Shear force of blood flow is crucial to embryonic heart development

In a triumph of bioengineering, an interdisciplinary team of California Institute of Technology researchers has imaged the blood flow inside the heart of a growing embryonic zebrafish. The results demonstrate for the first time that the very action of high-velocity blood flowing over cardiac tissue is an important factor in the proper development of the heart–a result that could have profound implications for future surgical techniques and even for genetic engineering.

In the January 9, 2003 issue of the journal Nature, the investigators report on two interrelated advances in their work on Danio rerio, an animal reaching only two inches in length as an adult but a model of choice for research in genetic and developmental biology. First, the team was able to get very-high-resolution motion video, through the use of confocal microscopy, of the tiny beating hearts that are less than the diameter of a human hair. Second, by surgically blocking the flow of blood through the hearts, the researchers were able to demonstrate that a reduction in “shear stress,” or the friction imposed by a flowing fluid on adjacent cells, will cause the growing heart to develop abnormally.

The result is especially important, says co-lead author Jay Hove, because it shows that more detailed studies of the effect of shear force might be exploited in the treatment of human heart disease. Because diseases such as congestive heart failure are known to cause the heart to enlarge due to constricted blood flow, a better understanding of the precise mechanisms of the blood flow could perhaps lead to advanced treatments to counteract the 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 exploited in the diagnosis of prenatal heart disease for early surgical correction, or even genetic intervention.

Hove, a bioengineer, along with Liepmann Professor of Aeronautics and Bioengineering Morteza Gharib, teamed with Scott Fraser, who is Rosen Professor of Biology, and Reinhardt K?ster, a postdoctoral scholar in Fraser’s lab, to study the heart development of zebrafish. Gharib, a specialist on fluid flow, has worked on heart circulation in the past, and Fraser is a leading authority on the imaging of cellular development in embryos. The new results are thus an interdisciplinary marriage of the fields 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 keyed their efforts on the zebrafish because the one-millimeter eggs and the embryos inside them are nearly transparent. With the addition of a special chemical to further block the formation of pigment, the team was able to perform a noninvasive, in vivo “optical dissection.” To do this, they used a technique known as confocal microscopy, which allows imaging of a layer of tissue. The images are two-dimensional, but they can be “stacked” for a three-dimensional reconstruction.

Concentrating on two groups of embryos–one group 36 hours after fertilization and the other at about four days–the researchers discovered that their deliberate interference with the blood flow through the use of carefully placed beads had a profound effect on heart development. When the shear force was reduced by 90 percent, the tiny hearts did not form valves properly, nor did they “loop,” or form an outflow track properly.

Because the early development of an embryonic heart is thought to proceed through several nearly identical stages for all vertebrates, the researchers say the effect should also hold true for human embryos. In effect, the research demonstrates that the shear force should also be a fundamental influence on the formation of the various structures of the human heart.

The next step for the researchers is to attempt to regulate the restriction of shear force through 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.

In addition to the lead authors Hove and K?ster and professors Gharib and Fraser, the team also included Caltech students Arian S. Forouhar and Gabriel Acevedo-Bolton.

The paper is available on the Nature Web site at http://www.nature.com/nature/links/030109/030109-1.html