Migration of medulla neurons

The concentric zones are formed in the larval brain.
What happens to the concentric zones during pupal development?


Concentric zones are disrupted due to radial migration of medulla neurons

Once the concentric zones are specified in the larval brain,
they are conserved early during pupal development until 8 hr after puparium formation, APF.
However, at 12 hours APF, the shapes of the concentric zones are somewhat disorganized.
For example, magenta cells are found in the outer-most area.
After this, distribution of cells keeps changing until 24 hours APF to become completely disorganized.
Same was true in the adult brain.
Since expression of each transcription factor is strong and stable,
medulla neurons are thought to migrate without changing gene expression.
We confirmed this idea by constitutively labeling subsets of medulla neurons with GFP as shown below. Intriguingly, green cells come to outer most and blue cells come to inner most areas.
So, medulla neurons are thought to migrate in radial orientation.

Medulla neurons migrate in tangential orientation as well

To examine if medulla neurons are indeed migrating, a group of medulla neurons were labelled with GFP and their distributions were examined before and after migration.
As explained earlier, each NB produce many neurons in radial orientation during larval development.
Once a NB is labelled with GFP, its daughter cells arranged in line would be labelled (top left).
You can see radial and linear arrangement of GFP positive cells in larva (top right).
At 12 hours APF at the beginning of migration, you can see linear arrangement of GFP positive cells (bottom left). However, after the completion of migration at 24 hours APF, GFP-positive cells were randomly distributed (bottom right). You can find no linear arrangement of GFP-positive neurons.
Relative locations of medulla neurons are significantly changed between 12 and 24 hours APF.

Migration pattern may be defined according to neuronal type

You may have an impression that medulla neurons migrate just randomly and stochastically.
So, I would like to show you an example of Bsh-positive neurons.
A part of Hth-positive neurons expressing Bsh are located in the inner area of the larval brain (top left). Please look at white cells that are Bsh-positive. Interestingly, they always move outward and finally positioned at the outer-most area of the brain as shown here.
Therefore, migration of Bsh-positive neuron is an organized and patterned process.




  Institute for Frontier Science Initiative