1. Schwann cells are the supporting cells of the peripheral nerves. One Schwann cell may envelop segments of several unmyelinated axons or provide a segment of a single myelinated axon with its myelin sheath. Each myelinated axon segment is surrounded by multiple layers of a Schwann cell process with most of its cytoplasm squeezed out; the remaining multilayered Schwann cell plasma membrane, called myelin, consists mainly of phospholipid. The gaps between the myelin sheath Se8ments are the nodes of Ranvier. Ovoid or flattened Schwann cell nuclei lie peripheral to the axon they support. They are usually more euchromatic than the nuclei of the fibrocytes scattered among the axons.
2. Satellite cells are specialized Schwann cells in craniospinal and autonomic ganglia, where they form a one-cell-thick covering over the cell bodies of the neurons (ganglion cells). Their nuclei are spheric with mottled chromatin. In sections, the nuclei typically appear as a "string of pearls" surrounding the much larger ganglion cell bodies.
Supporting Cells of the PNS:
SUPPORTING CELLS
A. Supporting Cells of the CNS: There are about 10 neuroglial cells per neuron in the CNS. Glial cells are generally smaller than neurons. Their processes, although abundant and exten sive, are indistinguishable without special stains. Identification is usually based on nuclear morphology. The major supporting cells in the CNS are the macroglia, including astrocytes and oligodendrocytes, the microglia, and the ependymal cells.
1. Astrocytes are the largest glial cells. Their nuclei, also the largest, are irregular, spheric, and pale-staining with a prominent nucleolus. Their branching cytoplasmic processes often have, at their tips, expanded pediclcs, or vascular end-feet. These surround capillaries of the pia mater and are important components of the blood-brain barrier . Proteplasmic astro cytes (messy cells) are more common in gray matter. They have ample granular cytoplasm and short, thick, highly branched processes. Fibrens astrecytes are more common in white matter. Silver stains show their cytoplasm to be full of fibrous material. Their long, thin processes are less branched than those of protoplasmic astrocytes. 2. Oligedendroglia or oligodendrocytes, the most numerous glial cells, are found in both gray and white matter. Their spheric nuclei fall between those of astrocytes and microglia in terms of size and staining intensity. Like the Schwann cells of the PNS, oligodendrocytes form myelin and occur in long rows as required to myelinate entire axons. Unlike a Schwann cell, each may have several cell processes and may provide myelin for segments of several axons. Unmyelinated axons of the CNS are not sheathed. 3. Microglia, the smallest and rarest of the glia, are found in both gray and white matter. Their nuclei are small and elongate (often bean-shaped), and their chromatin is so condensed that they often appear black in H&E-stained sections. Their processes are shorter than those of astrocytes and are covered with thorny branches. Microglial cells may derive from mes enchyme, or they may be glioblasts (immature oligodendrocytes) of neuroepithelial origin. Some microglia may be components of the mononuclear phagocyte system and have phago cytic capabilities. When neural injury is unaccompanied by vascular injury, phagocytic cells in the lesioned area appear to derive from macroglia. 4. Ependymal cells derive from ciliated neuroepithelial cells of the internal lining of the neural tube. In adults, they retain their epithelial nature and some cilia, and they line the remnants of the neural tube (ventricles and aqueducts of the brain and the central canal of the spinal cord). The lining resembles a simple columnar epithelium, but epcndymal cells have basal cell processes that extend deep into the gray matter. The ependymal lining is con tinuous with the cuboidal epithelium of the choroid plexus.
Neurons
ØA. Cell Body: The cell body (soma, perikaryon) is the synthetic and trophic center of the neuron. It can receive signals from axons of other neurons through synaptic contacts on its plasma membrane and relay them to its axon. The abundant free and RER-associated polyribosomes appear as clumps of basophilic material collectively called Nissl bodies.
ØB. Dendrites: These extensions of the soma are specialized to increase the surface available for incoming signals.
ØC. Axon: Each neuron has one axon, a complex cell process that carries impulses away from the soma. An axon is divisible into several regions. The axon billock, the part of the soma leading into the axon, differs from the rest of the perikaryon in that it lacks Nissl bodies.
ØD. Classification of Neurons
properties of nervous system - 2
H. Blood-Brain Barrier: Nerve tissue of the CNS receives oxygen and nutrients from capillaries in the pia mater. These capillaries are relatively impermeable because (1) their endothelial cells lack fenestrations and are joined at their borders by tight junctions, and (2) they are partly surrounded by the cytoplasmic processes of neuroglia called astrocytes. These features contribute to a structural and functional barrier that protects CNS neurons from many extraneous influences and prevents certain antibiotics and chemotherapeutic agents from reaching the CNS.
