Another difference between the CNS and the PNS in regard to the relationships between supporting cells and neurons is that unmyelinated neurons in the CNS are often found to be bare—that is, they are not embedded in glial cell processes. The lack of supporting cells around unmyelinated axons as well as the absence of basal lamina material and connective tissue within the substance of the CNS helps to distinguish the CNS from the PNS in histologic sections and in TEM specimens.
Microglia possess phagocytotic properties.
Microglia are phagocytotic cells. They normally account for about 5% of all glial cells in the adult CNS but proliferate and become actively phagocytotic (reactive microglial cells) in regions of injury and disease. Microglial cells are considered part of the mononuclear phagocytotic system (see Folder 6.4, page 185) and originate from granulocyte/monocyte progenitor (GMP) cells. Microglia precursor cells enter the CNS parenchyma from the vascular system. Recent evidence suggests that microglia play a critical role in defense against invading microorganisms and neoplastic cells. They remove bacteria, injured cells, and the debris of cells that undergo apoptosis. They also mediate neuroimmune reactions, such as those occurring in chronic pain conditions.
Microglia are the smallest of the neuroglial cells and have relatively small, elongated nuclei (Fig. 12.21). When stained with heavy metals, microglia exhibit short, twisted processes. Both the processes and the cell body are covered with numerous spikes. The spikes may be the equivalent of the ruffled border seen on other phagocytotic cells. The TEM reveals numerous lysosomes, inclusions, and vesicles. However, microglia contain little rER and few microtubules or actin filaments.
Ependymal cells form the epithelial-like lining of the ventricles of the brain and spinal canal.
Ependymal cells form the epithelium-like lining of the fluid-filled cavities of the CNS. They form a single layer of cuboidalto-columnar cells that have the morphologic and physiologic characteristics of fluid-transporting cells (Fig. 12.22). They are tightly bound by junctional complexes located at the apical surfaces. Unlike a typical epithelium, ependymal cells lack an external lamina. At the TEM level, the basal cell surface exhibits numerous infoldings that interdigitate with adjacent astrocyte processes. The apical surface of the cell possesses cilia and microvilli. The latter are involved in absorbing cerebrospinal fluid.
Within the system of the brain ventricles, this epithelium-like lining is further modified to produce the cerebrospinal fluid by transport and secretion of materials
FIGURE 12.16 • Photomicrograph of a nerve ganglion. a. Photomicrograph showing a ganglion stained by the Mallory-Azan method. Note the large nerve cell bodies (arrows) and nerve fibers (NF ) in the ganglion. Satellite cells are represented by the very small nuclei at the periphery of the neuronal cell bodies. The ganglion is surrounded by a dense irregular connective tissue capsule (CT ) that is comparable to, and continuous with, the epineurium of the nerve. 200. b. Higher magnification of the ganglion, showing individual axons and a few neuronal cell bodies with their satellite cells (arrows). The nuclei in the region of the axons are mostly Schwann cell nuclei. 640.
derived from adjacent capillary loops. The modified ependymal cells and associated capillaries are called the choroid plexus.
An action potential is an electrochemical process triggered by impulses carried to the axon hillock after other impulses are received on the dendrites or the cell body itself.