The subependymal layer in rodents: a site of structural plasticity and cell migration in the adult mammalian brain.

The persistence of neurogenesis and structural plasticity was believed until recently to be restricted to lower vertebrates and songbirds. Nevertheless, it has now been ascertained that these phenomena can occur in the adult mammalian nervous system, at least in three distinct sites: the olfactory neuroepithelium of the nasal mucosa and two brain regions, namely, the hippocampal dentate gyrus and the olfactory bulb. The newly generated cells of the olfactory bulb originate from the subependymal layer, a remnant of the primitive subventricular zone persisting in the adult forebrain. Besides being characterized by high rates of cell proliferation, the subependymal layer is a site of long-distance tangential cell migration, wherein migrating cells form chains enwrapped by a particular type of astrocytes. These glial cells give rise to channels (glial tubes) that separate single chains from the surrounding mature tissue. The cellular composition and the pattern of cell migration in the mammalian subependymal layer appear to be quite different in neonatal and adult animals, changing strikingly in the postnatal period. Other features of uniqueness involve the capability of neuronal precursors to divide while undergoing migration and the presence of multipotent stem cells. Thus, the subependymal layer is an area of the adult mammalian brain endowed with a cohort of phenomena proper of neural development, persisting into (and adapted to) the fully mature nervous tissue. Such features make this system an optimal model to unravel mechanisms permitting highly dynamic structural plasticity during adulthood, in the perspective of providing strategies for possible brain repair.