Proliferation is the first step in the production of new neurons and glial cells, and takes place between the fifth week and fifth month of pregnancy. The process takes place in the wall of the neural tube. This wall is divided into two parts:

- Ventricular zone: the inside of the wall, facing the ventricles

- Marginal zone: the outside of the wall, directed towards the meninges

The creation of new neurons and glial cells during proliferation is a process that can be divided into several steps:

1. A mother cell moves from the ventricular zone to the marginal zone.
2. A mother cell continues its journey to the surface of the marginal zone, reaching the meninges. Only at this place can the cell's DNA be copied, which is done then.
3. The mother cell slides back into the ventricular zone, retaining only a "tail" that remains attached to the meninges.
4. The cell releases completely from the meninges, including the tail.
5. The cell splits into two cells.

This process continues until there are enough cells. At that point, a built-in mechanism makes it impossible for the mother cell to divide any more. In fact, there are two ways in which a cell can divide. The first form is vertical division, in which the new cell becomes an exact copy of the mother cell. The two cells then also contain exactly the same substances, and can continue to divide. The second form of division is horizontal division. Here the important substances are divided between the mother cell and the new cell. For example, the mother cell keeps substance A, and the new cell gets substance B. When horizontal division has taken place, the cells migrate to their place in the cortex.

The question now remains as to how it is determined that a cell has divided often enough. We have seen that the DNA of the cell is copied on the meninges, and is therefore the same in all cells. DNA therefore cannot explain the change from vertical to horizontal division. This change is caused by differences in gene expression that occur after a certain time.

Auteur: Myrthe Princen (translated by Melanie Smekal)