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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 development of the nervous system in humans, or neural development, or neurodevelopment involves the studies of embryology, developmental biology, and neuroscience.These describe the cellular and molecular mechanisms by which the complex nervous system forms in humans, develops during prenatal development, and continues to develop postnatally.
It has also been proposed that always exists a subclinical phase in the beginning of every MS case, during which the permeability of the BBB can be used for diagnosis [142] It is also under investigation whether MS has a prodrome, i.e., a preliminary stage in which the disease exists with non-specific symptoms. Some reports point to a prodrome ...
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 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 ]
Myelin sheath reduces membrane capacitance and increases membrane resistance in the inter-node intervals, thus allowing a fast, saltatory movement of action potentials from node to node. [l] [m] [n] Myelination is found mainly in vertebrates, but an analogous system has been discovered in a few invertebrates, such as some species of shrimp.
Bmp2, 4, 5, and 6 are expressed during the process and can compensate for one another. For example, if Bmp-4 was absent from corticogenesis, very little would change in the cortex phenotype, due to the other Bmps helping accomplish the tasks of Bmp-4.
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 ]