In the bodies of sterile mice, rat spermatozoa have been created. To achieve this fantastic result, scientists used a technique called blastocyst complementation. Pluripotent stem cells (PSCs) are a powerful biomedical tool, but obtaining gametes (eggs or sperm) from PSCs is extremely challenging. Modified gametes are often less viable than animal host cells and therefore lose out to them in fertilisation competition.
One possible solution to the problem is the method of blastocyst complementation. The idea is that stem cells from one animal are implanted into a mutant embryo in which some organ does not develop. As a result, an organ develops in the mutant animal from the provided stem cells and, say, a mouse kidney develops in the rat.
The researchers now wondered if it was possible to obtain rat sperm in a mutant mouse, which is itself infertile. To test this idea, they injected rat PSCs into mutant mouse embryos (they had the Tsc22d3 gene, critical for sperm development, "turned off") to produce mouse-rat chimeras. The rat stem cells evolved with the mouse cells, resulting in a chimeric animal carrying DNA of the two species. As a result of a genetic mutation causing infertility, the mouse sperm itself did not develop, so their place was taken by rat PSCs developing into rat sperm.
The sperm were viable and were able to fertilize rat eggs (even if it took them longer than "normal" cells), but the scientists could not get live rats. Nevertheless, this work is the first evidence of the possibility of obtaining gametes of one species in the body of another.
If the researchers succeed in producing living offspring from pluripotent stem cells, the method could be used to preserve rare and endangered animal species and in biomedical research, such as the modelling of human inherited diseases.