Migration of Neurons in Vivo

Summary

Radial glial cells have been demonstrated by various methods in most developing mammalian brain regions, and their role in providing a suitable substrate for neuronal migration has become a generally accepted hypothesis. However, there remain several unresolved questions. The present morphological studies have dealt with two of these problems, the dynamics of the radial glial guidance system and the existence of other, non-radial glial cells which are connected to radial glial cells.

The rat dentate gyrus is characterized by two stages of cell production and migration, the first radial glial cells differentiate early enough to direct the stream of dentate precursor cells to set up the secondary proliferative zone. Here, new radial glial cells are produced together with the granule cells. This second set of radial glia is inserted into the lattice of its predecessor, but it is oriented radially with respect to the developing C-shape of the dentate gyrus. Migrating granule cells seem to migrate preferentially along the second, “dentate-intrinsic” set of radial glial cells.

During development of the neocortex of rat, attention was focused on non-radial glial cells. Since the time of their origin is difficult to determine using immunocytochemical markers or ³H -thymidine autoradiography, the contact relationship was investigated of cells with the surfaces of the neuroepithelium. By using astroglial-like contact relationships (contact with basal lamina but not with the ventricle) as a criterion, it has been possible to detect immature glial cells which are among the very first cells leaving the neuroepithelial zone and which can be traced until they express GFAP. During development, there appears a variety of morphological types of immature glial cells, most of which appear to form specialized intercellular contacts with radial glial cells. The resulting complex framework is anchored at the ventricular surface and at the mesenchymal surfaces of the cortex. It appears possible that the non-radial component of this framework plays a role in rerouting, delaying or terminating neuronal migration and has a similarly important effect on the developing architecture of the neocortex as has been demonstrated for the dynamics of the radial component in the case of the gyrus.