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Researchers, for the first time have found evidence that an adult human lung can grow back, at least in part, after it has been removed surgically.

Researchers used MRIs with hyperpolarized helium-3 gas to show that existing alveoli -- the tiny, air-exchange units of the lung -- actually increased in number after a 33-year-old woman had her entire right lung removed due to cancer, said the news release.The woman in the study showed a 64 percent increase in the number of alveoli in her lung 15 years after surgery.

"The research clearly shows that some form of lung growth can occur in the adult human," said study author James Butler, an associate professor of medicine in the department of medicine at Harvard Medical School in Boston in the news release.

All the new alveoli were similar in shape. "It's striking, the degree of homogeneity of the new alveoli, as if the lung was responding to something," Butler added.

He suggested that perhaps, the new growth was due to the stretching of the tissue because of exercise. "Could other bio-molecular growth be triggered by stretch? It's a wide-open question now."

After one and half years of the surgery, the women started an exercise regime that included walking, cycling and yoga. There have been studies conducted previously which have suggested that lung growth after pneumonectomy (removal of the lung) in dogs was possible, generally after periods of lung stress or strain.

After 15 years, the lung size and capacity was measured and it was checked as to how much air can be taken in and blown out with deep breaths. In the initial few months, as researchers expected, the total lung volume increased and the lung density fell below normal.

But then gradually, the tissue volume started increasing and the density also returned to normal, suggesting the growth of new tissue.

"When the lung develops in utero [when the fetus is developing], the pulling force of the diaphragm is an important stimulation for the lung to grow," Dr. Norman Edelman, a professor of medicine at Stony Brook University and chief medical officer of the American Lung Association said in the news release. "But, of course, the practical application of the research is a long way off."

A study involving more people is what is planned next, said Butler. "If we can discover the underlying bio-molecular mechanisms, they would suggest potential therapeutic options," he explained.