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Through careful breeding, scientists in Japan have created a see-through fish. The trasnparent medaka fish has virtually no pigmentation in its skin, revealing its innards for all to see.
The researchers took several strains of the medaka that were each missing one of the four pigments most fish have in their skin, and bred them to produce a strain that lacked all four.
The result is a transparent fish that provides a stunning view. The vividly colored brain, heart, liver and other organs make a beautiful display, one that also gives scientists an unprecedented opportunity to observe the internal machinery of a living animal.
"This means that you wouldn't have to kill the fish," says David Hinton, an aquatic toxicologist at Duke University in Durham, N.C. "You could visualize a lot of interesting phenomena."
Yuko Wakamatsu of Nagoya University in Japan and her colleagues wanted to study what happened inside the medaka when it was exposed to certain chemicals. Instead of killing the animals, the only option previously available, the researchers decided to create a see-through medaka.
"They will be a suitable alternative model to decrease the number of animals used for experiments, because many types of experiments can be conducted without having to kill the animals," says Dr. Wakamatsu, lead author of a study describing the fish. The study was published in a recent issue of Proceedings of the National Academy of Sciences.
She was particularly interested in how the reproductive organs would be affected by chemical exposure. So the researchers inserted a green fluorescent protein into one of the medaka's reproductive genes. When the gene is activated, the protein is released, lighting up the reproductive organs. Observers can study the development of the reproductive system from beginning to end and track any abnormal growth. "Now studies during the whole life on the same individuals are possible," says Dr. Wakamatsu.
The same technique could be used to tag any gene scientists want to study. "If you want to learn when a particular gene turns on and off, you can do that without killing the animal," says Lynn Lamoreux, a visiting researcher based at Texas A&M University in College Station who specializes in pigmentation.
Dr. Wakamatsu envisions many uses for the new fish, such as research on growth processes and aging, tracking the shrinking of cancerous tumors in response to potential new treatments or testing water for toxic pollutants. "They will also be an attractive viable model for classroom lessons," she says.
Medaka fish have been studied in Japan for more than 100 years. Along with the zebra fish, the 2-to-3-inc-long medaka is considered the white lab mouse of the fish world. "It's a hardy fish. It's been used for decades in the research arena," Dr. Hinton says.
Both zebra fish and medakas have transparent embryos, but the new medaka strain is the first animal that remains transparent as an adult. "The see-through medaka may be much more useful than the mouse in some cases," says Dr. Lamoreux. "There are things you cannot do with mice, and the see-through medaka fills that void beautifully."