FIGURE 12.11 • Electron micrograph of an axon in the process of myelination. At this stage of development, the myelin (M) consists of about six membrane layers. The inner mesaxon (IM) and outer mesaxon (OM) of the Schwann cell (SC) represent parts of the mesaxon membrane. Another axon (see upper left A) is present that has not yet been embedded within a Schwann cell mesaxon. Other notable features include the Schwann cell basal (external) lamina (BL) and the considerable amount of Schwann cell cytoplasm associated with the myelination process. 50,000. (Courtesy of Dr. Stephen G. Waxman.) processes must penetrate between the satellite cells to establish a synapse (there are no synapses in sensory ganglia). They help to establish and maintain a controlled microenvironment around the neuronal body in the ganglion, providing electrical insulation as well as a pathway for metabolic exchanges. Thus, the functional role of the satellite cell is analogous to that of the Schwann cell except that it does not make myelin.
Neurons and their processes located within ganglia of the enteric division of the ANS are associated with enteric neuroglial cells. These cells are morphologically and functionally similar to astrocytes in the CNS (see below). Enteric neuroglial cells share common functions with astrocytes, such as structural, metabolic, and protective support of neurons. However, recent studies indicate that enteric glial cells may also participate in enteric neurotransmission and help coordinate activities of the nervous and immune systems of the gut.
There are four types of central neuroglia:  Astrocytes are morphologically heterogeneous cells that provide physical and metabolic support for neurons of the CNS.  Oligodendrocytes are small cells that are active in the formation and maintenance of myelin in the CNS.  Microglia are inconspicuous cells with small, dark, elongated nuclei that possess phagocytotic properties.  Ependymal cells are columnar cells that line the ventricles of the brain and the central canal of the spinal cord.
Only the nuclei of glial cells are seen in routine histologic preparations of the CNS. Heavy-metal staining or immunocytochemical methods are necessary to demonstrate the shape of the entire glial cell.
Although glial cells have long been described as supporting cells of nerve tissue in the purely physical sense, current concepts emphasize the functional interdependence of neuroglial cells and neurons. The most obvious example of physical support occurs during development. The brain and spinal cord develop from the embryonic neural tube. In the head region, the neural tube undergoes remarkable thickening and folding, leading ultimately to the final structure, the brain. During the early stages of the process, embryonic glial cells extend through the entire thickness of the neural tube in a radial manner. These radial glial cells serve as the physical scaffolding that directs the migration of neurons to their appropriate position in the brain.
Astrocytes are closely associated with neurons to support and modulate their activities.
Astrocytes are the largest of the neuroglial cells. They form a network of cells within the CNS and communicate with neurons to support and modulate many of their activities. Some astrocytes span the entire thickness of the brain, providing a scaffold for migrating neurons during brain development. Other astrocytes stretch their processes from blood vessels to neurons. The ends of the processes expand, forming end feet that cover large areas of the outer surface of the vessel or axolemma.
Astrocytes do not form myelin. Two kinds of astrocytes are identified:  Protoplasmic astrocytes are more prevalent in the outermost covering of brain called gray matter. These astrocytes have numerous, short, branching cytoplasmic processes (Fig. 12.17).
 Fibrous astrocytes are more common in the inner core of the brain called white matter. These astrocytes have fewer processes, and they are relatively straight (Fig. 12.18).
Both types of astrocytes contain prominent bundles of intermediate filaments composed of glial fbrillary acidic protein (GFAP). The filaments are much more numerous in