They could be used to build an artificial artery in a test tube or the stem cells could be injected straight into the heart and they could form within it.

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Dr Reynolds, and others, have shown that under the influence of suitable growth factors, stem cells can lead full lives in a test tube.

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Sure enough, he found that the only cells that could produce new tumors in the test tube were the ones with CD133--strong circumstantial evidence that defective stem cells were the growth factor in brain cancer.

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Macchiarini performed the nine-hour operation on April 9 at the Children's Hospital of Illinois after carefully creating the windpipe using stem cells from Hannah's bone marrow that were saturated over a matrix of plastic fibers shaped into a tube.

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