Scientists are one step closer to being able to grow human replacement kidneys after they used rat incubators to grow kidneys from stem cells that were successfully tested in pigs, according to the Daily Mail.

Results from the Jikei University School of Medicine experiment are outlined in a study that was published in the Proceedings of the National Academy of Sciences.

One of the biggest problems with kidney transplants is providing them with a proper urine excretion pathway. This problem was solved by Takashi Yokoo and his team of researchers by creating the "Stepwise peristaltic ureter", or SWPU system, which is what allowed them to successfully transplant the kidneys into pigs.

"The SWPU system may resolve two important problems in the generation of kidneys from stem cells: construction of a urine excretion pathway and continued growth of the newly generated kidney," wrote the researchers.

The SWPU system is essentially an added pathway connected to an existing bladder, allowing for proper drainage of urine. Although it is far from being tested on humans, it makes significant progress towards eventually growing fully functional human kidneys from stem cells.

Despite this promise, some think that it will be some time before an effective system for human stem cells is developed, according to BBC News.

"This is an interesting step forward. The science looks strong and they have good data in animals," said Chris Mason, an expert in stem cells and regenerative medicine at University College London.

"But that's not to say this will work in humans. We are still years off that. It's very much mechanistic. It moves us closer to understanding how the plumbing might work. At least with kidneys, we can dialyse patients for a while so there would be time to grow kidneys if that becomes possible," he added.