U.S. scientists say they've identified a new growth factor that stimulates the expansion and regeneration of hematopoietic (blood-forming) stem cells.

Duke University Medical Center researchers say their discovery might help overcome one of the most frustrating barriers to cellular therapy -- the fact that stem cells are so few in number and so stubbornly resistant to expansion.

"Unfortunately, there are no soluble growth factors identified to date that have been proven to expand human stem cells for therapeutic purposes," said Dr. John Chute, who led the study.

But Chute, working with Heather Himburg, a post-doctoral fellow in his laboratory, discovered adding pleiotrophin, a naturally occurring growth factor, stimulated a ten-fold expansion of stem cells taken from the bone marrow of a mouse. They said they also found pleiotrophin increased the numbers of human umbilical cord blood stem cells in culture that were capable of engraftment in immune-deficient mice.

"Perhaps more importantly, systemic treatment with pleiotrophin may have the potential to accelerate recovery of the blood and immune system in patients undergoing chemotherapy or radiotherapy," Chute said.

The study is reported in the journal Nature Medicine.