Formation of the concentric zones

A neuroblast produces multiple types of medulla neurons

The structure of the larval medulla primordium is like a hemisphere.
You can find a round shape in a lateral view (left).
Neuroblasts (NBs), kinds of neural stem cells, are situated in the outer-most zones of the hemisphere.
Each NB produces many neurons inside the brain.
In a horizontal section (right), you can also find NBs, neurons and neuropile as shown here.
The larval neuropile is then transformed to become the adult medulla neuropile.

Global or Temporal?

it is still unknown how concentric gene expression is specified. There could be two possible mechanisms that form the concentric zones.
1) Global and spatial mechanisms governed by 'morphogen gradient' as shown in the vertebrate spinal cord.
2) Local and temporal mechanisms that control neuronal 'birth order' as shown in the embryonic CNS.

Neuronal type correlates birth order of neurons

As mentioned earlier, each NB produce many neurons toward the center of the medulla.
Neurons of the same lineage intersect the concentric zones.
Therefore, different types of medulla neurons may be produced in a lineage independent manner.
To demonstrate this possibility, clones containing single NB were labeled with GFP as shown here.
And we counted number of neurons that express concentric genes.
The results suggest many NBs have similar potential to produce a series of medulla neurons.
This result suggests birth order dependent mechanisms may regulate concentric gene expression.
For example, green cells are produced at first, magenta cells next, blue cells and so on.



  Institute for Frontier Science Initiative