Marrow cells take up residence in wounds.

Researchers have long known that immune cells derived from bone marrow provoke an infection-stopping inflammatory response in skin wounds. A study published in the September Stem Cells shows that some of these cells linger at the injury site indefinitely, bolstering the possibility that they could playa role in future wound-healing therapies.

Earlier research tracked bone marrow-derived cells by detecting a protein called CD45, which appears on the surface of some of these cells and provokes inflammation. Cells marked with the protein disappeared from the healing wounds within weeks.

Frank Isik of the University of Washington in Seattle and his colleagues have now investigated the fate of marrow-derived immune cells that don't express CD45. The team extracted bone marrow cells from mice genetically engineered so that their tissues glowed green under fluorescent light. Next, the scientists infused those readily visible cells into other mice that had been irradiated to kill their bone marrow. Finally, each infused animal's skin was subjected to an injury.

Glowing marrow-derived cells from the infusion clustered around the wound, the scientists report. As expected, those cells expressing CD45 disappeared in a few weeks. However, the cells that didn't express the CD45 protein could still be found at the injury site months later. The cells appeared to integrate into the wound and produce collagen, a primary, skin protein, rather than promote inflammation.

Isik says that his work doesn't yet show whether the bone marrow-derived cells actually help skin to regenerate or simply promote the formation of scar tissue, an inferior substitute for normal skin.--C.B.