Scientists developed a hybrid embryo by combining the sperm of the northern white rhino and the egg of the southern white rhino, which population is now estimated at around 21,000 individuals, according to the zoo statement.
But on Wednesday, Dr Hildebrandt and a team of colleagues reported in the journal Nature Communications that the story of the northern white rhino is not, in fact, over.
"You can't reach the ovaries by hand, so we developed a special device", Prof Hildebrandt explained to BBC News.
Because the sperm comes from a few rhinoceros bulls, limited genetic diversity could endanger the health of a newly bred northern white rhino population. No northern white rhinos remain in the wild.
"Our results are solid, reproducible and very promising".
The researchers froze their hybrid rhinoceros embryos so they can try implanting them into a female SWR surrogate using in vitro fertilization (IVF) at some point in the future. "These techniques have the potential to help the other endangered rhino species, including the Sumatran rhino and the Indian rhino and and other large mammals such as the Gaur, a large Asian cow that is also at risk of extinction". For safe collection of female bio-material, the zoologists used a recently patented instrument with a length of almost two meters.
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These researchers think so, and others around the world who have been involved in efforts to save the northern white rhino say it is an important step. If they thrive, they will be implanted to surrogate rhino mothers that could, later on, give birth to a rhino calf closely related to Sudan.
The key question of whether scientists can produce pure northern white rhino embryos using this technique remains unanswered.
However, the southern white rhino has been considered one of the hallmark conservation success stories, with a population estimated by some groups to now exceed 20,000 individuals. The embryos developed to the blastocyst stage, which increases the chances of pregnancy after implantation into a surrogate female.
Professor Cesare Galli, from the Avantea research laboratory in Cremona, Italy, which specialises in veterinary IVF, said: "In our lab we were able to develop procedures to mature the oocytes, fertilise them by Icsi and culture them".
They are the world's most endangered animals, with just two females left.
Of the 13 eggs injected, four developed into blastocysts (an early embryo), all of which showed signs of healthy embryonic stem cells. Scientists began to freeze northern white rhinoceros sperm in 2008, Hildebrandt said, storing 300 milliliters' worth, roughly the volume of a soda can.
"Many people working in conservation are strongly against using biotechnologies, which seems insane, because if you have the technology, why not use it for the good?" he said. "I do think that if there's one thing we should learn from this, it's that we shouldn't allow species to get to such a critical state that these high-tech approaches are the only ways of saving its genes".