The researchers, led by Robert Langer, created structures resembling young cartilage, liver, and neural tissues by growing cells on biodegradable polymer scaffolds -- spongelike structures that resemble the shape of the organ to be created. The scientists also exposed the cells to several hormones that normally stimulate the growth of these organs during embryonic development.

The newly forming tissues were implanted in mice whose blood vessels began to grow into the lab-made tissues, supplying oxygen and nutrients needed for further growth.

"This is a really important discovery," said Patricia D'Amore, a professor at Harvard Medical School, who studies how blood vessels grow into tissues but was not involved with the research. "We've known that one of the key limiting steps in this field is getting the host to vascularize the implanted tissue. Without that, the tissue can never grow large enough."

Almost 2,000 people died in 1999 while waiting for liver transplants alone, according to the journal Tissue Engineering. In their paper, the MIT scientists write that growing tissues in the lab "holds promise for addressing organ shortages."

Shulamit Levenberg, a scientist in Langer's lab and coauthor of the study to be published in today's Proceedings of the National Academy of Sciences, said the use of embryonic stem cells in tissue engineering seems to have great potential because these stem cells can develop into many types of cells. The cells used in the MIT test meet President Bush's criteria for research because they come from existing lines of stem cells, rather than new lines.

Much remains to be done before the findings can make a difference in the operating room. Langer and colleagues write that further research is needed to "allay concerns regarding the potential" of embryonic stem cells to turn cancerous. Levenberg said the MIT researchers had tested the cells for normalcy and found that none seemed to have the potential to turn cancerous.

But Harvey Lodish, a molecular biologist at the Whitehead Institute in Cambridge who was not involved in Langer's research, said it will be very difficult to prove the safety of engineered organ transplants. There is always the chance that the manufactured organs would contain "errant" cells that never fully develop and could turn cancerous.

But patients in desperate need of a new heart or liver probably would be willing to risk getting cancer, said Lodish, adding that he thinks the risks can be dramatically reduced or eliminated.

In previous tissue-engineering research, scientists used more-developed stem cells from adult organs. But because these cells proliferate relatively slowly and are specific to particular organs, researchers did not have much success generating new organs.

The lab-made organs also performed better because the mice's blood vessels treated them as normal tissue, sprouting new blood vessels that penetrated into the organs.

In similar experiments using adult cells, this did not occur.

"I think that the work is indeed significant, a very important step forward along a multistep path," said Jeffrey Hubbell, a biomedical engineer at the Swiss Federal Institute of Technology in Lausanne who is familiar with the MIT research.

Globe correspondent Boyce Rensberger contributed to this report.

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