Image copyright Science Photo Library

Scientists say they are a step closer to growing fully functioning replacement kidneys, after promising results in animals.

Getting the urine out has been a problem for earlier prototypes, causing them to balloon under the pressure.

The Japanese team got round this by growing extra plumbing for the kidney to stop the backlog, PNAS reports.

Although still years off human trials, the work helps guide the way towards making organs for people, say experts.

In the UK, more than 6,000 people are waiting for a kidney - but because of a shortage of donors, fewer than 3,000 transplants are carried out each year.

More than 350 people die a year, almost one a day, waiting for a transplant.

Growing new kidneys using human stem cells could solve this problem.

Image copyright SPL Image caption Stem cells are immature cells that can develop into different types of tissue

Dr Takashi Yokoo and colleagues at the Jikei University School of Medicine in Tokyo have previously used a stem cell method to grow kidneys, but encountered the ballooning problem.

So this time, they set about growing a drainage tube for the kidney, along with a bladder to collect and store the urine.

They used rats as the incubators for the growing embryonic tissue.

When they connected up this new plumbing to the animal's existing bladder, the system worked.

Urine passed from the transplanted kidney into the transplanted bladder and then into the rat bladder.

And the transplant was still working well when they checked again eight weeks later.

They then repeated the procedure on a much larger mammal - a pig - and achieved the same results.

This offers the possibility for plumbing any future lab-grown kidneys, a need that has not previously been addressed, say experts.

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

"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."

Other scientists have looked at rejuvenating old organs that would not normally be suitable for transplanting. Prof Harald Ott and colleagues have been testing out a method that washes away the tissue from dead organs to leave a scaffold that can be repopulated with healthy new cells.

They have built kidneys, hearts and lungs in this way.

Prof Ott says using a scaffold is a good short cut, rather than having to grow whole structures from scratch.