The (Real) Neurogenic/Gliogenic Potential of the Postnatal and Adult Brain Parenchyma

Table 1

Variables affecting the nature and features of adult neurogenesis.

Animal species (animal world)	General plasticity and persistent neurogenesis are usually reduced across phylogeny; in parallel, the reparative/regenerative potential is also reduced
Animal species (mammals)	Unlike previous belief and current bias, remarkable differences in the location and extension of adult neurogenesis do exist among mammals
Age	Some neurogenic processes are extensions of delayed developmental programs (postnatal/protracted neurogenesis) whereas others persist throughout life (persistent neurogenesis). All neurogenic processes are progressively reduced with age
Microenvironment (niche)	A well-defined neural stem cell niche sustains neurogenesis in neurogenic sites (SVZ, SGZ), whereas a niche has not been characterized in parenchymal neurogenesis
Origin of stem/progenitor cells	Neurogenic sites (SVZ, SGZ) directly derive from persistence and modification of preexisting, embryonic germinal layers, whereas for parenchymal cell genesis such direct link is not clear
Location in the CNS	Location either within a germinal layer-derived niche or in the parenchyma redirects to the two previous points; in parenchymal neurogenesis many variations are linked to local cues of the different CNS regions involved
Function	In physiology: linked to the different ecological niches of the animals (present in all animals)
	In repair: linked to the species; in invertebrates and nonmammalian vertebrates the physiological function is associated with function in repair, whereas in birds and mammals it is only linked to physiology/homeostasis of specific systems