Researchers have long known that hematopoeitic stem cells (HSCs)—which give rise to adult blood cells—migrate during development from the dorsal aorta and placenta to the fetal liver, and then to the bone marrow. In a study published in Science, Paul Frenette, M.D., describes for the first time the mechanisms that regulate the second leg of that journey. The study also provides evidence that the liver’s portal vessels (tributaries of the umbilical vein) serve as the niche, or microenvironment, that regulates HSC proliferation in the fetal liver. Understanding the HSC niche of the fetal liver--an organ in which HSCs expand naturally--may shed light on new ways to grow stem cells for transplantation. Dr. Frenette is professor of medicine and of cell biology and chair and director of Einstein’s Gottesman Institute for Stem Cell and Regenerative Medicine Research.