MILWAUKEE — Several years after scientists found a way to manipulate biology and send skin cells back to their embryonic origin, they are now learning that nature is not so easily tricked.

A reprogrammed skin cell retains a memory of its original identity as skin. Moreover, after the skin cell has returned to the embryonic state, it appears more willing to turn back into skin than to adopt a new identity.

The new findings by the lab of stem cell researcher George Daley at Children’s Hospital Boston were described Monday in a paper published online in the journal Nature and begin to address one of the mysteries surrounding reprogramming.

Cocktail of genes

Since 2007, when the labs of James Thomson at University of Wisconsin-Madison and Shinya Yamanaka at Kyoto University first used a cocktail of genes to create an alternative to human embryonic stem cells, scientists have been puzzled by subtle differences between actual embryonic stem cells and these engineered versions.

The differences are important because the engineered cells were hailed as an alternative to embryonic stem cells that would allow scientists to make all of the cells in the human body while bypassing the ethical controversy that surrounded embryonic stem cells.

Daley said his team’s work overthrows the assumption “that when you reprogram a skin or a blood cell you erase its memory of being skin or blood. … Researchers have to appreciate the potential for this memory and erase it further or exploit it.”

Working with mice, Daley’s group reprogrammed different kinds of cells. The scientists used different reprogramming methods, too, including the technique used by Yamanaka and the technique used to clone Dolly the sheep. The cloning method, known as nuclear transfer, involves removing the nucleus from an unfertilized egg and inserting in its place the nucleus of an adult cell.