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 unti l recently to be restricted to lower vertebrates and songbirds. Nevertheles s, it has now been ascertained that these phenomena can occur in the adult mammalian nervous system, at least in three distinct sites: the olfactory n euroepithelium of the nasal mucosa and two brain regions, namely, the hippo campal dentate gyrus and the olfactory bulb. The newly generated cells of t he olfactory bulb originate from the subependymal layer, a remnant of the p rimitive subventricular zone persisting in the adult forebrain. Besides bei ng characterized by high rates of cell proliferation, the subependymal laye r is a site of long-distance tangential cell migration, wherein migrating c ells 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 diffe rent in neonatal and adult animals, changing strikingly in the postnatal pe riod. Other features of uniqueness involve the capability of neuronal precu rsors 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, pers isting 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 provi ding strategies for possible brain repair. (C) 1999 Elsevier Science Inc.