WHEN the embryonic heart begins to beat, it kick-starts the production of blood from cells lining the growing aorta, two independent research teams have shown.
As the heart starts pumping a primitive blood-like fluid around the body of an embryo, the change in pressure from the flowing liquid is the cue for cells lining the aorta to change first into blood stem cells, then into all blood-cell types in the body. As they multiply and mature, these rapidly replace the initial embryonic 鈥渂lood鈥�, which is composed of embryonic red blood cells in a nutrient-rich serum.
Leonard Zon of the Howard Hughes Medical Institute in Boston and his colleagues demonstrated that the pressure of the embryonic fluid is what switches on the production of adult blood in zebra fish and mouse embryos. 鈥淭he finding answers an age-old question as to why the aorta makes blood stem cells at all,鈥� says Zon, whose findings appear in Cell (). 鈥淭he answer is that there must be a cue to start making adult blood cells, and that cue is the onset of circulation and blood flow,鈥� he says.
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The discovery could lead to a source of blood for people with leukaemia who need a transplant but don鈥檛 have a matched donor, by exposing stem cells to flowing liquid, says Zon.
In a separate study, George Daley of the Children鈥檚 Hospital Boston and colleagues found that blood cells form more readily in cultures of embryonic stem cells if they鈥檙e exposed to fluids mimicking the usual flow and pressure of blood (Nature, ). They also showed the phenomenon in embryonic aortic tissue from mice embryos engineered to have no heartbeat or circulation. Left alone, the aortic tissue made little blood, but blood production soared when Daley exposed the tissue to flowing fluid.
鈥淭he discovery underscores the critical importance of mechanical forces play in the development of blood and other functional tissue,鈥� says Robert Lanza, chief scientist at Advanced Cell Technology of Worcester, Massachusetts.
