Blood flow found crucial to heart development

In a triumph of bioengineering, Caltech researchers have imaged the blood flow inside the heart of a growing embryonic zebrafish. The results demonstrate for the first time that the action of high-velocity blood flowing over cardiac tissue is an important factor in proper heart development—a result that could have profound implications for future surgical techniques and genetic engineering.
In the January 9 issue of Nature, the investigators report on two related advances in their work on Danio rerio embryos. First, the team was able to get very-high-resolution video of the tiny beating hearts that are less than the diameter of a human hair. Second, by surgically blocking the hearts’ blood flow, they demonstrated that reducing “shear stress”—the friction imposed by a flowing fluid on adjacent cells—causes the growing heart to develop abnormally.

The result is especially important, says colead author Jay Hove, because it shows that more detailed studies of shear force might be exploited in the treatment of human heart disease.

Because diseases such as congestive heart failure constrict blood flow, causing hearts to enlarge, a better understanding of the mechanisms of blood flow might lead to advanced treatments to counteract enlargement.

Also, Hove says, a better understanding of genetic factors involving blood flow in the heart—a future goal of the team’s research—could eventually be used in early surgical correction of, or even genetic intervention in, prenatal heart disease.

For the study, Hove, a bioengineer, and Morteza Gharib, Liepmann Professor of Aeronautics and Bioengineering, teamed with postdoctoral scholar Reinhard Köster, the paper’s other lead author, and Rosen Professor of Biology Scott Fraser. A specialist on fluid flow, Gharib has worked on heart circulation in the past, and Fraser is a leading authority on imaging cellular development in embryos—making the study an interdisciplinary marriage of engineering, biology, and optics.

“Our research shows that the shape of the heart can be changed during the embryonic stage,” says Hove. “The results invite us to consider whether this can be related to the roots of heart failure and heart disease.”

The researchers focused on zebrafish because the one-millimeter eggs and the embryos inside are nearly transparent. Adding a special chemical to further block pigment formation, the team was able to perform a noninvasive, in vivo “optical dissection” using confocal microscopy. The technique allows two-dimensional imaging of a layer of tissue; images can also be “stacked” for a three-dimensional reconstruction.

Concentrating on two groups of embryos—one at 36 hours after fertilization and the other at about four days—the team found that interfering with the blood flow, using carefully placed beads, had a profound effect on heart development. When shear force was reduced by 90 percent, the tiny hearts neither formed valves nor “looped” (formed outflow tracks) properly.

Because early embryonic heart development is thought to proceed through several nearly identical stages in all vertebrates, the researchers say the effect should also hold true for humans. In effect, the study demonstrates that shear force should also fundamentally influence the human heart’s structural formation.

The team’s next step is to attempt to regulate shear-force restriction with new techniques to see how slight variations affect structural development, and to look at how gene expression is involved in embryonic heart development. “What we learn will give us directions to go and questions to ask about other vertebrates, particularly human beings,” Hove says.

Graduate students in bioengineering Gabriel Acevedo-Bolton and Arian Forouhar also contributed to the study, which is available at www.nature.com/nature/links/030109/030109-1.html. Movies and figures can be viewed at http://bicsnap1.caltech.edu/heart/start.htm.
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