Adult neurogenesis in the mammalian brain: significant answers and significant questions

Abstract

Adult neurogenesis, a process of generating functional neurons from adult neural precursors, occurs throughout life in restricted brain regions in mammals. The past decade has witnessed tremendous progress in addressing questions related to almost every aspect of adult neurogenesis in the mammalian brain. Here we review major advances in our understanding of adult mammalian neurogenesis in the dentate gyrus of the hippocampus and from the subventricular zone of the lateral ventricle, the rostral migratory stream to the olfactory bulb. We highlight emerging principles that have significant implications for stem cell biology, developmental neurobiology, neural plasticity, and disease mechanisms. We also discuss remaining questions related to adult neural stem cells and their niches, underlying regulatory mechanisms, and potential functions of newborn neurons in the adult brain. Building upon the recent progress and aided by new technologies, the adult neurogenesis field is poised to leap forward in the next decade.

Figure 1. Models of neural stem cells and lineage relationship in the adult dentate gyrus and subventricular zone

(A) A sagittal section view of an adult rodent brain highlighting the two restricted regions that exhibit active adult neurogenesis: dentate gyrus (DG) in the hippocampal formation (HP); the lateral ventricle (LV) to the rostral migratory stream (RMS) to the olfactory bulb (OB). (B) A schematic illustration of the neural stem cell niche in the subventricular zone (SVZ) and a model of potential lineage relationship under basal (solid arrows) and injury conditions (blue arrows). N: immature neurons (C) A schematic illustration of the neural stem cell niche in the subgranular zone (SGZ) in the dentate gyrus and a model of potential lineage relationship. (D) Three lineage models of neural precursors in the adult mammalian brain. In the first model (left), adult neural stem cells (S_1,2,3…) generated from primitive neural stem cells (S) are intrinsically diverse, exhibiting vastly different developmental potential depending on their regions of distribution and developmental origins. In the second model (middle), adult neural stem cells (S) are relatively homogenous and give rise to a heterogeneous population of lineage-restricted progenitors (P_1,2,3…). In the third model (right), only lineage-restricted neural progenitors (P_1,2,3…) are present in the adult brain; self-renewal and multi-lineage differentiation represent a collective property of a mixture of different lineage-restricted neural progenitors. N: neurons; O: oligodendrocytes; As: astrocytes.

Figure 2. Adult neurogenesis in the subventricular zone of the lateral ventricle and olfactory bulb

Summary of five developmental stages during adult SVZ neurogenesis: (1) activation of radial glia-like cells in the subventricular zone in the lateral ventricle (LV); (2) proliferation of transient amplifying cells; (3) generation of neuroblasts; (4) chain migration of neuroblasts within the rostral migratory stream (RMS) and radial migration of immature neurons in the olfactory bulb (OB); (5) Synaptic integration and maturation of granule cells (GC) and periglomerular neurons (PG) in the olfactory bulb. Also shown are expression of stage-specific markers, sequential process of synaptic integration, and critical periods regulating survival and plasticity of newborn neurons. GFAP: glial fibrillary acidic protein; DCX: doublecortin; NeuN: neuronal nuclei; LTP: long-term potentiation.

Figure 3. Adult neurogenesis in the dentate gyrus of the hippocampus

Summary of five developmental stages during adult hippocampal neurogenesis: (1) activation of quiescent radial glia-like cell in the subgranular zone (SGZ); (2) proliferation of non radial precursor and intermediate progenitors; (3) generation of neuroblasts; (4) integration of immature neurons; (5) maturation of adult-born dentate granule cells. Also shown are expression of stage-specific markers, sequential process of synaptic integration, and critical periods regulating survival and plasticity. ML: molecular layer; GCL: granule cell layer; SGZ: subgranular zone; GFAP: glial fibrillary acidic protein; BLBP: brain lipid-binding protein; DCX: doublecortin; NeuN: neuronal nuclei; LTP: long-term potentiation.

Figure 4. Basic circuit architecture of the olfactory bulb and hippocampal dentate gyrus and a unified model on how new neurons impact the local circuitry

(A) In the olfactory bulb, primary sensory neurons project to glomeruli where they synapse onto mitral and tufted cells, which in turn relay information to the olfactory cortex. Periglomerular neurons provide lateral inhibition between individual glomeruli, whereas granule cells provide lateral inhibition between mitral/tufted cells. Adult-born interneurons (green), although in small numbers, can have powerful inhibition of the local circuitry in the olfactory bulb. (B) In the hippocampus, layer II entorhinal cortical inputs innervate dentate granule cells, whereas dentate granule cells innervate CA3 neurons, which in turn innervate CA1 neurons. In addition, granule cells synapses onto hilar basket interneurons, each of which inhibit hundreds of mature dentate granule cells. Granule cells also synapse onto hilar mossy cells, which also innervate many mature dentate granule cells on the contralateral dentate gyrus. Adult-born dentate granule cells (green), although in small numbers, can have powerful influence in the local circuitry through basket interneurons and mossy cells.