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.
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.
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.