Glial Cells PDF

JABIR IBN HAYYAN MEDICAL Lecture By : Alaa AL Husainy UNIVERSITY COLLEGE OF MEDICINE Lecture:2 DEPARTMENT OF HUMAN ANATOMY Section of Histology...

JABIR IBN HAYYAN MEDICAL Lecture By : Alaa AL Husainy UNIVERSITY COLLEGE OF MEDICINE Lecture:2 DEPARTMENT OF HUMAN ANATOMY Section of Histology GLIAL CELLS & NEURONAL ACTIVITY Glial cells support neuronal survival and activities, and are ten times more abundant in the mammalian brain than the neurons. Like neurons, most glial cells develop from progenitor cells of the embryonic neural plate. In the CNS glial cells surround both the neuronal cell bodies, which are often larger than glial cells, and the processes of axons and dendrites occupying the spaces between neurons. Except around the larger blood vessels, the CNS has only a very small amount of connective tissue and collagen. Glial cells substitute for cells of connective tissue in some respects, supporting neurons and creating a microenvironment immediately around those cells that is optimal for neuronal activity. The fibrous intercellular network surrounding cells of the CNS may superficially resemble collagen with light microscopy, but it is actually the network of cellular processes emerging from neurons and glial cells. Such processes are collectively called the neuropil. There are six kinds of glial cells; 1) Oligodendrocytes Oligodendrocytes(Gr. oligos, small, few + dendron, tree + kytos, cell) produce the myelin sheaths around axons that provide the electrical insulation for neurons in the CNS. Oligodendrocytes extend sheet like processes that wrap around parts of several axons, producing myelin sheaths. These are the predominant glial cells in CNS white matter, which is white because of the lipid concentrated in the wrapped membrane sheaths. In Multiple Sclerosis (MS) the myelin sheaths surrounding axons are damaged by an autoimmune mechanism that interferes with the activity of the affected neurons and produces various neurologic problems. T lymphocytes and microglia, which phagocytose and degrade myelin debris, play major roles in progression of this disease. In MS destructive actions of these cells exceeds the capacity of oligodendrocytes to produce myelin and repair the myelin sheaths. 2) Astrocytes Astrocytes (Gr. astron, star + kytos) have a large number of radiating processes and are also unique to the CNS. Astrocytes are by far the most numerous glial cells of the CNS, as well as the most diverse structurally and functionally. Those with relatively few, long processes are called fibrous astrocytes and are typical in white matter; those with many shorter, branched processes are called protoplasmic astrocytes and predominate in the gray matter. Terminal branching of astrocytic processes is very extensive, allowing a single astrocyte to associate with over a million synaptic sites. The larger processes of all astrocytes are reinforced with bundles of intermediate filaments made of glial fibrillary acid protein (GFAP),which serves as a unique marker for astrocytes, the most common source of brain tumors, Most brain tumors are astrocytomas derived from those glial cells and characterized pathologically by their expression of GFAP. Functions associated with various astrocytes include the following: Extending processes with expanded perivascular feet that cover capillary endothelial cells and contribute to the Blood Brain Barrier BBB. Extending processes that associate with or cover synapses in the CNS, affecting the formation, function, and plasticity of these structures. Forming a barrier layer of expanded processes, called the Glial limiting membrane, lining the meninges at the external CNS surface. Finally, astrocytes communicate directly with one another via gap junctions, forming a very large cellular network for the coordinated regulation of their various activities in different brain regions. 3) Ependymal Cells Are columnar or cuboidal cells that line the ventricles of the brain and central canal of the spinal cord.In some CNS locations, the apical ends of ependymal cells have cilia, which facilitate the movement of cerebrospinal fluid (CSF), and long microvilli, which are likely involved in absorption. Ependymal cells are joined apically by junctional complexes similar to those of epithelial cells. However, unlike a true epithelium there is no basal lamina. Instead, the basal ends of ependymal cells are elongated and extend branching processes into the adjacent neuropil. 4)Microglia Less numerous than oligodendrocytes or astrocytes but nearly as common as neurons, microglia, macrophage of CNS are small cells with short irregular processes evenly distributed throughout gray and white matter. Unlike other glial cells, microglia migrate through the neuropil, scanning the tissue for damaged cells and invading microorganisms. They secrete a number of immunoregulatory cytokines and constitute the major mechanism of immune defense in the CNS. Microglia do not originate from neural progenitor cells like other glia, but from circulating blood monocytes, belonging to the same family as macrophages and other antigen-presenting cells. 5) Schwann Cells Schwann cells, sometimes called neurolemmocytes, are found only in the PNS and differentiate from precursors in the neural crest. Schwann cells have trophic interactions with axons and importantly allow for their myelination, like the oligodendrocytes of the CNS. As discussed with peripheral nerves, one Schwann cell forms myelin around a segment of one axon, in contrast to the ability of oligodendrocytes to branch and ensheath parts of more than one axon. 6) Satellite Cells of Ganglia Also derived from the embryonic neural crest, small satellite cells form an intimate covering layer over the large neuronal cell bodies in the ganglia of the PNS. Satellite cells exert a trophic or supportive effect on these neurons, insulating, nourishing, and regulating their microenvironments. CENTRAL NERVOUS SYSTEM The major regions of the central nervous system (CNS) are the cerebrum, cerebellum, and spinal cord. The CNS is covered by three connective tissue layers, the meninges, but contains very little collagen or fibrous tissue throughout its substance, making it relatively soft and easily damaged by injuries affecting its protective cranium or vertebral bones. The entire CNS displays organized areas of white matter and gray matter, differences caused by the differential distribution of myelin. The main components of white matter are myelinated axons, often grouped together as tracts, and the myelin-producing oligodendrocytes. White matter contains very few neuronal cell bodies, but astrocytes and microglia are present. Gray matter contains abundant neuronal cell bodies, dendrites, the initial unmyelinated portions of axons, astrocytes, and microglial cells. Gray matter is where most synapses occur, and it occupies the thick surface or cortex of both the cerebrum and the cerebellum; most white matter is found in deeper regions. Deep regions of the CNS also have darker aggregates called nuclei consisting of large numbers of neuronal cell bodies and surrounded by white matter. Neuroscientists recognize six layers of neurons with different sizes and shapes in the cerebral cortex. The most conspicuous of these cells are the efferent pyramidal neurons that come in many sizes. Neurons of the cerebral cortex function in the integration of sensory information and the initiation of voluntary motor responses. The cerebellar cortex, which coordinates muscular activity throughout the body, also has a layered organization: 1) Outer molecular layer, 2)Central layer of very large neurons called Purkinje cells. 3)Inner granule layer. The Purkinje cell bodies are conspicuous even in H&E-stained material, and their dendrites extend throughout the molecular layer as a branching basket of nerve fibers. The granule layer is formed by very small neurons, which are packed together densely, in contrast to the neuronal cell bodies in the molecular layer which are sparse. In cross sections of the spinal cord White matter is peripheral and gray matter is internal and has the general shape of the letter H. In the center is an opening, the central canal, which develops from the lumen of the embryonic neural tube. The canal is continuous with the ventricles of the brain, contains CSF, and is lined by ependymal cells. The gray matter forms the anterior horns, which contain motor neurons whose axons make up the ventral roots of spinal nerves, and the posterior horns, which receive sensory fibers from neurons in the spinal (dorsal root) ganglia. Spinal cord neurons are large and multipolar, especially the motor neurons in the anterior horns.