Zebrafish are an excellent model for studying the development of blood cells and blood vessels. This image shows blood cells (magenta) in blood vessels (yellow) on the yolk of an anesthetized, 2-day-old, transgenic zebrafish embryo, which is about the size of a grain of rice. The image was taken using a powerful microscope that uses lasers to illuminate the fish.
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Credit: Daniel Castranova, National Institute of Child Health and Human Development, NIH