Researchers said Wednesday that they have successfully used mouse stem cells to grow kidney kidney embryos using a technique that could one day help to grow human kidneys for transplantation.
But researchers warned that their success was only a first step and that "serious technical barriers and complex ethical issues" remain before the process can be used for human organs.
The technique has previously been used to grow mouse-derived pancreas in rats, but the new study is the first proof that one day it could provide a solution to the massive lack of donors for people with kidney disease.
The research, published Wednesday in Nature Communications journal, began with the development of a suitable "host" where the kidneys could be grown.
The researchers collected rat embryo structures that had been genetically modified so that they would not develop kidneys on their own.
The embryos were then injected with mouse pluripotent stem cells and implanted into rat glands so that they could be worn for term.
Pluripotent stem cells are a kind of "master" cell that can develop into any of the cells and tissues that make up the body.
The researchers found that the mouse stem cells apparently produced functional kidneys in the rats.
But the same did not hold when rat stem cells were injected into similarly modified mouse embryos.
"Rat stem cells are not easily distinguished into the two major types of cells necessary for kidney formation," said Masumi Hirabayashi, a lecturer at Japan's National Institute for Physiological Sciences, who oversees the study.
Conversely, mouse stem cells differentiated effectively … the formation of the basic structures of a kidney, "he told AFP.
The reason for the difference is still not quite clear, but the researchers believe that "environmental angles" inside the mice are probably due, rather than stem cells or technique.
But even in the rat embryos, the technique was not without problems.
While the rats apparently developed functional kidneys, including with proper connections to the urethra, which connect the kidneys to the bladder – they died shortly after birth because they had not sucked properly.
Removing the genes that allow the kidneys to develop in utero also seems to have removed their sense of smell so that the newborns failed to detect milk and died.
Their short life meant that limited testing could be performed on their kidney function, but Hirabayashi said the organs seemed functional "based on anatomical observations."
There are other concerns: Growing a kidney in a host from another species can lead to "contamination" of the organ with cells from the host.
And the process of cultivating human organs in animals constitutes an ethical assault because human stem cells can develop into brain or reproductive organ cells in the host.
"The main ethical concerns are the risk of consciousness and / or gamete (reproductive cell) production," Hirabayashi said.
"There are serious technical barriers and complex ethical issues to be discussed and resolved before producing human organs in animals," he added.
In the short term, further research is likely to focus on ways to genetically modify host rats without fatal side effects.
If successful, the researchers would like to run several tests on stem cell kidneys and try to transplant them from the hosts to other animals.
Eventually, testing can involve trying to grow human organs.
Pigs are usually regarded as the best hosts for human regeneration, but pig embryos only develop for 16 weeks, as opposed to the 40 weeks that human babies grow, meaning that the species may not be suitable for growing organs in embryos.
Cattle, with a 40-week gestation, could be another option.
Hirabayashi said he was hoping that he would see human bodies grown in animal warts during his lifetime.
"I don't know exactly the end of my life – tomorrow? Thirty years? But I expect a lot to hear news about the practical use of animal organs."
Published in Daily Times, February 6th 2019.