In the the quest to gain mastery of our biology, scientists say they've moved one step closer to developing the capability to replace human organs.

Researchers at Massachusetts General Hospital in Boston reported Sunday they've made functioning rat kidneys in the laboratory, a bioengineering feat of immense import to the tens of thousands of Americans and others on kidney donor waiting lists.

The regenerated rat kidneys effectively produced urine in the laboratory and when transplanted into rats.

Researchers made the organs by stripping cells within the kidneys to leave a skeleton of collagen and other compounds, called the extracellular matrix, to which they added new cells capable of regenerating tissue. Such a technique offers an advantage over attempting to build a kidney with an artificial base, according to Dr. Harald C. Ott, lead author of a paper describing the research published in Nature Medicine.

"The huge benefit would be that it's fully implantable in the shape of a kidney," he said.

Ott cautioned that the work was still in preliminary stages and much work remained still in creating fully functional human kidneys for transplantation. Some 17,000 people with end-stage kidney disease receive a kidney transplant every year in the United States, with nearly 100,000 on waiting lists. In 2011, nearly 5,000 of those people died waiting for a donor kidney. While 90 percent of Americans say they support organ donation, less than one-third make their donor intentions clear.

Other researchers have already conducted similar experiments using extracellular matrix in regenerative medicine, with hearts and other organs, said Dr. Stephan F. Badylak, a researcher at the University of Pittsburg, who was not involved with research. "The real value of this study is that it's the kidney and it's a proof of concept — and the clinical need is so great," he said.

One problem remains with the decellularized tissue technique, however. Previous experiments effectively ended when clotting occurred after the regenerated organ was reconnected to a blood supply. Though Ott and his colleagues saw no signs of bleeding or clotting, they ended the experiments after a "short time," before any sign of kidney malfunction.

The regenerated rat kidneys also failed to function as well as natural ones, producing only rudimentary urine — perhaps given the immaturity of the kidney cells used in the experiment. Now, the researchers are looking to identify the appropriate kinds of cells to repopulate the organ.

Regarding implications for human transplant, Ott said researchers might soon employ a chimeric combination of pig and man, seeding the skeletal remains of a decellularized pig kidney with human cells.

"[I]n an ideal world, you'd take cells from [the] patient and create a kidney on demand."

The study was reported in Nature Medicine on April 14