Ovarian xenotransplantation could offer hope for rare species

Xenotransplantation of ovarian tissue from endangered species may allow zoologists and conservationists to propagate endangered animals, according to a paper published today (September 27) in the journal Science.

Researchers at the Monash Institute of Reproduction and Development (Monash IRD) have demonstrated that eggs derived from ovaries grafted onto tissues in another species can give rise to live young.

Dr Shae-Lee Cox and PhD student Melanie Snow took ovarian tissue from FVB mice and implanted it under the kidney capsule of both male and female nude rats, an immunocompromised strain. Three weeks after the engraftment, the rats were given hormones to induce superovulation, and eggs were harvested a couple of days later.

The eggs were then matured in vitro before inseminating with sperm from FVB male mice, and after embryo culture, two-cell embryos were transferred into surrogate female mice to produce live young.

Five pups in total were born from females receiving embryos derived from the xenografted ovaries, and these pups all matured normally and were themselves fertile. While all of these pups came from the xenografts in female rats, Cox noted that failure for eggs from the male xenografts might have been due to the surrogate mother failing to get pregnant rather than embryo viability problems.

The technique could be applied to endangered animal species as long as there were established protocols for in vitro culture and fertilisation for the species and an available surrogate mother, said Cox.

"For example, with this technique we could take ovarian tissue from a live or recently dead female of an endangered species, put that tissue into a rat and the immature eggs, which are the most abundant eggs in the ovary, will become mature," Cox explained.

"We could then remove the eggs grown in the rat host, mix them with the sperm of the endangered species, and then transfer the embryos into a surrogate mother from that endangered species or possibly a closely related species."

Cox said the group was currently measuring the outcome of the xenotransplantation technique in the tamar wallaby and the common wombat, as well as continuing experiments with rats and mice.

"Wallaby tissue does grow in mice and eggs can be collected," she said.

According to Cox, the technique may be important in saving ovaries from endangered species that die prematurely, as the tissue can be grafted from recently deceased animals. She said that the Animal Gene Storage Resource Centre of Australia, based at Monash IRD, had stored sperm, eggs, embryos and ovarian tissue from endangered species and could theoretically be used to increase the endangered populations.

But Cox cautioned against human use of the technique for assisted reproduction purposes, explaining that transfer of retroviruses from graft hosts to humans was one potential risk. In addition, she said that not much was known about whether xenotransplants caused genetic changes to the eggs.

While other research groups have attempted autologous transplants of human ovaries, no pregnancies had been achieved to date, Cox said. Transplant to human recipients was unlikely to work as the immunosuppressive drugs required often killed the eggs.

For more information: M Snow, S-L Cox, G Jenkin, A Trounson and J Shaw (2002). 'Generation of Live Young from Xenografted Mouse Ovaries', Science 297:2227.