The scientists transplanted pig bone marrow cells into the mice.

BBC: NEWS | Health | Pig organs may be safe for humans

The transplanted bone marrow contains stem cells that can turn into skin cells.

CNN: A mother's plea: Heal my children's skin

Another possibility is a bone marrow transplant - the existing bone marrow cells are destroyed using chemotherapy, then new, healthy ones are transplanted in to take over their job.

BBC: Q&A: Test-tube lifesaver