Neurocytology Lecture Objectives PDF

Neurocytology Page 1 of 20 Dr. Jean Peduzzi Nelson Neurocytology Lecture Objectives I. Describe the neuronal features in the CNS and PNS A. Contrast the differences between the PNS and CNS....

Neurocytology Page 1 of 20 Dr. Jean Peduzzi Nelson Neurocytology Lecture Objectives I. Describe the neuronal features in the CNS and PNS A. Contrast the differences between the PNS and CNS. B. Describe the Plasma membrane of Neurons. C. Recall the features of the Cell body of Neurons. D. Recognize the morphology of Dendrites of Neurons E. Describe characteristics of Axons and Myelin F. Summarize the Presynaptic terminals of Neurons. G. Classify Local circuit neurons (interneurons) II. Differentiate the characteristic of neuroglia in CNS and PNS. A. Summarize the general characteristics of Glia. B. List the morphology and function of Astrocytes. C. Describe the morphology and function of Oligodendrocytes. D. Recall the morphology and function of Microglia. E. Describe the morphology and function of Ependymal and Tanycytic. F. Contrast the differences between Oligodendrocytes and Schwann cells in myelination.  Neurocytology Page 2 of 20 Dr. Jean Peduzzi Nelson I. Describe the features of neurons in the central and peripheral nervous systems. A. Contrast the differences between the PNS and CNS. Neurons: List the function and morphology of general features of neurons: Long-lived, electrically excitable cells specialized for the reception, integration, and transmission of coded information (often over long distances) to specific cellular targets. Neuron morphology 1. A cell body (perikaryon or soma), containing the nucleus (Figure 1), 2. A variable number of processes originating from the cell body; these being termed: a. Dendrites – processes specialized for receiving contacts, and b. Usually an axon (Figure 1) – the axon is a process specialized for conducting information (nerve impulses). Axon usually originates from the cell body, but on occasion may issue from dendrites. Fig. 2 Fig. 1 Based on the number of processes arising from the cell body, neurons may be classified as bipolar (e.g., retinal bipolar cells), unipolar (pseudo- unipolar (e.g., dorsal root ganglion cells) or multipolar (e.g., cortical pyramidal cells, spinal motor neurons). See Figure 2. Differences between the peripheral nervous system (PNS) and the central nervous system (CNS).Nervous system composition 1. In PNS, there is large amount of connective tissue which forms the sheaths of nerves and ganglia. Nerves are bundles of axons. 2. In CNS, tissue is Fig. 3 Unstained tissue composed of gray and white matter. a. Gray matter is rich in neuronal cell bodies, dendrites, and supporting cells (glia). Clusters of cell bodies are called nuclei. Neurocytology Page 3 of 20 Dr. Jean Peduzzi Nelson b. White matter consists of bundles of axons (both myelinated & unmyelinated) & supporting cells. Organized bundles of axons in the white matter sharing same origin, destination and function are called tracts, columns, fasciculi, peduncles, lemniscus. In unstained brain as in Fig. 3, white matter appears lighter in color. Many sections in course are Weigert stained where white matter is stained black. B. Describe the Plasma Membrane of Neurons The phospholipid bilayer contains a large variety of integral membrane proteins to act as receptors for neurotransmitters and growth factors, ion channels (K+, Na+, Ca++, Cl-, etc.), pumps and adhesion molecules (NCAM, cadherins, fasciculation molecules). Many of these proteins must be replaced on a regular basis. Aggregations of integral membrane proteins form the following membrane specializations: 1. Synapse (Figure 4). a. Characterized by distinct pre- and postsynaptic membrane thickenings. b. Crowding of synaptic vesicles against the presynaptic thickening. 2. Punctum adherens (Figure 5) (adhesion dot) – analogous to the zonula adherens of epithelia. 