Endangered white rhinos' cells turned into sperm and egg stem cells

Cells with the potential to form sperm and eggs have been derived from northern white rhino skin cells, raising hopes that lab-grown embryos could save the animals from extinction.

The only two remaining northern white rhinos (Ceratotherium simum cottoni) are females – Najin and her daughter Fatu at the Ol Pejeta Conservancy, a reserve in Kenya. The last male northern white rhino, called Sudan, died in March 2018.

The following year, researchers attempted to save this subspecies by using sperm from deceased male northern white rhinos, including Sudan, to fertilise egg cells collected from Najin and Fatu.

This has so far produced 22 northern white rhino embryos that have been frozen for future implantation in southern white rhinos (C. simum simum), a closely related subspecies of which about 20,000 remain in southern Africa.

Another possible way to produce embryos would be to turn skin cells from southern white rhinos into stem cells, which have the ability to turn into various types of specialist cell. These could then be coaxed to form egg and sperm cells – an approach that has worked in mice.

To test this method for northern white rhinos, Katsuhiko Hayashi at Osaka University in Japan and his colleagues first bathed skin cells from a female northern white rhino who died in 2015, called Nabire, in a cocktail of chemicals that turned them into stem cells with the potential to form any type of cell.

The team then tested different mixtures of growth factors and chemicals to make such stem cells from southern white rhinos turn into the precursors of sperm and egg cells, called primordial germ cell-like cells. Once they found the precise cocktail that successfully did this in southern white rhinos, they applied it to the northern white rhino stem cells and found it worked for them too.

“This was a lot of hard work, and it will be very beneficial to the field,” says Jeanne Loring at the Scripps Research Institute in California. But further work is needed to see if such egg and sperm cells result in fertilised embryos, she says.

Even then, it is unclear whether southern white rhino surrogates would successfully carry the embryos to produce northern white rhino offspring, says Loring.

“There is the additional challenge of acquiring the genetic diversity that is necessary to sustain a population. Inbreeding invariably has bad consequences,” she adds.

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