Polar body diagnosis

Polar body diagnosis - what is it?

Polar body diagnosis belongs to prefertility diagnostics, among other things. which are genetic tests designed to discover chromosomal defects before artificial fertilization has taken place. This must be differentiated from pre-implantation diagnostics (PID), for which ethical principles prohibit its application in some places. In the case of PID, the embryos that will be implanted into the uterus of the woman are selected only after artificial fertilization ( for instance, during in-vitro fertilization).

Polar body diagnostics in Germany

The reason why polar body analysis is performed is to detect and rule out chromosomal abnormalities of the embryo during ICSI or IVF treatment. In Germany, the Embryo Protection Act prohibits genetic testing of any embryo, although testing of the fertilized egg cell in its pronuclear phase is allowed. It is at this point in time when the polar bodies, which are specific hereditary subtrates of the egg cell, are analyzed, providing data about potential chromosomal aberrations.

What are polar bodies?

Each human being's genes - or hereditary traits - are encoded in chromosomes. We all have 46 chromosomes, divided into pairs of 23, each of which has been passed on from our mother and our father.

During the first meiosis (reduction division) of the egg cell, first the double (diploid) set of chromosomes is being reduced to a single (haploid) set of chromosomes, so it goes from 46 to 23 chromosomes. As one set remains in the egg cell’s nucleus, the other set is being discarded from the cell as a so-called polar body. In case the egg cell is now fertilized by a sperm, the second meiosis is initiated. During this process, each chromosome is divided into two chromatids and once again, once set of chromatids is to remain in the egg cell while the other set is discarded as the second polar body.

What is chromosomal maldistribution?

Normally, the amount of chromosomes is identical in the first polar body and the egg cell. However, in some cases, the number of chromosomes is not the same inside and outside of the egg cell. If so, a maldistribution of chromosomes has occurred, referred to as aneuploidy. Such maldistributions arise during the first meiosis 80% of the time and are only rarely associated with the second meiosis. The chances of chromosomal maldistributions increase with maternal (as well as paternal) age. Already after the mother is 34 years of age, the risk increases dramatically so that it amounts to a 50 . 70% chance of chromosomal maldistributions once the female turns 40. It is evident, that the likelihood of a successful pregnancy is severely stunted, while the rate of abortions is greatly increased.

Procedure of polar body diagnosis

To minimize the risk or appearance of genetic defects, polar bodies are extracted from both male as well as female. The biopsy is mostly carried out during the pronuclear stage. Firstly, the egg cell’s protective membrane called zona pellucida needs to be opened in order to access the genetic material. The zona pellucida can be opened with extremely precise laser systems or with microsurgery.

Once the polar bodies have been extracted, procedures such as FISH can be employed to analyze the distribution of the important chromosomes X, 22, 21, 18, 16 and 13. However, the chromosomes can also be examined by means of array cGH, which enables assessment of whether the chromosomes have been adequately distributed in the egg cell or a maldistribution took place. Reaching a diagnosis is very likely and only in 15% of cases, no certain diagnosis can be made. The aim of polar body diagnosis is to eliminate all disruptive factors for a successful pregnancy early on and increase the likelihood of a healthy birth.

Who can benefit from a polar body diagnosis?

Polar body diagnosis is reasonable for the following criteria, among others:

• The mother‘s age is 37 years or more
• The mother has a history of chromosomal maldistribution
• Unsuccessful IVF or ICSI procedures during which embryos capable of development have been implanted
• Earlier pregnancies during which aneuploidy has been detected
• At least 5 egg cells capableof fertilization should be available
• The fertility oft he sperm should be sufficient

It is important to note, that the probability of pregnancy is not reduced after a polar body diagnosis. Also damages to embryos as a result of polar body diagnosis have not been reported in medicine as of yet. It should be mentioned, however, that PBD does not consider the chromosomal distribution of sperm, as at this time only the pronuclear stage of the egg cell is examined, which happens before it fuses with the sperm.

One should into account, that the scientific benefit of PBD has not yet been sufficiently proven. In case suspicious findings are present, the question remains whether or not the parents want to discard the embryo. PBD can possibly uncover genetic disorders, which may put a great burden on the couple and therefore each couple that decides to undergo polar body diagnosis has to be aware of the consequences, because the desired event may not always set in.

Cost of polar body diagnosis

Typically, insurance companies do not cover the costs of polar body diagnosis. In case of five examined egg cells, the costs are estimated to be around 2500€ for FISH and 4000€ for array CGH.