Preimplantation Genetic Diagnosis: Facts and Flaws

Preimplantation Genetic Diagnosis for IVF Baby

Preimplantation genetic diagnosis (PGD) is performed on IVF generated Blastomeres, totipotent cells of human embryo.

Each daughter cell formed by the division of fertilized ovum contains exactly the same genetic information as the "parent" cell, and has the same genetic potential to further divide.

Blastomere biopsy

Blastomere biopsy is prerequisite of PGD.
It is performed on a 3 days old IVF embryo.
A 3 days old embryo is made of 8 cells, known as blastomere.
1-2 blastomeres are gently separated from rest of the embryo and pipetted out of the covering shell, zona pellucidia, for genetic screening.

Can cell selection affect preimplantation genetic diagnosis?

All the 8 cell are expected to be identical in a 3 days old embryo. Therefore, it doesn't matter which of the eight blastomeres are removed for genetic screening.

However, studies show that IVF generated embryos present almost 50 percent chances of mosaicism, where chromosome representation of blastomeres differ.

To minimize erroneous genetic screening results, PGD is performed on 2 cells of 8 celled embryo.

Frequently asked questions on embryonal biopsy

Does biopsy for preimplantation genetic diagnosis affect fetal development?

Removal of a cell from an embryo at this early stage does not jeopardise the fetal growth.
Rest of the developing embryo compensates for the removed cells, and resumes normal fetal growth and development under appropriate in vitro conditions and after implantation in mother’s womb.
Though considered a safe procedure, its long term impact on children born of biopsied embryos yet remains to be studied.
Like in any procedure, here also we do not expect all win situation.

Possible flaws in preimplantation genetic diagnosis (PGD)

Risk for loss of the embryo how ever small is there.
Some studies mention that there is 60 percent chances of embryo loss.
Post blastomere biopsy an embryo’s potential to implant in mother’ womb can be reduced.
IVF generated embryos present high chances of mosaicism; and If it gets missed, the entire purpose of preimplantation genetic diagnosis is lost.

What is mosaicism?

Mosaicism result from mutation, crossing over, or nondisjunction of chromosomes during cell division. Consequently, the daughter cells thus formed will show variation in the number of chromosomes. 8 celled embryo is formed by the mitotic division of the fertilized ovum. Therefore all 8 cells are expected to have identical genetic representation.
Click here to see the intricacy of cell divisions.

Preimplantation genetic screening analysis can also have error in final diagnosis.
Human, procedural or technical error can risk missing the target diagnosis.
False negative results are possible in PGD.
This would mean that genetic make up of the embryo is reported normal when it actually is not. With the result the parents accept an abnormal embryo.
False positive results of PGD leads to rejection of a normal embryo.
Damage of the embryo during blastomere biopsy can occur.
If such accidental damage of the embryo does occurs, it can be recognized almost immediately by microscopic examination.
Blastomere biopsy & preimplantation genetic diagnosis (PGD):Video clip