Most insights into human disease are a result of experiments that would be unethical or unfeasible to perform on humans. Instead we can use models to look at the functions of genes involved in maintaining healthy organisms in order to obtain vital clues about the causes and progression of human diseases.
Similarity between species
Building on Charles Darwin’s pioneering insights, we now appreciate the remarkable similarity that exists between the genetic codes of different species. Many genes are shared between humans and other animals, but as we learn more about how animals are built it is not that surprising.
Although humans may appear to be very different to mice, flies, chicks or fish, the reality is that they’re not that different at all. All of them have two eyes, a mouth and a gut, for a start. And when we look closer – at all the cells inside them – there really are more similarities than differences. To make an eye requires the information stored in many different genes, but since your eyes and those of many other animals work in much the same way the genes involved are pretty much the same. Only subtle changes in the genetic code are necessary to create the differences between them.
This similarity between the genes of different organisms, which scientists call conservation, or genetic homology (from an ancient Greek word meaning ‘to agree’), is the fundamental reason why we can use animals – even fish – to learn about human diseases.
Why different model organisms?
Because there are so many different questions in biology there are a range of model organisms to suit different topics.
Most people are familiar with the use of mice and rats as model organisms. As mammals they are fairly similar to humans; therefore they can be used to study complicated processes underlying normal human development and disease.
Often we want to know something simple that is likely to occur in all living organisms – in which case scientists will use a basic organism such as bacteria or yeasts as they are easy and cheap to look after and they are very well understood. Sometimes they can be too simple – as you can imagine it is difficult to look at how eyes form in something that doesn’t have any!
A simple multicellular organism that is used as a model by scientists all over the world is the microscopic worm Caenorhabditis elegans. There are just 1031 cells in the adult, which makes understanding how the animal develops much easier. It is so simple that we know where every single one of its 302 neurons connects with each other, something that is impossible to do with the estimated 100 billion neurons in the human brain!
Worms and bacteria can still be too simple for some laboratory work and mice and rats can be too complicated and expensive to use. Zebrafish are the ideal model organism to bridge this gap.
Zebrafish are vertebrates and have similar body plans, tissues and organs to humans, but they are much easier and cheaper to look after than mice and rats. They have lots of other advantages over other model organisms.