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OSAKA — In what is reportedly a world’s first, a group of Japanese scientists said Wednesday they have developed rat cardiac muscle that can strengthen rats’ hearts when transplanted.

The muscle, which was created in sheets, can beat automatically outside the body, and is derived from the heart-muscle cells of newborn rats.

The group, led by Teruo Okano, a professor of regenerative medicine at Tokyo Women’s Medical University, and Yoshiki Sawa, a cardiovascular surgeon at Osaka University, successfully cultivated newborn rats’ cells to form a sheet of cardiac muscle.

They used the sheet to strengthen the functions of the rat’s heart, which had suffered cardiac muscle cell degeneration.

Okano said the development will pave the way for enhanced treatment of heart diseases.

The scientists said they hope to develop a cardiac muscle sheet from human stem cells in bone marrow or embryonic stem cells, which are believed to be capable of developing into nerves, liver tissue, heart muscle or any other tissue.

The results of the study are to be reported at an general meeting April 18 and 19 of the Japanese Society of Regenerative Medicine in Kyoto.

Okano, Sawa and others grew the rat cardiac muscle cells to form a 1 cm × 1 cm sheet 0.05 mm thick.

After confirming that the muscle could beat by itself, they combined four such sheets. The beat rhythm of each was different at first, but they later synchronized, the scientists said. The organism continued to beat for several days before the scientists forced it to stop for further examination.

After the sheet was transplanted into the rat’s weak heart, the strength of its beats grew by up to 40 percent, they said.

The group said veins to infuse the cardiac muscles with blood must be developed for practical application of the technique.

“Our achievement has proved that a weakened heart function can be improved through the use of cultured cardiac muscle sheets,” Okano said. “The technique has a wide range of potential, to be applied in the treatment of heart diseases in the near future.

“However,” he added, “prior to the practical application, infrastructural development at medical institutions should be made so that the culture of cardiac muscle sheets and the transplant operations can be done at the same medical facility.”

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