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Myelin is formed by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.Therefore, the first stage of myelinogenesis is often defined as the differentiation of oligodendrocyte progenitor cells (OPCs) or Schwann cell progenitors into their mature counterparts, [4] followed by myelin formation around axons.
The process of generating myelin is called myelination or myelinogenesis. In the CNS, oligodendrocyte progenitor cells (OPCs) differentiate into mature oligodendrocytes, which form myelin. In humans, myelination begins early in the 3rd trimester, [ 11 ] although only little myelin is present in either the CNS or the PNS at the time of birth.
The most accepted sequence of events is first NAWM appearance, then the so-called pre-active lesions, with activated microglia, and finally the BBB (blood-brain barrier) breakdown, which enables the entry of T-cells to the CNS. This marks the beginning of an autoimmune attack which destroys myelin in active lesions. [50]
The maturation of myelination in intracortical layers coincides with critical period closure in mice, which has led to further research on the role of myelination on critical period duration. [ 37 ] Myelin is known to bind many different axonal growth inhibitors that prevent plasticity seen in critical periods. [ 38 ]
A repair process, called remyelination, takes place in the early phases of the disease, but the oligodendrocytes are unable to completely rebuild the cell's myelin sheath. [90] Repeated attacks lead to successively less effective remyelinations, until a scar-like plaque is built up around the damaged axons. [ 90 ]
When a nerve axon is severed, the end still attached to the cell body is labeled the proximal segment, while the other end is called the distal segment. After injury, the proximal end swells and experiences some retrograde degeneration, but once the debris is cleared, it begins to sprout axons and the presence of growth cones can be detected.
Beginning in the future neck region, the neural folds of this groove close to create the neural tube. The formation of the neural tube from the ectoderm is called neurulation. The ventral part of the neural tube is called the basal plate; the dorsal part is called the alar plate. The hollow interior is called the neural canal. By the end of the ...
RGCs are the primary stem cells of the mammalian CNS, and reside in the embryonic ventricular zone, which lies adjacent to the central fluid-filled cavity (ventricular system) of the neural tube. [ 5 ] [ 6 ] Following RGC proliferation, neurogenesis involves a final cell division of the parent RGC, which produces one of two possible outcomes.