Researchers in Japan say they have overcome an obstacle that has prevented the use of transplanted kidneys grown from human stem cells.

If at first you fail, dust yourself off and try again.

Takashi Yokoo and his team of colleagues at the Jikei University School of Medicine in Tokyo did just that.

With the number of patients with end-stage kidney failure increasing globally due to a limited supply of donor organs, Yokoo and his team set out to find a way to grow a kidney to full size, as well as a way for the new kidneys to pass urine.

Of late, researchers have succeeded in growing functional kidneys from human stem cells. However, because there was no pathway for excreting urine, the kidneys were not able to grow to full size.

The Japanese research team was looking to fix that condition, called hydronephrosis, by running tests on lab rats.

“Several groups, including us, have reported regeneration of kidney, which produces urine, but there is no report which shows to let urine out from the body,” Yokoo, professor and chair of the division of nephrology and hypertension at the Department of Internal Medicine at the university, told Healthline. “This step seemed to be easy, and several trials have been made to use artificial tube or native ureter to connect the regenerated kidney and native bladder, but such trials ended in failure.”

So, Yokoo explained, they needed to create a novel system for excreting urine from regenerated kidneys.

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That system, called the stepwise peristaltic ureter system, was used to implant embryonic rat kidneys with bladders into adult rats.

After four weeks, the team connected one of the rat’s ureters to the transplanted bladder. That allowed the urine from the transplanted kidneys to go into the transplanted bladder and then into the host bladder. That process helped avoid hydronephrosis.

Eight weeks after the transplantation, the kidney tissues were beginning to resemble mature kidneys.

Just to make sure of the system’s feasibility, the team repeated the experiments using pigs with similar results. The entire process, Yokoo said, took five years.

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Allowing a urine excretion pathway and allowing long-term growth of newly generated kidneys, the team concluded, can lead to overcoming challenges in generating functional kidneys from stem cells.

Yokoo is pleased with his team’s results. He is optimistic about the possibilities going forward and is already onto the next step of the process.

“We already started the next stage to show that this system can be used in primates, using marmosets for the application to human,” he said.

The hope for Yokoo and the Jikei University team is to complete the experiments within the next 10 years.

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The American Cancer Society estimates there will be 61,000 new cases of kidney cancer in the United States this year.

About 14,000 Americans will die from the disease in 2015.

Kidney cancer is rare in people under the age of 45. The average age in the United States for diagnosis is 64.

There are more than 100,000 people in the United States awaiting kidney transplants, according to the National Kidney Foundation. The median wait-time is 3.6 years.

In 2014, there were slightly more than 17,000 kidney transplants done in the United States.

Every month, 3,000 people are added to the kidney transplant waiting list. On average, 12 people die each day while waiting for a kidney transplant.