Oocyte Vitrification

Oocyte Vitrification

December 01, 2007
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While it has been possible to preserve sperm for many years (the famed Dutch microscopist Anton von Leeuwenhoek allegedly cooled and then recovered sperm using snow and ice in the 17th century), reliable methods for oocyte preservation have been elusive.

We previously discussed some of the problems with oocyte freezing (see Fertility Flash, January 2005, Volume 3, Issue 1), and now report our success in overcoming some of the obstacles.

Traditionally, preservation of sperm and embryos has been achieved with the use of a technique called slow freezing. This process incubates the sperm or embryos in low concentrations of cryoprotectants (antifreeze) to draw water out of the cells. After this incubation, they are cooled very slowly to sub zero temperatures. Typically this slow freezing technology just works for cells that exist individually (such as sperm), or together in small numbers (embryos), as the water must be extracted from every cell. Tissues, which are made up of many hundreds of thousands of cells, cannot be dehydrated successfully and therefore cannot be frozen intact. Cells in the tissue can burst when the water remaining in the cells expands as it turns to ice. For example, it is not possible to freeze a whole ovary, but some success has been achieved with ovaries that were cut into tiny pieces.

Frustrated by the lack of progress with slow freezing, scientists have more recently moved towards a technology called vitrification for oocyte preservation. Vitrification, which was described in detail in September’s Fertility Flash (Volume 5, Issue 8 ) works by using higher concentrations of cryoprotectants and much faster cooling rates. Cells are typically cooled in tiny straws (see article heading). This process allows us to achieve cooling rates of several thousand degrees per minute.

When vitrification straws and cryoprotectants were first approved by the FDA for human embryos, PFC began the process of adapting the technology to oocytes. Our embryologists attended training courses and became proficient with the technology by practicing on mouse and hamster oocytes and embryos. Even though we have been handling oocytes and embryos for many years, this technology provided many new challenges, mainly due to the tiny size of the straws and the speed at which the cells had to be cooled. Once we became proficient with the procedure, we began to freeze high quality oocytes from donors who had proven fertility. In this way, we knew that if the procedure did not work, it would be the vitrification technology and not the oocytes that were to blame. In addition, we satisfied ourselves that the technology was safe by looking at the exhaustive work by Dr. Gary Smith at the University of Michigan, which showed that vitrified/warmed oocytes were both physically and genetically normal and that the resulting pregnancies and babies were healthy.

We recruited five oocyte donors and vitrified all of their oocytes immediately after their oocyte retrieval procedures. We then offered the oocytes to individuals who were on our waiting list to accept donated embryos. Typically, these individuals were unable to get pregnant with their own oocytes or financially unable to proceed to an egg donor cycle. The availability of the vitrified oocytes was a great alternative to accepting donated embryos as it allowed couples to choose their own sperm source. Furthermore, the immediate availability of vitrified oocytes was an attractive alternative to what may be a very long wait for donated embryos.

Pacific Fertility Center had immediate success with the first recipient. We had vitrified 16 oocytes from the first donor, and for the first recipient we warmed only 7 of these. Four hours later we injected a single sperm into each of the 6 oocytes that appeared alive and healthy (1 oocyte had not come through the process successfully). The next morning, 3 of the oocytes fertilized normally. After 2 more days, we had 3 nice embryos for transfer. The positive pregnancy test 11 days later, and a singleton pregnancy confirmed by ultrasound at 7 weeks were great rewards for our efforts and thrilling news for the recipient. Our second recipient used a different donor and although her pregnancy started out well, she miscarried in the first trimester. Our disappointment over this loss was compounded when we discovered the oocytes from 2 of the donors did not survive well when warmed. In these particular donors, we recovered high numbers of oocytes (each had close to 40) and for unknown reasons their oocytes were overly sensitive to vitrification. The next three donor cycles proceeded well and resulted in pregnancies. These 3 pregnancies are all ongoing at the time of writing. We will update readers with their outcomes at a future date.

Although we were warming relatively small numbers of oocytes (typically 6 or 7), we began to have more embryos than could be safely transferred to recipients. Our first pregnancy had been achieved after transferring 3 embryos. It is more typical, however, to transfer only 1 or 2 embryos when donor oocytes are used. Even when using only 2 embryos, multiple pregnancy rates were unacceptably high. Understandably, few patients are willing to risk a decreased chance of conceiving by transferring only a single embryo. In order to avoid high multiple pregnancy rates in a typical IVF cycle, embryos are usually cultured for 5 days to determine which embryos in a cohort have the best chance of establishing a pregnancy. However, if a patient has only a few embryos, the benefits of extended culture are less, and the transfer is typically done after only 3 days growth. With our recipients of the vitrified oocytes, we began by doing 3-day transfers. Once high success rates were evident, we elected to implement day-5 transfers, in an effort to decrease high order multiples. The last 2 pregnancies both resulted from day-5 transfers of 2 embryos each, and they are both twin gestations.

In summary, we have had 7 out of 10 embryos implant after transfer (excluding the 2 failed donors with the high oocyte numbers). This implantation rate (70%) is comparable to the implantation rates that our patients have when using fresh embryos from donor oocytes.

We are moving forward cautiously with our oocyte vitrification program and hope to use the remaining oocytes soon. While these results are encouraging and have brought great joy to a small number of our patients, there are more issues to resolve before we declare complete success. The 70% success rate was obtained with the use of the highest quality oocytes from young donors who were known to be fertile and healthy. We have already seen that some oocytes are less tolerant of the procedure, as evidenced by the results from the 2 donors with high oocyte numbers. We also fully anticipate that the results for older women using their own oocytes will be worse, as they are for these same patients using a fresh IVF cycle. In fact, at this time, we do not have any idea if the oocytes from women in their 30’s will be able to tolerate vitrification.

Going forward, we will offer oocyte vitrification unconditionally to women with cancer who are likely to be left sterile by their treatment. For these women, and for others who elect to vitrify oocytes for social reasons, we will exercise great caution in our estimates of future pregnancy potential with the warmed oocytes. Until we have more data with oocytes from a variety of women, we will have no way of telling if there is any hope from anything other than donor oocytes. That data will accumulate more slowly because women who elect to preserve oocytes are not likely to be using them for some time. For now, until there is more data, we continue to believe that embryo freezing has the greatest potential for those wishing to preserve future fertility. However, for those who are single and in their late 30’s, we will be reluctant to recommend oocyte vitrification as a reliable fertility preservation method. Hopefully, they will find Mr. Right before we have objective data.

Joe Conaghan, PhD, HCLD

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