The findings of the study suggest that human or porcine fetal tissue might take on the shape and function of a healthy kidney if transplanted into humans as well. Israeli scientists have successfully grown miniature human kidneys in mice, in a breakthrough that might one day help save thousands of patients waiting for transplants.

A team at the Weizmann Institute of Science has induced human stem cell tissue to grow into functional kidneys, and have accomplished the same with porcine stem cell tissue.

The method, which will be published in this month’s issue of Nature Medicine, could lead to a promising solution to the severe shortage of kidney donors.

According to the U.S National Kidney Foundation and the United Network for Organ Sharing, more than 50,000 people in the United States alone are on the waiting list for kidney transplants and more than 2,000 died this year waiting for a match. The wait can last years. And after a kidney is transplanted patients run the risk of transplant rejection.

The findings of the study headed by Prof. Yair Reisner suggest that human or porcine fetal tissue might take on the shape and function of a healthy kidney if transplanted into humans as well.

Pig tissue, as opposed to pig organs, is not expected to cause hyperacute rejection (common in cross-species transplants), as has been demonstrated by recent transplants of insulin-producing cell clusters taken from porcine fetal tissue that did not induce such rejection. The scientists hope that porcine stem cells might thus provide a ubiquitous source for those in need of a kidney.

Reisner and Ph.D. student Benny Dekel of the Weizmann Institute’s Immunology Department, with Prof. Justen Passwell, the head of the pediatric department at the Sheba Medical Center, transplanted human and porcine “kidney precursor cells” (stem cells that are destined to become kidney cells) into mice.

Both human and porcine tissues grew into perfect kidneys, the size of the mice’s kidneys. The miniature human and pig kidneys were functional, producing urine. In addition, blood supply within the kidney was provided by host blood vessels as opposed to donor blood vessels, greatly lowering the risk of rejection.

After growing the human and porcine kidney tissue in mice, the scientists checked how human lymphocytes (fighter cells in the immune system) might react to it. They injected human lymphocytes into immunodeficient mice (that have no immune system and thus do not interfere with the immune response).

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. Immune rejection was also tested in normal mice and was shown to be reduced compared to that induced by precursors from later time points.

The procedure is now in the pre-clinical study stage. According to Reisner, if all goes well, a treatment may ensue within a few years.