Postnatal development of radial glia and the ventricular zone (VZ): a continuum of the neural stem cell compartment.

The germinal neuroepithelium, or ventricular zone (VZ) of the developing fetal brain, was once thought to transform into the non-germinal ependymal zone of the postnatal and adult brain. Persistence of neural stem cells and neurogenesis throughout postnatal life, however, suggests a continuum between embryonic and adult germinal brain centers. Here, we suggest that developmental changes in anatomy and molecular marker expression in the ventricular walls (the principal germinal centers of the brain) may have misled us into current interpretations of VZ transformation from a germinal to a non-germinal epithelium. We review previous studies and present new data indicating that a germinal layer with characteristics similar to those of the embryonic VZ persists in lateral ventricular walls of the postnatal mouse brain, a region where the adult subventricular zone (SVZ) develops and where neurogenesis persists into adult life. The early postnatal VZ is largely composed of radial glial cell bodies that remain proliferative, display interkinetic nuclear migration and serve as progenitors of new neurons. Ependymal cells then progressively populate the walls of the lateral ventricle but a subpopulation of astrocytes, derived from radial glia, remain in contact with the ventricle lumen, into which they extend a single cilium similar to that found on neuroepithelial cells and radial cells. We propose that a VZ 'compartment' is retained postnatally and that this niche may be essential for stem cell function.