As a young medical student three decades ago, Mark Magnuson learned the basic facts of human development. Among those supposed facts was this one: Adult cells can’t change what they are. A heart cell is always a heart cell, a skin cell is always a skin cell.
That’s not the case with embryos, whose cells eventually create the entire human body. As embryonic cells divide, they develop distinct identities, becoming heart cells and brain cells and blood cells and every other kind of cell.
It’s a process called differentiation. And once it happens, there is no going back. “When I was a medical student, I was taught that a differentiated cell was a differentiated cell,” said Magnuson, a professor of medicine and director of the Vanderbilt Center for Stem Cell Biology in Nashville. “That was the end of the line.”
Then along came the induced pluripotent stem cell (iPS cell), and everything changed.
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