Researchers at Children’s Hospital Boston and the Dana-Farber Cancer Institute (DFCI) report a surprising finding about embryonic development: The blood system begins to form not only in the embryo itself, but also in the placenta, the organ that nurtures the baby in utero.
Experiments in mice revealed that the placenta harbors a large supply of hematopoietic (blood-forming) stem cells. These cells, which appear very early in development, are able to generate more blood stem cells and can give rise to a complete blood system when transplanted into an adult. Unlike other sites where blood stem cells are found during embryonic development, such as the liver, the stem cells in the placenta can increase in number without giving rise to mature, specialized cells.

“There must be something unique about the placenta that nurtures blood stem cells and discourages them from differentiating,” says Stuart Orkin, a Howard Hughes Medical Institute investigator at Children’s and DFCI, and a senior investigator of the study. “If we figure out what’s special about the placental environment, we may learn how to grow blood stem cells in large numbers for clinical application.”

Blood stem cells are used in treating blood cancers like leukemia and other blood diseases, and in patients receiving transplants, but growing them in quantity is difficult. The cells don’t multiply readily in the laboratory, so they must be harvested from bone marrow by needle aspiration, a painful procedure, or coaxed into the blood and then collected. Both methods yield only a limited number of blood stem cells.