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. Over the past eight years, a quiet revolution has taken place in stem cell biology as researchers have discovered that they can actually teach old cells new tricks.
They have learned how to reprogram adult cells so that they can do many things an embryonic cell can do. No human embryos are destroyed in the process. Along the way, embryonic stem cells—just a decade ago hailed as the future of medicine—have largely been bypassed. Some researchers still use them, but for now, the future belongs to adult stem cells and iPS cells, which are adult cells genetically reprogrammed to express specific genes.
Every year for the past 10 years, the National Institutes of Health (NIH) has funded more adult stem cell research compared with embryonic research. For 2012, NIH grants totaled $146.5 million ...1