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Broken Symmetry

Scientists seek mechanisms explaining development of the body’s left-right pattern

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12:30pm, September 11, 2009

On the outside, people’s right and left sides look pretty much the same. On the inside, though, such superficial symmetry gives way to an imbalanced array of organs: The heart, spleen and stomach sit on the left side of the body, while the liver and pancreas take up the right. Even organs that at first glance appear as perfect mirror images of each other, such as the kidneys, lungs and testicles, turn out to have telltale left-right differences.

Figuring out how a body with such internal asymmetry develops from an egg (and later an embryo) with near-perfect symmetry has long stymied developmental biologists. But in recent years, scientists have identified a structure that seems to explain how mice — and possibly humans — break left-right symmetry early in development. In an 8-day-old mouse embryo, researchers discovered a short-lived, shallow pit covered with cilia. The whiplike protrusions paddle through the surrounding embryonic fluid, creating a microcur

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