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  Heartbeat key for blood growth in embryos
For blood cells to grow in an embryo, they need the pressure of a beating heart, say US researchers.

The finding explains why an embryo's heart starts beating just weeks after conception, and it points the way to new stem cell-based treatments for a host of blood disorders such as leukaemia.

"In learning how the heartbeat stimulates formation of embryos, we've taken a leap forward in understanding how to direct blood formation from embryonic stem cells in the petri dish," says Dr George Daley of Harvard Medical School and Children's Hospital Boston, whose study appears in the journal Nature.

"These observations reveal an unexpected role for biomechanical forces in embryonic development," says Guillermo Garcia-Cardena of Brigham and Women's Hospital in Boston, who also worked on the study.

Stem cells are the body's master cells, providing a renewable source of brain, bone, muscle, blood and other cells.

An embryo in the first days after conception is made up entirely of these cells, each of which can give rise to all of the cells and tissues in the body.

Daley's team looked at how the stress of friction and the flow of fluid affected the formation of blood cells from mouse embryonic stem cells.
Promoting blood

The study showed that these forces promote blood formation and increase the production of colonies of cells that give rise to specific types of blood cells.

Mouse embryos with a mutation that kept the developing heart from beating had far fewer of these so-called progenitor blood cell colonies.

In a separate study, a team led by Dr Leonard Zon of Harvard and Children's Hospital Boston, looked at the effects of the embryonic heartbeat in zebrafish, which have transparent embryos.

"We were looking in real aortas in real vertebrate embryos to see the actual stem cells," says Zon.

The study published in journal Cell found that levels of nitric oxide, which is known to play a key role in blood vessel formation, increase when blood is flowing.

Mutant embryos that had no heart beat or circulation were found to have far lower levels of blood-forming stem cells known as hematopoietic progenitor cells.

Researchers think nitric oxide may work as a type of signal to start the process of blood stem cell production.

"This finding connects the change in blood flow with the production of new blood cells," says Zon.

The findings may lead to new ways to coax iPS cells - which look and act like embryonic stem cells - into producing blood-forming cells that could be used by people with leukaemia who need bone marrow transplants, he says.
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