A fish with a tiny brain

Last update: February 21, 2024
Reading time: 2 minutes
By Brain Matters

A human's brain contains about 100 billion neurons that are interconnected in many ways. This makes it rather complicated to study. It would be handy for scientists if there was a smaller and simpler brain to work with. Fortunately, there is: the brain of a zebrafish.

Zebrafish are small, striped, tropical fish. They have become increasingly popular in scientific research in recent years. Mainly because zebrafish are related to humans: they are both vertebrates and 70% of the genes in a zebrafish have a corresponding gene in humans. As a result, a zebrafish can suffer from the same diseases, such as Parkinson's. In addition, zebrafish are easy to maintain and very suitable for genetic modification. Thus, the zebrafish is an excellent candidate for brain research.

The brain of a young zebrafish is smaller than one cubic millimetre and contains about 100 thousand neurons. Human brains, by comparison, are 10 million times bigger. Because young zebrafish are transparent, scientists can use a very precise method to measure their brain activity. This method is called "light sheet fluorescence microscopy". This is a hefty term, but thankfully, it can be divided into three pieces:

1. "Microscopy"

Microscopy simply means: Looking at something that cannot be seen with the naked eye.

2. "Light sheet"

To see the brain cells, a flat laser is shone through the transparent fish. With this, a layer of the brain can be illuminated very precisely, at the cell level. Thus, the entire brain can be illuminated by the laser, layer by layer, in a single second.

3. "Fluorescence"

The calcium ions in brain cells are made fluorescent. As a result, they emit light when shone upon by the laser. A brain cell normally contains very few calcium ions. But when the brain cell activates, a lot of calcium ions flow in. So by measuring calcium ions, you also measure brain activity!

With this amazing technique, the brain of a zebrafish can be mapped down to the brain cell. This will give science a better understanding of the zebrafish, and more importantly the brain.

Author: Boudewijn van Gils


  • Howe, K., Clark, M., Torroja, C. et al. The zebrafish reference genome sequence and its relationship to the human genome. Nature 496, 498–503 (2013). https://doi.org/10.1038/nature12111
  • Ahrens, M., Orger, M., Robson, D. et al. Whole-brain functional imaging at cellular resolution using light-sheet microscopy. Nat Methods 10, 413–420 (2013). https://doi.org/10.1038/nmeth.2434
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