Researchers at Children's Hospital Boston and the Dana-Farber Cancer Institute 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.
Meticulous 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 Dr. 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.
For more than a decade, scientists have believed that blood stem cells are made only in the embryo itself, within the region of the developing aorta. No role was suspected for the placenta, which has been seen as simply a place for nutrient exchange and waste removal between mother and fetus. But rather than merely providing nutrients, Orkin says, the placenta may also provide an ''infusion'' of blood stem cells to the fetus.
''This research reveals a new organ for blood development,'' Orkin says. ''It is surprising that this role for the placenta has been overlooked for so many years.''
The study, published in the March issue of the journal Developmental Cell, found that blood stem cells appeared in the placenta early, with numbers peaking mid-gestation. Only the fetal liver, where blood stem cells are known to expand tremendously, had greater numbers of blood stem cells.
Children's Hospital Boston is home to the world's largest research enterprise based at a pediatric medical center, where its discoveries have benefited both children and adults for over 136 years. More than 500 scientists, including eight members of the National Academy of Sciences, nine members of the Institute of Medicine and 10 members of the Howard Hughes Medical Institute comprise Children's research community. Founded in 1869 as a 20-bed hospital for children, Children's Hospital Boston today is a 325-bed comprehensive center for pediatric and adolescent health care grounded in the values of excellence in patient care and sensitivity to the complex needs and diversity of children and families. Children's also is the primary pediatric teaching affiliate of Harvard Medical School. For more information about the hospital visit: http://www.childrenshospital.org/research.
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