In contrast to other central neuroglia, microglia cells are derived from mesodermal macrophage precursors, specifically from Granulocyte/monocyte progenitor (GMP) cells in bone marrow. They infiltrate the neural tube in the early stages of its development and under the influence of growth factors such as colony stimulating factor-1 (CSF-1) produced by developing neural cells undergo proliferation and differentiation into motile ameboid cells. These motile cells are commonly ob-
FIGURE 12.22 • Ependymal lining of the spinal canal. a. Photomicrograph of the cental region of the spinal cord stained with toluidine blue. The arrow points to the central canal. 20. b. At higher magnification, ependymal cells, which line the central canal, can be seen to consist of a single layer of columnar cells. 340. (Courtesy of Dr. George D. Pappas.) c. Transmission electron micrograph showing a portion of the apical region of two columnar ependymal cells. They are joined by a junctional complex (JC) that separates the lumen of the canal from the lateral intercellular space. The apical surface of the ependymal cells has both cilia (C) and microvilli (M). Basal bodies (BB) and a Golgi apparatus (G) within the apical cytoplasm are also visible. 20,000. (Courtesy of Dr. Paul Reier.) served in the developing brain. As the only glial cells of mesenchymal origin, microglia possess the vimentin class of intermediate flaments, which can be useful in identifying these cells with immunocytochemical methods.
PNS ganglion cells and peripheral glia 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 from migrating neural crest cells that become associated with the axons of early embryonic nerves. Several genes have been implicated in Schwann cell development. Sex-determining region Y (SRY) box 10 (Sox10) is required for the generation of all peripheral glia from neural crest cells. Axon-derived neuregulin 1 (Nrg-1) sustains the Schwann cell precursors that undergo differentiation and divide along the growing nerve processes. The fate of all immature Schwann cells is determined by the nerve processes with which they have immediate contact. Immature Schwann cells that associate with large-diameter axons mature into myelinating Schwann cells, while those that associate with small-diameter axons mature into nonmyelinating cells.
The peripheral nervous system (PNS) consists of peripheral nerves with specialized nerve endings and ganglia containing nerve cell bodies that reside outside the central nervous system.
A peripheral nerve is a bundle of nerve fibers held together by connective tissue.
The nerves of the PNS are made up of many nerve fibers that carry sensory and motor (effector) information between the organs and tissues of the body and the brain and spinal cord. The term nerve fber is used in different ways that can be confusing. It can connote the axon with all of its coverings (myelin and Schwann cell), as used above, or it can connote the axon alone. It is also used to refer to any process of a nerve cell, either dendrite or axon, especially if insufficient information is available to identify the process as either an axon or a dendrite.