WASHINGTON — Scientists have transformed one type of fully developed adult cell directly into another inside a living animal, a startling advance that could lead to cures for a plethora of illnesses and sidestep the political and ethical quagmires that have plagued embryonic stem cell research.

Through a series of painstaking experiments involving mice, the Harvard biologists pinpointed three crucial molecular switches that, when flipped, completely convert a common cell in the pancreas into the more precious insulin-producing ones that diabetics need to survive.

The feat, published online Wednesday by the journal Nature, raises the tantalizing prospect that patients suffering from not only diabetes but also heart disease, strokes and many other ailments could eventually have some of their cells reprogrammed to cure their afflictions without the need for drugs, transplants or other therapies.

“It’s kind of an extreme makeover of a cell,” said Douglas Melton, co-director of the Harvard Stem Cell Institute, who led the research. “The goal is to create cells that are missing or defective in people. It’s very exciting.”

The findings left other researchers in a field that has become accustomed to rapid advances reaching for new superlatives to describe the potential implications.

“I’m stunned,” said Robert Lanza, chief scientific officer of Advanced Cell Technology in Worcester, Mass., a developer of stem cell therapies. “It introduces a whole new paradigm for treating disease.”

Even the harshest critics of embryonic stem cell research hailed the development as a major, welcome development.

“I see no moral problem in this basic technique,” said Richard Doerflinger of the U.S. Conference of Catholic Bishops, a leading opponent of embryonic stems cells because they involve destroying human embryos. “This is a ‘win-win’ situation for medicine and ethics.”

Melton and other researchers cautioned that many years of research lay ahead to prove whether the development would translate into cures.