Window to a fish

In a sentence: Researchers at Boston Children’s Hospital are inducing fluorescent cancer in transparent zebrafish in order to study the spread of the cancer cells. Now for a little background info to help that sentence make sense!
Zebrafish are one of the so-called “model organisms” because they fit a list of criteria that make them easy to study: fast growth, prolific reproduction, easy to keep in a lab, small genome without too much “junk DNA”, and so on. They are transparent during development, so we can see how their organs take shape over time. As vertebrates, their bodies aren’t all that different from human bodies, so they can give us valuable information about human disease and developmental problems
Once the fish get to be about 4 weeks old, their scales darken, closing the “window” into the fish. Richard White, one of the researchers, said they solved the problem “by mating two existing zebrafish breeds, one that lacked a reflective skin pigment and the other without black pigment. The offspring had only yellow skin pigment, essentially appearing clear [LiveScience].”
What do you do with a transparent fish? White put a fluorescent tumor in the fish’s abdomen. Unfortunately, my university does’t have access to Cell Stem Cell, but the abstract says: “transplantation of GFP-labeled marrow cells allowed for striking in vivo visual assessment of engraftment from 2 hr–5 weeks posttransplant… both transparent and wild-type fish had equal engraftment, but this could only be visualized in the transparent recipient.” In other words, the clear skin of the new breed of fish allowed the researchers to actually watch the cancer spreading. “Viewing the fish under a microscope, White saw the cancer cells begin to spread within five days. He even saw individual cells metastasize, something that has not been observed, so readily and in real-time, in a living organism [EurekAlert].”
So, what’s GFP? Green fluorescent protein occurs naturally in some species of jellyfish. The molecule produces a green glow when exposed to certain wavelengths of blue light. It’s completely non-toxic, so can be used to illuminate living organisms. It sounds strange, but GFP has allowed researchers to view biological processes that would not otherwise be visible. When tissue specific promoters (like light switches for genes) are used to drive expression of the gene for GFP, only that type of tissue glows green. In the picture, zebrafish embryos are expressing GFP controlled by a promoter for a circulatory system gene. A video showing a zebrafish heartbeat, visible via GFP, can be seen here. Now, imagine how much the researchers were able to learn from watching a tumor spread in real time.