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A model for heart development
What do you see? Watch this zebrafish embryo grow from a single cell to a young fish. Can you find the eyes, backbone, and spots of this developing fish?

Why look at a fish?

Since the 1990s, zebrafish have proven to be a powerful tool in trying to understand how the human body develops. In addition, its genome is being sequenced (and should be complete by the end of 2008), making this model organism even more useful. Adults are relatively easy to keep in the lab: they grow up and reproduce in about three months, and, perhaps most importantly, they are vertebrates that lay small eggs (about 1 millimeter across) with clear shells that develop rapidly into translucent embryos. By peering at these embryos through a microscope, researchers can watch for abnormal development of vital organs such as the heart.

Zebrafish embryos are particularly well-suited to investigating the mysteries of heart disorders, because even embryos that develop severely malformed, nonbeating hearts can survive and continue to grow for several days. Because of the embryo's small size, it does not need a working heart. As it turns out, the cells of this model organism do not depend on the circulation of blood for oxygen during its first few days of development; they can survive with the oxygen that diffuses in from the watery surroundings for the first week of life. Luckily, this is long enough for researchers to make crucial observations about how these defective hearts form and malfunction over time.

Next: How is your heart like that of a zebrafish? »