A model organism is a living thing which scientists study to learn more about biological processes. Then, they can apply this knowledge to humans (and other animals). When studying reproduction and embryo development it is much better to use a model organism because of ethical concerns. Plus, humans take a really long time to reproduce, they are expensive to feed, their embryos are inside their mother at all times, and it is hard to keep them in a small lab space.
This is why scientists turned to the zebrafish, or Danio rerio. In fact, it has contributed more to scientific research than most scientists. Zebrafish gained popularity in the 1980's as a research tool.
Zebrafish are inexpensive and easy to maintain in a small lab space. They are primarily used as a model organism in developmental biology because they are vertebrates with a quick reproduction time. Many organs begin to develop within 24 hours. After three days they are already hatched and swimming. At five days, most cells types have differentiated.
A zebrafish showing off his stripes.
Best of all, the embryo develops externally and it is transparent. This means that scientists can look at different cells and see which cells become brain cells and which cells become muscle cells, etc. During embryo development, different types of cells move and travel to special zones inside the embryo. It is almost like a gene inside the embryo calls out, "All future brain cells over here pronto."
Scientists use zebrafish to learn which genes are in charge of organizing the cells. They look at fish with mutations in certain genes. Then, they look to see if those mutations caused disorder in the embryo. If mutating the gene caused developmental problems then they know the gene must normally be important.
Zebrafish have been helpful in understanding the genetic control of embryo development. They can be just as helpful in understanding many other types of health problems as well such as cancer and heart defects. If you ever wander into a laboratory and something fishy is going on…it is probably a good thing.