are identified.
• The test result is received within days.
• The appropriate embryo/s are thawed and transferred back to the uterus.
IVF live success rate
• Studies have shown that the use of PGD/PGS at the blastocyst stage improves clinical outcome.
• There is recent evidence from multiple studies that trophectoderm biopsy and PGS using one of the newer genetics technologies increases IVF success rates.
• Day 3 embryo biopsy process weakens the embryo and well controlled studies have shown lower chances for a live birth when those biopsied embryos are transferred.
