In an improbable experiment, they are trying to resurrect an entire species using dead males, two infertile remaining females, and some closely related southern white rhinos.
And they say they also plan to use stem-cell technology to try to create a population of pure northern white rhinos in the lab.
The European team of researchers created a few early-stage rhino embryos using a technique called intracytoplasmic sperm injection or ICSI. They used sperm from the last northern white rhino males before they died. Some of this sperm was injected into egg cells taken from females of the southern white rhino subspecies.
They ended up with a handful of embryos, some of which could potentially be implanted into surrogate rhino mothers.
While any resulting baby rhinos would be hybrids — half northern and half southern white rhino — the experiment is a first step to re-creating an extinct species in the lab, the researchers report in the journal Nature Communications.
Northern white rhinos were driven to extinction by poaching and war in the countries they once roamed — Chad, Sudan, Uganda, Congo and Central African Republic.
The last living male white northern white rhino, named Sudan, died last March. Two females remain in captivity, but they are infertile. No northern white rhinos remain in the wild.
Thomas Hildebrandt of the German Leibniz Institute for Zoo and Wildlife Research in Berlin and colleagues in the Czech Republic and Italy, as well as in Japan, started an all-out effort to create new white rhinos in the lab.
“We didn’t have many options so we had to be realistic."
They've succeeded in their first step - making hybrid embryos. "These are the first in-vitro produced rhinoceros embryos ever. They have a very high chance to establish a pregnancy once implanted into a surrogate mother," Hildebrandt said in a statement.
It's a tricky process. Rhinos are big and it's not easy to make them cooperate. The females had to be anesthetized and a special tool used to collect their egg cells. "We … were highly afraid that something unexpected would happen during this procedure," Hildebrandt told a telephone briefing.
And the frozen sperm was not the greatest. "The amount of semen is limited, is of poor quality, and has been obtained from only three bulls," the team wrote in their report.
They hope to eventually implant their embryos into the more common southern white rhino females and then breed any resulting rhinos to try and concentrate their northern white rhino genes. For now, these early embryos, called blastocysts, have been frozen.
This will probably not be enough to save a whole subspecies, so the team is also using stem cell and cloning technology to try to make pure northern white rhinos. They pulled embryonic stem cells — the body's master cells, from four of the blastocysts.
What they want to do is turn these embryonic stem cells into eggs and sperm using stem cell technology.
Chemical and genetic techniques can be used to get embryonic stem cells to turn into any cell type desired, including egg and sperm cells.
There's a second way to make them, also. The team has tissue samples from northern white rhinos. They plan to use chemical and genetic techniques to transform these ordinary skin cells into more cells resembling embryonic stem cells. These lab-produced cells are called induced pluripotent stem cellsor iPS cells.
In theory, these laboratory-made sperm and egg cells could be combined in a lab dish via in vitro fertilization (IVF) to make a pure northern white rhino embryo, which could be implanted into a southern white rhino surrogate mother.
The scientists hope they could make enough rhino babies this way to have a genetically diverse population that could, eventually, be re-introduced into their traditional range.
"Our results suggest that these methods could play a valuable role in the effort to save rhinoceros populations on the brink of extinction," they wrote.
Don't get your hopes up, caution Terri Roth and William Swanson of the Center for Conservation and Research of Endangered Wildlife (CREW) at the Cincinnati Zoo & Botanical Garden.
"Impressive results in a Petri dish don't easily translate into a herd of healthy offspring," the two experts, who were not involved in the research study, wrote in a commentary.
"It remains unlikely that a viable population of Northern white rhinos will be restored."
Using assisted reproductive techniques in wild species of animals have not worked as well as they have in domesticated animals such as horses and cattle, they pointed out.
"In almost all cases, they result in lost pregnancies and/or neonatal mortalities, efficiency is extremely low and overall conservation value questionable," they wrote.
But Cesare Galli of Italian biotechnology lab Avantea, which specializes in livestock breeding in the lab, said there were not many other choices. "We didn't have many options so we had to be realistic," he said.