Sterile Mice Can Produce Rat Sperm With New Stem Cell Injection

The mouse-rat chimeras demonstrate that sterile animals can be used as hosts for the generation of sperm from other species.


Rachael Funnell

Social Editor and Staff Writer

clockAug 4 2022, 15:10 UTC
mouse rat chimera
This psychedelic background actually shows a mouse-rat chimera testis cryosection. Image credit: Joel Zvick/ETH Zurich,

Researchers have created sterile animals capable of producing the fertile sperm of a different species through a technique called blastocyst complementation. The approach meant they were able to collect rat sperm from mice that had been genetically engineered to be sterile.

Stem cell research harnesses the versatility of cell types to produce tissue and organs but creating gametes such as sperm and egg have proven difficult to achieve (though researchers did recently make a mouse embryo from stem cells). Now, new research published in Stem Cell Reports has been able to promote the generation of rat sperm in a sterile mouse by injecting them with pluripotent stem cells (PSCs).


A chimera is defined as an animal whose body is made up of cells that are genetically distinct from one another. It’s happened in humans, including a woman who was her own twin, and in the wider animal kingdom (such as this kitten called Apricot).

By that definition, the study created mouse-rat chimeras by introducing cells made up of rat genetic material inside the testes of mice.

“We were surprised by the relative simplicity by which we could mix the two species to produce viable mouse-rat chimeras,” said senior author and stem cell biologist at ETH Zurich Ori Bar-Nur in a statement.  


“These animals, by large, appeared healthy and developed normally, although they carried both mouse and rat cells in a chimeric animal. The second surprise was that indeed all the sperm cells inside the chimeras were of rat origin. As such, the mouse host environment, which was sterile due to a genetic mutation, was still able to support efficient sperm cell production from a different animal species.”

The achievement demonstrates that it’s possible to get a sterile animal to create the sperm cells of a different species, but it wasn’t perfect.

Sperm created by the mouse-rat chimeras was immotile, meaning in real-life reproductive circumstances it wouldn’t be able to fertilize an egg. In lab settings, it was able to fertilize eggs but at a significantly lower rate compared to natural rat sperm.


Furthermore, of the eggs that were fertilized, none of the embryos developed normally or gave rise to live offspring.

That said, the achievement represents a novel method for generating species’ gametes which could prove useful in conservation, as well as wider stem cell research.

“Our study shows that we can use sterile animals as hosts for the generation of germ cells from other animal species,” said Bar-Nur. “Aside from a conceptual advancement, this notion can be utilized to produce endangered animal species gametes inside more prevalent animals.”


“However, it is important to note that several scientific hurdles will need to be overcome to adapt this technique to other animal species. In addition, one still needs to showcase the production of female reproductive cells (i.e., eggs) in female sterile mice, especially if we envision utilizing this technology for species conservation efforts.”

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