Blastomere biopsy (also known as embryo biopsy) is a technique that is performed during IVF when an embryo has reached the six to eight cell stage (about 72 hours or day 3 of embryo culture). One or two cells, or blastomeres, are separated from the rest of the embryo and removed from the zona pellucida which is the shell surrounding the developing embryo. After removal of the cell(s), the developing embryo is placed back into the culture media and returned to the incubator where it can resume its normal growth and development. preimplantation genetic diagnosis – PGD can be performed separately on the removed cell(s).
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In this picture, you can see the aspiration pipette (the glass tube on the right) gently pulling the single blastomere off of an 8 cell embryo.
At this early point of embryo development, all of the cells should be identical and thus, removal of a cell from the embryo at this stage should not remove anything critical for normal development. An embryo should be able to compensate for the removed cell and should continue to divide following blastomere biopsy. However, a recent study suggested that a biopsy performed at the blastomere stage was responsible for a decreased chance that the embryo would be able to implant into the uterus later.
After obtaining cells from the embryo, they can then be analyzed using a variety of different techniques. It doesn’t matter which of the eight cells was removed because as the embryo divides, each subsequent generation of cells contains exactly the same genetic information as the “parent” cell. This, each of the eight cells should be identical.
However, at times there can be an aberration in the cell division in which one or more of the “daughter” cells ends up being slightly different from the parent cell. This is called mosaicism. Mosaicism is important when performing preimplantation genetic diagnosis – PGD via blastomere biopsy because it means it is possible that the cell that is biopsied may not be representative of the entire embryo. For example, if during PGD, a blastomere biopsy is performed and the cell that is obtained is abnormal, the entire embryo would considered abnormal even though the remaining cells in the embryo may be normal. The opposite is also true. An embryo with 7 abnormal cells and one normal can be considered normal if the “eighth” cell happens to be the one that is biopsied.