Stem cells greatly improve their therapeutic effectiveness in replacing damaged tissue when a molecule is added to their surface, U.S. researchers said Friday.

By attaching a molecule called SLeX, known to play a vital role in cell-to-cell recognition processes, to the surface of stem cells, researchers changed the stem cells from passive bloodstream travelers to active therapeutic contributors that migrate out of the vessels into the surrounding tissue, a study published in the journal Bioconjugate Chemistry said.

"Delivery remains one of the biggest hurdles to stem-cell therapy," explained senior author Jeffrey Karp, an instructor at the Harvard-MIT Division of Health Sciences and Technology.

"The bloodstream offers a natural delivery vehicle, but stem cells don't move through blood vessels normally after being expanded in culture," he said. "Our procedure promises to overcome this obstacle."

Adult stem cells "resemble couch potatoes if they hang out and divide in a dish for too long," a statement from the researchers said.

"They get fat and lose key surface proteins, which interferes with their movement and reduces their therapeutic potential," the statement said.

But by adding SLeX, or Sialyl LewisX, the researchers found a way to "get these cells off the couch and over to their therapeutic target," their statement said.