Icon module 22


The histogenesis of the nervous systems proceeds in three consecutive steps:

  • Cell proliferation
  • Cell differentiation
  • Cell migration 

Proliferation of the neuroepithelial stem cells

As has already been presented, the entire nervous tissue stems from the ectoderm that at first has the form of simple highly prismatic epithelium. In the ectoderm the chorda-mesoderm complex induces then the development of the neural plate. This caves in and closes itself to form a neural tube. Through this an epithelium out of bipolar neuroepithelium cells arises that apically (that is toward the lumen) are bound together by cell collections (zonulae adhaerentes). In this all the cells along the border of the lumen are involved whereby, though, not all attain to the externally lying basal membrane. Thus the wall of the neural tube consists of double rows of highly prismatic neuroepithelium. These neurectodermal cells divide mitotically.

Fig. 19 - Schematic view of the neural tube coated with the basal membrane

Fig. 19

Due to the folding of the neurectoderm into the neural tube the basal membrane of the neuroepithelium (pink) comes to lie on its outer side. For this reason it is also termed a membrana limitans externa.
In contrast, the membrana limitans interna (light blue) is formed by the cell masses of the neuroepithelium at the boundary to the central canal.

Over the course of a cell's cycle characteristic shifts of the cell's nucleus within the neuroepithelium (interkinetic nucleus migration) occur. During the G1 phase (G1) the nucleus shifts from its juxtaluminal position in the direction of the periphery. During DNA synthesis (S phase of the interphase) the cell stands in a broad connection with the lamina limitans externa. Subsequently, in the G2 phase (G2), the nucleus returns to the juxtaluminal position and the connection of the cell with the lamina limitans externa is lost. The cells roll themselves up, also detach themselves from the lamina limitans interna and in this position cell division takes place (M). Afterwards, in the subsequent G1 phase, the two daughter cells stretch out until they reach the inner boundary of the neural tube and then the lamina limitans externa. Through the mitosis activity of the epithelium the cell density increases continuously in the wall of the neural tube.

Fig. 20 - Cell division (schematic diagrams of the various phases of a single cell)

Fig. 20

The schematic cross-section through the neural tube shows the characteristic positional changes of the cell nucleus of a neuroepithelial cell over the course of a cellular cycle.

Soon, the wall of the neural tube consists of multi-row epithelium that gradually loses its epithelial character (see below). In parallel to this the closure of the neural tube takes place through the fusing of the neural folds.