8/23/2023 0 Comments Adult neural stem cellsA glial origin for adult generated neuronal cells in the SVZ and SGZ has received further support ( 26, 27). One theory contends that they originate from differentiated ependymal cells in the lateral ventricle, while another contends that they originate from astrocyte-like cells ( 9). The origin of newly generated neuronal cells in the adult brain remains the source of controversies and to be fully determined. Origin of newly generated neuronal cells in the adult brain However, some of these reports have been contradicted by other studies, and need to be further evaluated ( 23- 25). Neurogenesis may also occur, albeit at lower levels, in other areas of the adult mammalian brain, like the Ammon’s horn CA1, neocortex, substantia nigra, and the 3rd ventricle in certain species ( 19- 22). The newly generated neuronal cells that survived to maturity may be very stable, and may permanently replace cells born during development, as adult-generated neuronal cells have been reported to survive for extended period of time ( 17, 18), at least 2 years in human DG ( 6). Among them, a significant proportion undergoes programmed cell death rather than achieving maturity ( 14- 16). Approximately 9,000 new neuronal cells or 0.1% of the granule cell population are generated per day in the DG, and 65.3-76.9% of the bulbar neurons are replaced during a 6 weeks period, in young adult rodents ( 12- 14). Newly generated neuronal cells establish functional connections with neighboring cells ( 5, 10), particularly GABAergic innervations in the DG, soon after their migration is completed ( 11). In the SVZ, cells are generated in the anterior part of the SVZ and migrate to the OB, where they differentiate into interneurons ( 9). In the DG, newly generated neuronal cells in the subgranular zone (SGZ) migrate to the granular layer, where they differentiate into mature neuronal cells, and extend axonal projections to the CA3 region. However, there is much debates, controversies and questions to be answered. Over the past decades, significant progresses have been made in the field of research. The advent of adult neurogenesis and NCS research has tremendous implications for our understanding of the physio- and pathology of the CNS, as well as for therapy. With the advent of new procedures for labeling dividing cells in the CNS, like bromodeoxyuridine (BrdU), retroviral labelings and magnetic resonance nuclear, new studies have since confirmed that neurogenesis occurs in the adult mammalian brain, primarily in two regions: the DG of the hippocampus and the subventricular zone (SVZ), in several species ( 3- 5), including human ( 6- 8). The authors reported, using -thymidine autoradiographic labeling, the generation of new neuronal cells in the dentate gyrus (DG), and cell proliferation in the ventricular zone, migration and persisting neurogenesis in the olfactory bulb (OB) of adult rodents ( 1, 2). Seminal studies in the 60s, conducted by Altman and Das, were the first to report the generation of new neuronal cells in the adult mammalian brain.
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