When cells have not yet fully developed, they are called neuroblasts, rather than neurons. The cells that emerge from the proliferation phase during brain development are all neuroblasts. The cells must first find their location in the brain, and only then can they continue to develop. These two processes are sequentially called migration and differentiation.
The first neuroblasts to migrate from the neural tube form the subplate. This is a layer of cells that exists only during brain development. After this, these cells die off and the subplate disappears.
Once the subplate is formed, the cortex is built. This involves working from the inside out. This means that layer 6 of the cortex is formed first. The neuroblasts that migrate after this go through layer 6, and form layer 5 on top of it. This process repeats until finally (the outermost) layer 1 of the cortex is formed.
However, we know that the cortex is not only composed of layers, but also of columns. This division is formed early in development and is explained by the radial unit hypothesis. In this theory, there is a special role for a specific type of glial cell, the radial glial cell. The radial glial cells form a kind of web in the brain. Neuroblasts that lie side by side in the ventricular zone during proliferation follow the same route in the web of radial glial cells. In this way, they move to the columns in the cortex.
Author: Myrthe Princen (translated by Melanie Smekal)