Know More about Embryo Biopsy, PGS & PGD
What is Embryo Biopsy?
Removal of one or two cells from the preimplantation embryo for the purpose of the genetic analysis is called an embryo biopsy. The procedure is similar to chorionic villus sampling or amniocentesis, as these also aim at genetic testing by means of removal of an adequate number of embryonic cells. Couples, who undergo an IVF procedure in order to get pregnant, may opt for an embryo biopsy, to allow genetic screening of their embryos for chromosomal errors, before the embryos are transferred back into the patient.
In vitro fertilization involves fertilizing the female egg with sperm in a petri dish. Multiple embryos may be formed in this process. A biopsy can be performed at any one of the following 3 developmental stages of an embryo: polar body, day 3 or day 5. Even though, traditionally, embryo biopsy used to be predominantly performed in an 8-cell embryo on day 3, now, clinical evidence is mounting in favour of performing biopsy at the blastocyst stage on day 5.
Process of a Blastocyst stage Embryo Biopsy:
- A laser is used to create a small opening in the covering layer, also known as zona pellucida, of a blastocyst stage embryo on day 5.
- As the blastocyst begins to expand in order to hatch out of the zona, the trophectoderm slowly begins to herniate out of the zona pellucida through the opening created.
- With the help of advanced micro tools and laser, the embryo is held in place and 4 – 5 herniating cells are detached from the embryo. During the biopsy procedure, special care is taken to safeguard the embryo.
- Embryo biopsy is performed on all the embryos generated during IVF. After biopsy, the embryos are immediately frozen for subsequent use.
3rd day vs 5th day biopsy
Biopsy can also be performed on the 3rd day of embryonic development when the embryo is at the 8-cell stage. In this case, 1 to 2 cells can be removed from an 8-cell embryo. This procedure requires the introduction of an opening in the zona pellucida through which a single cell can be gently aspirated for genetic analysis. However, since removal of a single cell from an 8-cell embryo may lead to a higher loss in total embryonic mass as compared to that for a blastocyst stage embryo (where 4-5 cells are removed from an embryo with ~ 150 cells), this approach may lead to a reduction in the implantation potential of the embryo.
In addition, a biopsy from a blastocyst on day 5 involves removal of trophectodermal cells (that later develop into the placenta) but not cells that form the inner cell mass, a portion of the embryo that eventually develops into the foetus. The implantation rate is higher with this approach. Trophectoderm biopsy also results in a more accurate analysis, as there is more genetic material (from 4-5 cells) available for genetic testing.
Preimplantation Genetic Diagnosis (PGD)
PGD is a procedure that is performed in combination with In vitro fertilization (IVF). PGD involves removal of a single cell or few cells from the preimplantation embryo and testing it for specific genetic conditions. More than 1000 types of single gene mutations can be diagnosed with this technology. PGD is recommended for the patients who are carriers of inherited genetic disorders such as thalassemia, haemophilia, cystic fibrosis etc.
Preimplantation Genetic Screening (PGS)
Also known as comprehensive chromosome screening, PGS is one of the most effective tools for testing the chromosomal status of an embryo. The PGS technology involves screening of embryos, for abnormal number of chromosomes or aneuploidies, in infertile patients undergoing IVF, in order to improve outcomes. A human cell contains 46 chromosomes, which carry genetic information in the form of DNA. An embryo will receive 23 chromosomes from each parent. If the egg or sperm possess a missing or extra chromosome, then the embryo created will also have a missing or extra chromosome. This condition is called aneuploidy and can be identified in PGS. While majority of aneuploid embryos fail to implant, a significant proportion of such embryos may result in miscarriages and some may even result in a live baby with congenital birth defects and mental retardation such as in Down’s syndrome, Edwards’s syndrome, Turner’s syndrome etc.
Benefits of PGS
- Embryos can be screened on the basis of their chromosomal status which allows embryos with correct number of chromosomes to be transferred resulting in higher number of pregnancies and a reduction in number of miscarriages and aneuploid conceptions. Consequently, PGS may lead to reducing the overall time to achieving a live healthy baby.
- PGS technology can benefit patients who are at a high risk of producing aneuploid embryos.
- Couples with multiple miscarriages have a very high rate of aneuploidy in their embryos, and therefore are prime candidates for PGS treatment.
- Couples seeking parenthood at an advanced maternal age (>37 years) also have a very high aneuploidy rate in their embryos and hence can benefit from this technology.
- Another group of couples that may have a high rate of aneuploid embryos are those who have previously failed multiple IVF cycles and thus may benefit from the PGS technology.