Human kidneys grown in mice raise transplant hopes

Scientists grown entire kidneys in laboratory mice using human stem cells in a development that raises the prospect of growing full-sized human organs in pigs – a breakthrough that would alleviate the worldwide shortage of kidneys for transplant operations.

An Israeli team led by Professor Yair Reisner of the Weizmann Institute of Science in Rehovot grew miniature human kidneys inside the body cavities of mice in which human kidney stem cells had been transplanted. The kidneys were fully functional and produced urine.

The scientists were also able to produce pig kidneys with the same technique although in both cases the kidneys were the size of the normal mouse organ. The scientists hope now to attempt to grow human kidneys inside pigs to produce organs of a comparable size to those for human transplants.

Alternatively, they might be able to grow functioning pig kidneys inside human patients using pig foetal tissue, although this would require more careful ethical consideration because of the possibility of transferring pig viruses to people.

The study, published in the journal Nature Medicine, pinpointed the ideal time during embryonic development in which the stem cells had the best chance of forming well-functioning kidneys with minimal risk of immune rejection.

Their findings suggest that tissue seven to eight weeks old in humans and four-week-old pig tissue offers the best opportunity for transplantation. If taken earlier, the tissue could include non-kidney structures such as bone, cartilage and muscle. If taken later, then the risk of rejection by the immune system is substantial.

The work is part of a series of studies on growing entire organs using stem cells. In 1998, Marc Hammerman of Washington University in St Louis announced he had managed to grow miniature rat kidneys inside the body cavities of mice.

Professor Reisner's team also studied how the human immune system might respond to a kidney grown from human stem cells inside an animal. The scientists injected human lymphocytes – the "killer cells" of the immune system – into mice that lacked an immune system of their own.

"The findings were encouraging: as long as the kidney precursors were transplanted within the right time range, the lymphocytes did not attack the new pig or human kidneys – despite the fact that lymphocytes and kidney precursors originated from different donors," a spokesman for the Weizmann Institute said.

This suggests that such organs may not be rejected so readily if they were ever used in transplant medicine.

The team said the research was in a pre-clinical study stage, but that if all went well, a treatment could follow within a few years.

In SeptemberAmerican researchers said they had managed to grow teeth in rats, which suggested the existence of dental stem cells, and there was no reason why the technique used in rodents would not work in humans.

The shortage of kidneys for transplants is getting worse each year, according to the UK Transplant Authority. There are about 1,600 kidney transplants each year, with more than 5,000 people on the waiting list at any one time.