Fishy Brain Cells Take Sides

One of our fish has an unnaturally long fin on its left side. I call him Flag Fin. Our tank houses ten zebrafish: small, cheap and hearty. Our fish were intended for conditioning the tank -to prepare the environment for more sophisticated creatures- but we never got around to flushing and replacing them so here they are, swimming.

Most things have direction in life -with reference sidedness, not purpose. The snail collective dutifully scrapping algae off the tank's walls all have shells that spiral to the right (though I understand the shells can coil counterclockwise). To notice this lopsidedness in yourself, look at the reflection of your reflection. It really brings out facial asymmetries.

Most days, I walk by the tank without taking notice. But today, I sit next to it and look closely. One fish has a bulging belly. One looks almost orange between the dark stripes. And then there's Flag Fin. All of them have equally small heads. Tiny, pea-sized heads.

That brains have sides is no news. How our right hand corresponds to our left brain, or how our verbal skills are distinct from our musical talents, has scientists linking regions with function left and right.

Only recently did I make the connection that the zebrafish used in lab experiments are the same feeder fish swimming in my living room. A study published last week in Neural Development used the fish to show that the brain's asymmetry coincides at the level of single neurons.

Two pea sized nodes in the human brain (the habenula) have been implicated in functions including control of the circadian rhythm, behavior and mood. In the zebrafish, this structure is notably lopsided.

The researchers based at University College London embedded the fish embryos (no longer than the width of a toothpick) in a gelatinous substance and injected DNA corresponding to a green or red fluorescent protein using a series of electric pulses. This enabled them to visualize individual brain cells that display elaborate shapes. What they saw was telling. Looking at cells from the left and right habenula, the team noticed an unprecedented feature. The axons crossed over to the other side -not once, not twice, but lots of times. In fact, they made spirals.

The team also noticed that most of the cells on the left looked different than those on the right. The shape typical of left cells were “domed crowns” and the right-typical looked more flattened and shallow.

This experiment exemplifies reductionism on two levels: 1. the brains microscopic building blocks dictate or “encode” the macroscopic shape and 2. primitive critters are important to advanced concepts.

Zebrafish have primed so much more than a fish tank for the introduction of more sophisticated content. How our brain cell morphologies dictate asymmetry and how that corresponds to differences in behavior and cognition will keep scientists whirling in experiments.

Inside the glowing tank, Flag Fin waves. I imagine a spiraling green cell deep in his little brain. “Good night Flag Fin,” I say, flipping off the light.


The images displayed above show cells from the larval zebrafish habenula that extend into spirals. They correspond to Figures 1C and 2A from the article referenced.

Isaac H Bianco et al. 2008