FIGURE 12.19 • Distribution of glial cells in the brain. This diagram shows the three types of glial cells—astrocytes, oligodendrocytes, and microglial cells—interacting with several structures and cells found in the brain tissue. Note that the astrocytes and their processes interact with the blood vessels as well as with axons and dendrites. Note that astrocytes also send their processes toward the brain surface, where they contact the basement membrane of the pia mater, forming the glia limitans. In addition, processes of astrocytes extend toward the fluid-filled spaces in the CNS, where they contact the ependymal lining cells. Oligodendrocytes are involved in myelination of the nerve fibers in the CNS. Microglia exhibit phagocytotic functions. pia mater myelin oligo-dendrocyte ependyma astrocyte microglial cell pericyte neuron nerve fibers myelin axons node of Ranvier (showing axon in contact with extracellular space) oligodendrocyte
FIGURE 12.20 • Three-dimensional view of an oligodendrocyte as it relates to several axons. Cytoplasmic processes from the oligodendrocyte cell body form flattened cytoplasmic sheaths that wrap around each of the axons. The relationship of cytoplasm and myelin is essentially the same as that of Schwann cells.
next. The speed of saltatory conduction is related not only to the thickness of the myelin but also to the diameter of the axon. Conduction is more rapid along axons of greater diameter.
In unmyelinated axons, Na and K channels are distributed uniformly along the length of the fiber. The nerve impulse is conducted more slowly and moves as a continuous wave of voltage reversal along the axon.
CNS neurons and central glia, except microglial cells, are derived from neuroectodermal cells of the neural tube.
Neurons, oligodendrocytes, astrocytes, and ependymal cells are derived from 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. However, in the adult mammalian brain, a very small number of cells left from development called neural stem cells retain the ability to divide. These cells migrate into sites of injury and differentiate into fully functional nerve cells.
Oligodendrocyte precursors are highly migratory cells. They appear to share a developmental lineage with motor neurons migrating from their site of origin to developing axonal projections (tracts) in the white matter of the brain or
FIGURE 12.21 • Microglial cell in the gray matter of the brain. a. This diagram shows the shape and characteristics of a microglial cell. Note the elongated nucleus and relatively few processes emanating from the body. b. Photomicrograph of microglial cells (arrows) showing their characteristic elongated nuclei. The specimen was obtained from an individual with diffuse microgliosis. In this condition, the microglial cells are present in large numbers and are readily visible in a routine H&E preparation. 420. (Reprinted with permission from Fuller GN, Burger PC. Central nervous system. In: Sternberg SS, ed. Histology for Pathologists. Philadelphia: Lippincott-Raven, 1997.) spinal cord. The precursors then proliferate in response to the local expression of mitogenic signals. The matching of oligodendrocytes to axons is accomplished through a combination of local regulation of cell proliferation, differentiation, and apoptosis.
Astrocytes are also derived from cells of the neural tube. During the embryonic and early postnatal stages, immature astrocytes migrate into the cortex, where they differentiate and become mature astrocytes. Ependymal cells are derived from the proliferation of neuroepithelial cells that immediately surround the canal of the developing neural tube.