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Neurogenesis in Cerebrovascular Disease
Introduction
Tissue repair is as old as tissues themselves, as it can be observed in the most primitive multicellular organisms. However, evolution confers tissue complexity and cellular specialization at a price, which includes less effective repair capacity. As the most complex and most specialized tissue, brain is also the most difficult to regenerate after injury, such as that associated with cerebrovascular disease. Nevertheless, mechanisms for brain regeneration and repair exist. One such mechanism involves the manufacture of new cerebral neurons, or neurogenesis, which occurs at select sites in the brain and may contribute to functional recovery from stroke.
Adult Neurogenesis
Developmental neurogenesis associated with brain maturation proceeds with a time course that varies across brain regions. However, neurogenesis continues in the adult brain, most prominently at two sites. These are the subgranular zone (SGZ) of the hippocampal dentate gyrus, which adds neurons to the adjacent granule cell layer and may thereby participate in memory function, and the subventricular zone (SVZ) along the anterior horns of the lateral ventricles ( Fig. 17.1 ). In rodents, neuroblasts arising in the SVZ migrate along a pathway known as the rostral migratory stream to the olfactory bulb, where they replenish interneurons. A similar pathway exists in humans, but loses prominence after infancy, and populates not only the olfactory bulb but also the prefrontal cortex . Precursor cells from a variety of brain regions, including neocortex, can also generate neurons under certain in vitro conditions, suggesting that a similar phenomenon might also occur in vivo, perhaps in connection with injury or disease.
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