CNS neurons are derived from neuroectodermal cells of the neural tube
After developing neurons have migrated to their predestined locations in the neural tube and have differentiated into mature neurons, they no longer divide. Astrocytes and oligodendrocytes are also derived from cells of the neural tube, but studies with tritiated thymidine indicate that these neuroglial cells undergo a slow turnover.
As noted above, microglia are derived from mononuclear phagocytotic cells of blood (mononuclear phagocy-totic system [MPS]) along with other macrophages of the body. It is still not known whether they are able to divide after they have reached the CNS. As the only glial cells of mesenchymal origin, microglia possess the vimentin class of intermediate filaments, which can be useful in identifying these cells by using immunocytochemical methods.
The ependymal cells are derived from the proliferation of neuroepithelial cells that immediately surround the canal of the developing neural tube.
PNS ganglion cells are derived from the neural crest
The development of the ganglion cells of the PNS involves the proliferation and migration of ganglion precursor cells from the neural crest to their future ganglionic sites, where they undergo further proliferation. There, the cells develop processes that reach the cells' target tissues (e.g., glandular tissue or smooth muscle cells) and sensory territories. Initially, more cells are produced than are needed. Those that do not make functional contact with a target tissue undergo apoptosis.
Schwann cells also arise originally from the neural crest but undergo mitosis along the growing nerve. Most Schwann cells are formed by mitosis of parent Schwann cells in the peripheral nerves rather than by the migration of cells from the neural crest.
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