Embryo biopsy is a step in the process of PGD which consists of removing one or two cells from the embryo without compromising its normal growth so that an assessment and a genetic study of the embryo can be conducted. The biopsy can be performed before or after fertilisation, the former by biopsying the polar body (PB), and the latter by biopsying a blastomere from the embryo or cells from the trophoblast.
Polar body biopsy
Polar bodies (PB) are a set of cells with no definite function which are produced during the meiosis and instantly disappear after fertilisation. Thus, a polar body biopsy is performed before fertilisation. This allows choosing only those eggs with greater chances for achieving pregnancy.
Analysing one of these cells won’t have an impact on the embryo development. However, it will be impossible to detect possible abnormalities which may occur after fertilisation or to obtain genetic information from the father.
Embryo biopsy on day 3 (D3)
In order not to affect the embryo when removing a cell, embryo biopsy on D3 can be performed as long as it is at least at the 6-cell stage. Using this type of biopsy provides us with genetic information both from the mother and the father.
To perform an embryo biopsy on day 3, it is necessary to make an orifice will on the embryo’s zona pellucida by means of laser dots or chemical agents such as Tyrode’s solution. Once the hole has been performed, a blastomere or cell from the embryo will be obtained by aspiration. Then, it will be evaluated at molecular and genetic biology laboratories.
The embryo, once the biopsy has been performed, will be kept under observation up until the moment of the transfer.
Embryo biopsy on day 5 (D5)
On D5, the embryo is at blastocyst stage. In this case, between 4 and 5 cells from the trophoblast (epithelium which will give raise to the placenta) will be biopsied. The inner cell mass, which will give raise to the embryo, will stay intact.
The embryo biopsy will be conducted by performing a hole on the zona pellucida using a laser. Since the zona pellucida has reduced its thickness due to its own enlargement processes, using a laser is considered as the most accurate method. If other methods were used, the embryo development could be compromised. A fragment of the trophoblast will be removed out of the zona pellucida and it will be aspirated for finally being biopsied. For the cells to be divided, laser dots will be needed in order to obtain an appropriate result from the aspiration process.
In these cases, it may be necessary to freeze the embryos while the diagnosis is being performed up to the moment of the transfer.