https://en.wikipedia.org/wiki/Subgranular_zone

Neural stem cells and neurons[edit]

Structure and features of the neurogenic niche. Adapted from a paper by Ilias Kazanis, et al., 2008.

The brain comprises many different types of neurons, but the SGZ generates only one type: granule cells—the primary excitatory neurons in the dentate gyrus (DG)--which are thought to contribute to cognitive functions such as memory and learning. The progression from neural stem cell to granule cell in the SGZ can be described by tracing the following lineage of cell types:[5][6]

1. Radial glial cells. Radial glial cells are a subset of astrocytes, which are typically thought of as non-neuronal support cells. The radial glial cells in the SGZ have cell bodies that reside in the SGZ and vertical (or radial) processes that extend into the molecular layer of the DG. These processes act as a scaffold upon which newly formed neurons can migrate the short distance from the SGZ to the granule cell layer. Radial glia are astrocytic in their morphology, their expression of glial markers such as GFAP, and their function in regulating the NSC microenvironment. However, unlike most astrocytes, they also act as neurogenic progenitors; in fact, they are widely considered to be the neural stem cells that give rise to subsequent neuronal precursor cells. Studies have shown that radial glia in the SGZ express nestin and Sox2, biomarkers associated with neural stem cells, and that isolated radial glia can generate new neurons in vitro.[7] Radial glial cells often divide asymmetrically, producing one new stem cell and one neuronal precursor cell per division. Thus, they have the capacity for self-renewal, enabling them to maintain the stem cell population while simultaneously producing the subsequent neuronal precursors known as transiently amplifying cells.[8]

2. Transiently amplifying progenitor cells. Transiently amplifying (or transit-amplifying) progenitor cells are highly proliferative cells that frequently divide and multiply via mitosis, thus "amplifying" the pool of available precursor cells. They represent the beginning of a transitory stage in NSC development in which NSCs begin to lose their glial characteristics and assume more neuronal traits. For instance, cells in this category may initially express glial markers like GFAP and stem cell markers such as nestin and Sox2, but eventually, they lose these characteristics and begin expressing markers specific to granule cells such as NeuroD and Prox1. It is thought that the formation of these cells represents a fate-choice in neural stem cell development.

3. Neuroblasts. Neuroblasts represent the last stage of precursor cell development before cells exit the cell cycle and assume their identity as neurons. Proliferation of these cells is more limited, although cerebral ischemia can induce proliferation at this stage.

4. Postmitotic neurons. At this point, after exiting the cell cycle, cells are considered immature neurons. The large majority of postmitotic neurons undergo apoptosis, or cell death. The few that survive begin developing the morphology of hippocampal granule cells, marked by the extension of dendrites into the molecular layer of the DG and the growth of axons into the CA3 region, and subsequently the formation of synaptic connections. Postmitotic neurons also pass through a late maturation phase characterized by increased synaptic plasticity and a decreased threshold for long-term potentiation. Eventually, the neurons are integrated into the hippocampal circuitry as fully matured granule cells.

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