Don't call them stem cells just yet, but researchers say they have discovered a rare and long-sought class of cell in adult mice that is responsible for patching up an injured pancreas. If equivalent cells were found in the human pancreas, the hope is that they would point the way to therapies for growing new insulin-secreting beta islet cells, which cause diabetes when they break down.

"That's the far away dream, but the data we find in mice gives us the hope this kind of reasoning makes sense," says biologist Harry Heimberg of Vrije University Brussels in Belgium, who the led the research, published in the journal Cell.

Unlike stem cells, the newly discovered cells, referred to as progenitors, do not reproduce repeatedly in the lab. But according to the study, they are capable of differentiating into all cell types of the islets of Langerhans, pancreatic structures that include beta cells and cells that secrete hormones such as glucagon and somatostatin. Heimberg and his co-workers first injured the islets of adult mice by clamping shut the ducts that carry digestive enzymes out of the pancreas, causing a destructive backup.

The injured pancreases, forced to repair themselves, soon swelled with double the normal number of beta cells. The researchers note that the only previously known source of new beta cells in adult mice was the slow cell division of preexisting beta cells. Seeking fresh sources, they looked for cells that express the gene neurogenin 3, a potential sign of cell differentiation because it is the first gene to only switch on in pancreatic islets during embryonic development. The team found an estimated 5,000 such cells, some of which were isolated and tested for their ability to restore beta cell activity.

The researchers did this by injecting the cells into embryonic pancreatic tissue taken from mice engineered to lack neurogenin 3. The embryonic pancreas was incapable of secreting insulin and other hormones by itself. But the extracted tissue began producing insulin, glucagon and other hormones after the newly identified cells were added, indicating that they were indeed progenitors capable of differentiating into all the islet cell types, including beta cells.

The source of these progenitors, however, remains a mystery. "It's necessary now to look for the precursors of these progenitors," Heimberg says, which may be more like true stem cells.