3. Gap junction (Figure 5) – acts as an electrical synapse for ultra-rapid communication (no delay). Rare in mammalian brains except in development. Fig. 4 Fig. 5 C. Recall the Features of the Cell Body (see Figures 6A&B) 1. Cell bodies vary in diameter from 4m Fig. 6B (e.g., cerebellar granule cells) to over 50m Electron (cortical pyramidal cells). microscope 2. Nucleus (Fig.6 ‘A’ or Fig. 6B ‘N’) Usually large and pale – most chromatin is dispersed (active transcription-euchromatin). Large (1-3 m), centrally placed nucleolus, (function is rRNA synthesis). Fig. 6A ‘B’ Fig.6A Light microscope Neurocytology Page 4 of 20 Dr. Jean Peduzzi Nelson Figure 6 (previous page) Left image. The nerve cell body (soma) of a motor neuron that is Nissl stained. A, nucleus; B, nucleolus; C, axon hillock; large arrowheads:1st order dendrites; small arrowheads: axon initial segment; *, artefactual spaces created by fixation. The nucleus is usually centrally located in the cell body and shows little heterochromatin. The nucleolus of most neurons is very prominent (1-3 microns in size). The Nissl granules in this cell type are prominent. Right Image is an electron micrograph (EM) showing profiles of two cortical pyramidal neurons. D: dendrites. Arrows point at plasma membrane of cell body. N, nucleus. 3. Cell body (soma or perikaryon) contains cytoplasm surrounding the nucleus and bounded by the plasma membrane of cell body. Contains numerous organelles such as: a. Nissl bodies: Aggregates of rough endoplasmic reticulum (rER) for synthesis of enormous amounts of protein destined both for the plasma membrane and cytosol of the same neuron. b. Smooth endoplasmic reticulum (sER) is found throughout cell body, dendrites, and axon. The sER functions mainly to sequester Ca++. Golgi functions in lysosome and glycoprotein production, as well as packaging membrane-bound vesicles for constitutive (e.g., receptor, channel, pumps, adhesion) or regulated release (e.g., synaptic vesicle) proteins. Figure 7 EM profiles (details) of the cell body of two neurons (upper and lower left) at higher magnification. N, nucleus; nu, nucleolus; rER, rough endoplasmic reticulum; G, Golgi apparatus. rER. Composed of membranous cisternae arranged like stacks of pancakes with their associated ribosomes, polyribosomes, and free ribosomes. Polyribosomes are sites for the synthesis of proteins to be inserted into the neuron’s cell membrane while free ribosomes synthesize proteins mostly for its cytosol. Aggregations of rER form the Nissl granules. The Golgi apparatus serves to process post-translational proteins and to sort out different types of proteins to be delivered to the dendrites or the axon. c. Mitochondria: Function in the production of ATP. As neurons do not store energy (e.g., in the form of glycogen), ATP is quickly depleted in states of O2 deprivation such as during ischemia.  Neurocytology Page 5 of 20 Dr. Jean Peduzzi Nelson d. Lysosomes. Function in autophagocytosis of organelles and membrane components. e. Microtubules (25nm): Found throughout cell body, dendrites, and axon. Function in cytoskeletal support and transport of integral proteins via microtubule associated proteins (MAPs) such as tau protein. f. Neurofilaments (10nm): Unique to neurons. Function in cytoskeletal rigidity by maintaining the shape of dendrites and axons. g. Lipofuscin: An inclusion probably derived from the lysosome which tends to accumulate in aging neurons. h. Melanin: Another inclusion found only in certain neuron classes such as in the catecholaminergic neurons (dopamine, NE, epinephrine) of the substantia nigra and locus coeruleus (by-product of catecholamine degradation) Clinical Correlation Tauopathy: aggregation of tau protein into neurofibrillary or gliofibrillary tangles found in Alzheimer’s disease and chronic traumatic encephalopathy (CTE, e.g., in some boxers) D. Recognize the Morphology of Dendrites (Figures 8 & 9) 1. Taper and branch at acute angles to increase receptive membranes of the neuron. 2. Membranes contain a biochemical mosaic of transmitter receptors. Both excitatory and inhibitory contacts occur on the dendritic shafts. 