Kidneys obtained from embryonic stem cells, which know how to produce and expel urine like their natural "models": they are the result of the work of a group of Japanese researchers, who implanted them and successfully tested them in rats and pigs.

The study published in the journal PNAS opens up new hopes for those who are waiting for a transplant, even if - the authors warn - human trials are still several years away.

Swollen with pee. The number of patients suffering from kidney disease is constantly increasing in the world, also due to the scarcity of donors. Recently it has been possible to cultivate functioning kidneys from human stem cells, but these prototypes cannot grow properly because they are unable to expel urine (a condition known as hydronephrosis). Due to the pressure and stagnation of liquid, they swell like balloons and slow down their development.

A free organ. As a potential solution, a team of doctors and researchers from the Jikei University School of Medicine in Tokyo cultivated not only kidneys but also an "extra" bladder in the laboratory, starting from rat embryonic cells: the tissues of the new organs still unripe were implanted in the bodies of adult "host" rats.

The urine ways. After four weeks, one of the ureters (the ducts that convey the urine produced by the kidney into the bladder) of the host rat was connected to the transplanted bladder. The new apparatus worked: the urine produced by the transplanted kidneys successfully passed to the transplanted bladder, and from this to the "natural" bladder of the rodent.

Good results. Eight weeks after transplantation, the kidney tissues contained all the characteristic structures of mature kidneys. The same technique (which the experts call stepwise peristaltic ureter, SWPU), has been successfully replicated on pigs.

Although it is too early to assert that it can work on humans, the technique has made important progress in understanding the mechanics of the urinary tract.