Although no living male northern white rhinos remain on Earth, experimental assisted reproduction techniques treatments still offer the functionally extinct species a fighting chance. Scientists are now reporting a promising breakthrough in the battle for their survival, producing first-of-a-kind test tube embryos they claim can offer a high probability of pregnancy in a surrogate mother.

The last male northern white rhino was euthanized at a Kenyan conservancy in March, as a result of age-related complications. Only two female northern white rhinos remain, but the living 21,000 southern white rhinos is where conservationists see a glimmer of hope. This is because they could serve as surrogate mothers if and when early-stage embryos developed from the egg and sperm of their northern cousins can be successfully implanted to establish a pregnancy.

Scientists at the German Leibniz Institute for Zoo and Wildlife Research in Berlin (Leibniz-IZW) have landed on a promising stepping stone, successfully creating a hybrid embryo using an egg from a southern white rhino and sperm from a northern white rhino.

They achieved this by adapting techniques already used in horses, that saw immature egg cells called oocytes collected from the females and then matured and fertilized with sperm from now deceased northern white males.

"Our results are solid, reproducible and very promising," says Professor Thomas Hildebrandt, Head of the Department of Reproduction Management at Leibniz-IZW. "Now we are well prepared to go to Kenya and collect oocytes from the last two NWR females in order to produce pure NWR blastocysts where both eggs and sperm are from NWR."

While this would be significant advance, it wouldn't necessarily mean the return of the northern white rhino. Because only two females remain and preserved semen is only available from four males, there is not enough genetic diversity to spawn a self-sustaining population of the species.

But the scientists have plans to expand the genetic foundations by turning to stem cell technologies to convert other cryopreserved northern white rhino cells into the necessary ingredients.

"In the future our goal is to produce in vitro primordial germ cells from iPS cells (induced pluripotent stem cells) obtained from somatic cells, cryopreserved in the past, of several northern white rhino individuals. In a second step these germ cells will then be transformed into eggs and sperm," explain Dr Sebastian Diecke, stem cell expert at the Berlin's Max Delbrück Center for Molecular Medicine and Professor Katsuhiko Hayashi from Department of Stem Cell Biology and Medicine, Kyushu University, Japan.

The research was published in the journal Nature Communications.

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