3. Contain virtually all the organelles found in the cell body. 4. Microtubules and neurofilaments extend longitudinally (parallel to the dendritic long axis). 5. Appendages (called spines, seen better in Figure 9) are numerous in some classes of forebrain (i.e., cerebral cortex, striatum) neurons and cerebellum (e.g., Purkinje cell). Function to isolate certain synapses from those found in other regions of the neuron. Their number and integrity (plasticity) fluctuate with changes in their input from perturbations in the outer and inner environments. In addition, neural activity at spines increases their size and conduction efficiency which may underlie learning and memory formation. Figure 8 basilar dendrites: green arrowhead apical dendrite: black arrowhead dendritic branches: red arrowheads axon originating from cell body: large blue arrowhead axon collaterals: small blue arrowheads Neurocytology Page 6 of 20 Dr. Jean Peduzzi Nelson sp Figure 9 (above) Left. The large dendritic tree of the Purkinje Cell extends into the molecular layer of the cerebellum while the single axon (arrowhead) projects through the granular layer into the underlying white matter and finally terminates in the deep cerebellar nuclei (positioned deeply in the white matter of the cerebellum). Golgi stain. Upper right. The 3rd and 4th order Purkinje Cell dendrites are covered with dendritic spines (approx. 200,000 spines per cell) and each spine serves as the postsynaptic process for a single presynaptic axon of a parallel fiber (axon of the granule cells). The shape of Purkinje cell dendritic tree is similar to a flat disk (its short axis in parallel and long axis at right angle to the long axis of cerebellar folia). Lower right. EM of a Purkinje Cell small dendrite (Den) giving rise to a dendritic spine (sp). The tip of spine is postsynaptic to an axon terminal (At1). The structure of spine is susceptible to changes in the external and internal environments, such changes presumably underlying cognitive processes such as memory and learning. Arrows, postsynaptic membrane densities. E. Describe the characteristics of Axon and Myelin 1. Axons contain the initiating (hillock/initial segment) and the conducting (axon beyond initial segment) regions of the neuron. Hillock contains no Nissl bodies. Membrane of initial segment has voltage-gated Na+ channels and the lowest threshold to initiate the action potential. 2. Most neurons have a single axon, although some neurons (e.g., local circuit interneurons) have multiple axons and, a few have no axon (e.g., retinal amacrine cell, granule olfactory neuron). 3. Branches (at right or obtuse angles to parent branch) are termed collaterals.  Neurocytology Page 7 of 20 Dr. Jean Peduzzi Nelson 4. Unmyelinated axons are organized into fascicles by processes of astrocytes. Fig. 10. This is the axon hillock- initial segment portion of the axon of an aspiny cell in basal ganglia (Golgi stain). Arrowhead points at the axon initial segment. * marks the end of the Golgi stain (impregnation) and probably the beginning of myelination. 5. Saltatory Conduction: Some axons beyond initial segment become myelinated (myelin is an extension of the membranes of the oligodendrocyte and thus extrinsic to the neuron). Periodic breaks in the myelin along the axon are called nodes of Ranvier. Conduction of the nerve impulse occurs from node to node (“saltatory”) and its speed along myelinated axons is proportional to the thickness of the nerve fiber (axon+myelin). Khan academy (site below) gives an excellent explanation of how saltatory conduction occurs. https://www.khanacademy.org/science/biology/human-biology/neuron-nervous- system/v/saltatory-conduction-neurons 6. Transport along axons. Microtubules are major conduits for trafficking proteins and vesicles away from, and toward, the perinuclear microtubule-organizing center. Transport motors are macromolecular ATP-hydrolyzing complexes, which bind to specific types of cargo or more general cargo and “walk” along microtubules away from the nucleus (kinesin mediated anterograde