Neurogenesis in the mammalian neocortex occurs from the inside out
through the precise spatiotemporal coordination of the birth,
migration, and differentiation of neurons across the cortical plate.
The central nervous system is divided into different regional zones or layers
from its most apical luminal surface to its most superficial pial surface,
and these different layers are built in an apical or inside
to basal or outside direction over time.
The early neural tube is first made up of Neuroepithelial cells,
which span its full apical to basal axis.
Initially, these neuroepithelial cells will divide symmetrically
to produce more copies of themselves
and establish the ventricular and subventricular zones.
As the neural tube matures,
the progeny of the neuroepithelial stem cells become radial glial cells,
which similarly span the full apical to basal axis of the neuroepithelium.
Consequently, these radial glial are known as Ventricular Radial Glia.
The Ventricular Radial Glia serve as the parental stem cell type
that undergo self-renewing divisions.
Some divisions are symmetrical to expand the stem cell pool,
while other divisions are asymmetric and serve to generate neurons.
In addition to directly generating neurons,
Ventricular Radial Glia will also give rise to two other progenitor cell types,
The first type is known as intermediate progenitor cells,
which can also proliferate and give rise to neurons.
The other progenitor cell that Ventricular Radial glial generate
is called the “outer Radial glial cell,”
so named because it’s apical most surface
uniquely resides in the outer subventricular zone.
Importantly, Outer Radial Glia also divide asymmetrically
to give rise to more intermediate progenitor cells
that expands this progenitor pool
which will have real impacts on the capacity of neurogenesis
in the brain across mammalian species.
Lastly, late neurogenesis is marked by an important transition
where Radial Glia will start to generate glial cells in addition to neurons,
and often these stem and progenitor cells
will demonstrate symmetrical differentiating divisions
during these later stages of neurogenesis in the embryonic brain.