Fertilisation Of Giant Oocytes Biology Essay


IVF success rates are relatively low despite ongoing research in this field (28.6% live birth rate for women aged under 35, HFEA 2008). There are numerous reasons why an IVF cycle may fail, ranging from oocyte quality to a poor sperm sample. Oocyte quality can contribute largely to the outcome of some IVF cases. Immature and poor quality eggs are unlikely to result in good quality embryos, which in turn are less likely to produce a healthy pregnancy (Navot et al, 1991; Serhal et al, 1997). 22.1% of oocytes were found to be karyotypically abnormal (23 X) in a study carried out by Pellestor et al (2002) who examined 1397 in-vitro unferiltilised oocytes. This case study examines a subset of abnormal oocytes often referred to as giant eggs. Giant eggs are often diploid or triploid and can lead to blighted ova, spontaneous miscarriage and congential malformations.

Clinical background

The couple have had a previous miscarriage at 6 weeks in 2008 but no other pregnancies. The subfertility was classed as unexplained. The patients FSH level was 10.0 prior to treatment at age 36. The patient was undergoing antagonist (Cetrotide) down-regulation and was prescribed high dose stimulation (Menopur 450) from day 2 to12 of her cycle.

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8 eggs were collected by ultrasound-guided trans-vaginal oocyte aspiration and inseminated with the husband's sperm. At fertilisation check 19 hours later there was no apparent fertilisation. Of 5 mature oocytes only one showed any signs of fertilisation; the pronuclei were very faint and became indistinct within a minute of the initial fertilisation check. During removal of the cumulus cells using a 140µm stripper at fertilisation check the zona pellucida of the oocytes was damaged and the cytoplasm leaked from the zona. This damage is very likely to have been caused due the large size of the oocytes, which unfortunately was not apparent until after stripping. It is unclear whether the failed fertilisation occurred because of the oocyte abnormality or was due to other factors such as the semen sample.

The initial semen analysis showed good numbers of sperm (125 x 106/ml), good motility (60%), good progression (2-3/4), but slightly raised abnormal forms (87%, with cut off of 85%). It is possible that the raised abnormal forms could have been a factor in the failed fertilisation. During the fertilisation check 90% of the sperm in the dish were moving with a progression of 1-2/4, ie twitching only. There appeared to be no sperm-oocyte binding.

In this particular case ICSI could be attempted in order to overcome the uncertainty regarding the sperm function and the lack of sperm-egg binding. The patient was karyotyped with a "normal" result. It was advised that the couple could undergo PGS in order to select a normally fertilised oocyte (diploid) or consider oocyte donation in future cycles.

A second cycle was undertaken whereby ICSI was performed and 5 out of 7 mature eggs achieved fertilisation, the other two eggs degenerated post ICSI. The oocytes appeared to be slightly larger than normal, but not the extent of traditional giant eggs and could not be easily distinguished by eye. The couple opted to go ahead with the cycle on the basis that these may not be giant eggs. On day 3 the embryos reached the 2 cell stage, and by day 5 only one embryo had gone onto the 4 cell stage. The couple agreed to stop the treatment cycle at this point and not go ahead with embryo transfer.


Giant oocytes are approximately 1.3 fold larger than normal oocytes and are frequently aneuploid. It is thought that giant oocytes undergo atresia during natural folliculogenesis and are very rarely ovulated (Gougeon 1981). Balakier et al (2002) found that giant oocytes can be found in patients of all ages and all aetiological groups of infertility including male factor, tubal abnormality, PCOS, unexplained and endometriosis. It is possible that the development of giant oocytes in assisted reproduction may be related to augmented response to gonadotrophin therapy. Balakier et al (2002) also reports that patients who produced giant oocytes have significantly higher levels of oestradiol (P<0.01) and have significantly more oocytes collected (P<0.01) compared to patients who did not produce giant oocytes. However regardless of the high dosage of stimulation (450 Menopur) during both oocyte retrievals for this patient 8 and 7 oocytes were collected. These figures are not similar to those described by Balakier et al (2002) who reports an average of 19.4 oocytes from patients who produced giant oocytes.

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Older reports have previously linked the occurrence of aneuploid oocytes and abnormal embryos with hormonal stimulation used in assisted reproduction (Tarin and Pellicer, 1990). In a small study by Rosenbusch et al, (2002) 6 out of 6 giant oocytes were karyotyped as diploid. In addition to this Munne and Cohen (1998) found that giant oocytes with diameters greater than 220µm resulted in embryos that were invariably triploid or triploid mosaic with a higher contribution of maternal chromosomes. Triploids formed from maternal origin are likely to be created by monospermic fertilisation of a diploid oocyte (Balakier et al, 2002; Rosenbusch et al, 2002). Diploid oocytes arise often due to failure to expel the 1st or 2nd polar body, leading to retention of extra maternal chromosomal material. Alternatively a lack of cytokinesis during mitotic division of oogonia or other failures during oogenesis can result in unusually large diploid oocytes known as giant oocytes (Austin 1960). In this particular case during the first IVF cycle 5/8 oocytes were at the metaphase two stage and so the first polar body was evident in perivitilline space and during the second cycle 7/7 oocytes were at the metaphase two stage prior to ICSI. Therefore it seems reasonable to reject the theory that the large eggs were a manifestation of failure to expel a polar body.

It is reported that fertilisation of giant oocytes is compromised and often results in zygotes containing ≥ 3 PN (Balakier et al, 2002). However some reports confirm normal fertilisation of giant oocytes (Balakier et al, 2002 and Rosenbusch et al, 2002). In this case there were no signs of fertilisation using IVF and 5 normally fertilised oocytes using the ICSI technique. Of the 12 oocytes mixed with sperm no oocytes showed any more than 2PNs. This may indicate that the failed fertilisation from IVF arose from a problem with the sperm-oocyte binding and not the size or chromosomal complement of the oocytes. Zygotes derived from giant oocytes which are classed as normally fertilised, containing 2PN, are capable of cleaving and continuing on until the blastocyst stage (Balakier et al, 2002). This means it would not be possible to select a "normal" embryo for transfer from a cohort of embryos derived from giant oocytes on the basis of morphology. In total 29% of morphologically normal embryos are chromosomally abnormal (Munne and Cohen 1998). In this particular case the resultant embryos showed abnormal development and reinforces the theory that the oocytes were chromosomally abnormal. The patient also has had a previous miscarriage at 6 weeks (conceived naturally), which may also be due to the chromosomal complement of the oocyte.

Embryos developed from giant oocytes have the potential to result in blighted ova, spontaneous miscarriage and congential malformations. Therefore it can be concluded that any embryos resultant from giant oocytes, regardless of apparent normal fertilisation, should be excluded from treatment in IVF and ICSI cycles.