In one study, a team at Yale University in Connecticut implanted engineered lung tissue into rats that worked like the real thing, helping the animals breathe and supplying their blood with fresh oxygen.

In another, a team at Harvard University in Massachusetts developed a tiny lung device from human tissue and synthetic materials to test for environmental toxins or see if new drugs work.

Both studies published on Thursday highlight advances in tissue engineering, in which researchers combine synthetic materials and human cells to work like natural organs.

"This is an early step in the regeneration of entire lungs for larger animals and, eventually, for humans," said Dr. Laura Niklason of Yale, whose study appears in the journal Science.

Niklason's team stripped away cells from rat lung that could cause organ rejection, and used the remaining shell as their starting point.

They infused this tissue with lung-specific stem cells and placed them in a bioreactor -- a kind of incubator built to resemble fetal-like conditions.

"What we found very much to our surprise was the cells generally landed in their correct anatomical location. We think that means the decellularized matrix has (postal) zip codes," Niklason said in a telephone interview.

When implanted into rats, the lung functioned much like a normal lung for up to two hours, Niklason said.

At first, "it was pretty close to perfect," she said.

But after a while, blood clots began to form, suggesting some of the cells that coat the blood vessels were sticky.

Niklason said they plan to refine the process.

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