Scientists in Israel say they've found a possible new way to produce embryos for use in assisted reproduction. In a report published in mid-September in the journal Human Reproduction,¹ the scientists described the successful transplantation of previously frozen ovaries in a group of sheep. Once the transplant procedures were completed, the researchers were able to retrieve healthy eggs and triggered them into early embryo development using a special lab experiment.

"There is a lot of research still to be done, but we hope that it will not take more than a few years for this to become a practicable option for women, such as young cancer patients, who would otherwise be left infertile after their treatment," explained Yehudit Nathan, program manager at Core Dynamics, the biotech firm that funded and provided the scientific and technological expertise for the project.

Nathan was also one of the investigators in the study.

In addition to this feat, the scientists found that the ovaries in two sheep from which eggs were recovered were still functioning normally three years later.

Feasible Option for Cancer Patients?
The results of this study show for the first time that it is possible in a large animal species to remove, freeze, thaw, and replace ovaries, obtain eggs, and maintain normal long-term ovarian function, said the study's chief investigator, Amir Arav, DVM, PhD, a senior scientist at the University of Animal Science, part of the Agriculture Research Organization in Israel. This also establishes a foundation for an approach that could become a feasible treatment for women facing premature ovarian failure.

Nathan says the next step will be to attempt to transplant ovaries in women at risk of losing their fertility due to potentially damaging cancer therapy.

Other Trials of Ovary Transplantation
Whole ovary transplants have been attempted twice in women, once in 1987, and again in 2004.² Both procedures involved a transplant to the upper arm.

Other earlier research involved the transplant of frozen-then-thawed ovarian tissue.^3,4 However, adhesions and the loss of blood to the ovarian follicles that occurs during the transplant process remain major hurdles to achieving success using this technique, experts have reported. Further, egg freezing as a method to preserve fertility has been investigated in recent years, but it is only successful less than 2% of the time, they wrote.

The researchers in the latest study thus defend whole-ovary transplantation as a preferred approach due to the limitations of these related procedures.

Testing Follicle Survival
The scientists in this study wanted to attempt a vascular transplant with the ovary, so that the cutoff of blood to the follicles was minimized. "A vascular transplant would prevent ischemic follicular loss, and thus the functional lifespan of a vascular ovarian transplant would be considerably extended," they wrote.

The Israeli team wanted to first test whether whole ovaries taken from sheep, together with their associated blood vessels, could survive the freezing process after thawing using a technique that allows precise control over the formation of ice crystals. This reduces the damage to cells that has been shown using more conventional methods. This technique worked in this study. There were no differences in follicular survival, the Israeli team wrote, when comparing previously frozen ovaries with fresh ones.

Viable Ovaries After Transplant?
Next, the researchers wanted to see if they could remove, freeze and thaw the right ovary from eight sheep, and replace the ovary up to two weeks later, either at the original site or by attaching it on to the pedicle (the attachment that contains the main blood vessels) of a previously removed left ovary.

Of five sheep that had normal bloodflow restored, indicating the transplant had been a success, one had severe adhesions, and it was not possible to attempt egg retrieval. But two other animals yielded one egg each, and four months later, four more eggs were taken from one of those sheep.

All six eggs were transformed into embryos using a special process that simulates fertilization (parthenogenesis). The process was used "as we had only a low number of oocytes [eggs] and because IVF success in sheep depends on the quality of the ram's sperm," explained Arav. "This way, we knew that the development of the embryo depended solely on the quality of the oocyte."

Normally Functioning Ovaries Found
Two years after the transplantation, the investigators performed magnetic resonance imaging (MRI) tests on one sheep with a transplanted ovary, and compared the outcome with that of a sheep that underwent no transplants. It showed that the transplanted ovary contained small egg follicles, and although it was a bit smaller than the ovary in the sheep without treatment, its size was within the normal range, the investigators stated. The blood vessels were also intact.

Hormone testing revealed that two sheep maintained their normal cycles after the transplants, as well.

"Adhesions on the transplant might interfere with natural conception, so pregnancy may require IVF techniques," Arav said. "But we have been able to demonstrate long-term intact organ cryopreservation with restored function following thawing and transplantation in a large animal for around 36 months post-transplantation."

"This approach could revolutionize the field of cryopreservation for diverse human applications, such as organ transplants, as well as helping women who face the loss of their fertility," he said.

1. Arav A, Revel A, Nathan Y et al. Oocyte recovery, embryo development and ovarian function after cryopreservation and transplantation of whole sheep ovary. Hum Reprod 2005 Sep 20; [Epub ahead of print].
2. Hilders CG, Baranski AG, Peters L, Ramkhelawan A, Trimbos JB. Successful human ovarian autotransplantation to the upper arm. Cancer 2004 Dec 15;101(12):2771-8.
3. Donnez J, Dolmans MM, Demylle D et al. Livebirth after orthotopic transplantation of cryopreserved ovarian tissue. Lancet 2004 Dec 4;364(9450):2020.
4. Meirow D, Levron J, Eldar-Geva T et al. Pregnancy after transplantation of cryopreserved ovarian tissue in a patient with ovarian failure after chemotherapy. N Engl J Med 2005 Jul 21;353(3):318-21.

John Martin is a long-time health journalist and an editor for Priority Healthcare. His credits include overseeing health news coverage for the website of Fox Television's The Health Network, and articles for the New York Post and other consumer and trade publications.