The majority of primitive supporting cells, the gliablasts, are formed by neu-roepithelial cells after production of neuroblasts ceases. Gliablasts migrate from the neuroepithelial layer to the mantle and marginal layers. In the mantle layer, they differentiate into protoplasmic astrocytes and fibrillar astrocytes (Fig. 19.11).
Another type of supporting cell possibly derived from gliablasts is the oligo-dendroglial cell. This cell, which is found primarily in the marginal layer, forms myelin sheaths around the ascending and descending axons in the marginal layer.
In the second half of development, a third type of supporting cell, the microglial cell, appears in the CNS. This highly phagocytic cell type is derived from mesenchyme (Fig. 19.11). When neuroepithelial cells cease to produce neuroblasts and gliablasts, they differentiate into ependymal cells lining the central canal of the spinal cord.
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