UCLA researchers said their finding solves a long-standing biological mystery and might allow scientists to mimic the specific embryonic microenvironment necessary for development of blood stem cells in cell culture.

Such cells could then be grown for use in treating diseases such as leukemia and aplastic anemia, said Dr. Hanna Mikkola, a researcher at the university's Broad Center of Regenerative Medicine and Stem Cell Research.

"It was a big mystery, where these cells originated," said Mikkola, an assistant professor of molecular, cell and developmental biology. "This is the first time we can really say definitively that blood stem cells are generated in the placenta. There's no more speculation."

The study is reported in the March 6 issue of the journal Cell Stem Cell.