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Published: Friday, March 11, 2011 - 17:03

Vitrification: What does it mean?

First the background, briefly: In the last few years, fast freezing of the oocytes (female eggs) has been adopted as the preferred way of preservation. This is what is known as vitrification. Vitrification involves dropping the temperature at a super-fast rate of 23,000 °C per minute, a rate that is almost 70 thousand times faster than the conventional slow freezing of 0.3°C per minute. The slow freezing was always associated with the problem of ice crystals forming within the eggs, something which led to damage of some of the eggs or embryos. With vitrification, the speed of cooling is such that ice crystals simply cannot form.

Just as good

The concern that has lingered with the vitrification process is that these oocytes (eggs) may not perform as well as fresh oocytes. Now, in a report appearing in the February 2011 online edition of the journal Human Reproduction, Peruvian researchers reiterate findings earlier studies  showing that fertilization outcomes from vitrified oocytes are similar to those obtained from fresh oocytes.

Vitrified oocytes had a survival rate of 89.4%, the researchers report. The fertilization rate in the vitrification group was 76.1% compared to 87.5% seen in the fresh group, a difference that was not statistically significant.

Drs Javier I. García, Luis Noriega-Portella and Luis Noriega-Hoces of the Concebir Clinic, Lima, and colleagues note that an alternative to slow freezing, the vitrification procedure has allowed improvements in oocyte survival, fertilization, embryo development and clinical outcomes.

To gain further information, the researchers studied outcomes in 1098 oocytes obtained from 78 donors. Of these oocytes, 312 were vitrified and 768 were used when fresh. There were 34 recipients who received blastocysts obtained from vitrified oocytes and 58 who received blastocysts from fresh oocytes.

Note: A blastocyst is a stage reached at around 5-6 days after fertilization and it consists of around 70-100 cells following the repeated division  of the fertilized egg.

There was no significant difference between the day 2 cleavage rate (96.3% and 98.0%) and the blastocyst formation rate (41.3% and 45.3%). The pregnancy rates were also similar (61.8% and 60.0%), as were implantation rates (43.9% and 42.9%) and miscarriages (9.5% and 5.9%).

These results, the researchers conclude, "confirm that oocytes conserve their capacity for fertilization and potential to reach the blastocyst stage after being vitrified, similar to fresh oocytes."

Moreover, they assert that "the vitrification procedure represents the best current alternative for creation of reliable oocyte banks."

The legal issues

For those where there is a clear medical indication, this is clearly a very important medical advance. Women who are at risk of losing their ovaries because of disease, usually cancer, or as a result of treatment can take advantage of this. For individuals where there is no obvious medical indication, it is not so straight-forward. There are, of-course, legal issues to be considered with this evolving and advancing technology. Should you consider taking advantage of this technology to postpone having children?

Here is the current regulatory clause: The European Society for Human Reproduction and Embryology Task Force on Ethics and Law published ethical considerations for the cryopreservation of gametes and reproductive tissues for self-use in 2004. In clause 2.2, it stated, ‘‘In view of the lack of success and clinical application in the case of ovarian tissue, this application (reproductive tissue cryo-preservation) should not be offered to women as a means to preserve their fertility potential when there is no immediate threat to their fertility.’’ In addition, ‘‘According to similar reasoning, oocyte freezing for fertility preservation without a medical indication should not be encouraged.’’ In a similar vein, the Practice Committee of the American Society for Reproductive Medicine released a statement in late 2007 that ‘‘oocyte cryo-preservation should not be a means for women to delay reproduction’’.  

That is where we are now but, as medical advances keep coming, the regulators will be called upon to keep reviewing their stance.

Oocyte vitrification outcomes as good as with fresh eggs.

By Dr Joe Kabyemela, MD