Postnatal neurogenesis: of mice, men, and macaques.

Over the past 20 years, the conception of brain development has radically changed from a fixed and limited hierarchical process to a more plastic and continuous one. Most surprising, the field has learned that postnatal neurogenesis is not just a seasonal phenomenon in songbirds but a process that occurs across species and seasons. Astrocytes, whose primary role in the central nervous system was thought to be strictly supportive, have emerged as a heterogeneous population, a subset of which is the neural stem cell. Postnatal neurogenesis persists in specialized niches within the rostral subventricular zone and hippocampal dentate gyrus and, for a limited period, within the white matter tracts and external granular layer of the cerebellum. These specialized microenvironments are influenced by factors in the blood, cerebrospinal fluid, and local extracellular matrix. This article reviews the current understanding of adult neurogenesis, which is conserved across many vertebrate species, underscoring the value of animal models in past and present studies of human neurogenesis and neurogenic disease.