transport) or toward the nucleus (dynein mediated retrograde transport). Anterograde transport Cell body to synapse, kinesin Retrograde transport Synapse to cell body. Dynein Very short video https://www.facebook.com/therealcrediblehulk/videos/this-kinesin-protein-walks-a- vesicle-along-a-microtubule-via-a-hand-over-hand-me/2557615100955890/ Fig. 11 on right Neurocytology Page 8 of 20 Dr. Jean Peduzzi Nelson Peripheral Nerves Figure 12. Myelinated axons in a peripheral nerve. Osmic acid stain. A, Connective tissue sheath; B, myelin surrounding two unstained axons. The myelin sheath is entirely extraneous to the axon since it is produced and maintained by the Schwann cell. Myelin insulates the axon membrane from excitation, thus allowing current flow to pass only from node to node of Ranvier (saltatory conduction). The thicker the nerve fiber (axon+myelin sheath) the faster the conduction of the impulse along the fiber. The cross-sectional diameters of these fibers range from 0.7 to 25 microns. Figure 13. EM of a peripheral nerve fiber. Note that the axon contains microtubules (mt), neurofilaments (nf), sacs of sER. The innermost membrane to the myelin sheath is the axolemma (arrow). The membranes making up the myelin sheath belong to the Schwann cell. Note the outer cytoplasmic pockets of Schwann cell on the upper left and lower right of the myelin sheath. Also note the basal lamina (bl) surrounding the Schwann cell. It plays a role in regeneration and is used to identify peripheral axons. Figure 14. PNS. EM of myelinated fibers from an axon that has a cross sectional diameter greater than 0.6 microns (such as Ax1) it will be myelinated by contiguous Schwann cells. However, axons with diameters of 0.6 and less (such as Ax2, Ax3) will not be myelinated but rather will be ensheathed in individual cytoplasmic troughs of the Schwann cell (an example of such cell is seen in the lower 1/3 of the figure). A single Schwann cell may provide as many as 12-20 troughs containing unmyelinated axons. Axons of this type are categorized as C fibers (autonomic and pain fibers) by neurophysiologists and generally have slow conduction velocities. In the CNS, unmyelinated axons are ensheathed by astrocytes rather than by oligodendrocytes, with the membranes of adjacent axons apposing each other, thus forming large bundles of these fibers. Neurocytology Page 9 of 20 Dr. Jean Peduzzi Nelson F. Summarize the Presynaptic Terminals (transmitting region, see Figure 15) 1.The distal most part of the axon forms a highly branch structure called the axonal tree (telodendria or arborization). The branches of this tree contain small varicosities (swellings) that contain mitochondria and synaptic vesicles. 2.The varicosities contact dendrites, dendritic spines, cell bodies and/or axon hillock/initial segments of other neurons. These contacts are called synapses. 3.Using EM images, the synapse is seen to consist of: a. Presynaptic membrane density (part of the axon varicosity membrane). b. Synaptic vesicles crowded against the presynaptic membrane. Vesicles contain synaptic transmitter and/or modulatory peptides. a. Postsynaptic membrane density (part of the membrane of the target cell). b. A synaptic cleft (intervening space) between pre- and postsynaptic membranes that fluctuates between 20-30 nm in width. c. Usually (but not always) excitatory synapses (e.g., glutamatergic) have rounded synaptic vesicles and a wide (close to 30 nm) space, whereas inhibitory synapses (e.g., GABAergic) have pleomorphic (many shapes) vesicles and a narrow (close to 20 nm) cleft. d. Types of synaptic contacts (Figure 6) include: axosomatic, axodendritic, axospinous, axoaxonic, dendrodendritic, somatodendritic (rare) and somatosomatic (rare). Figure 15 Upper left Axon terminals. Climbing fiber, cerebellar cortex (Golgi stain). Large arrowhead: preterminal branch of climbing fiber; small arrowheads: presynaptic varicosities (boutons in passing); *: terminal varicosity. The long axes of the varicosities measure 1-2 microns. Varicosities contain synaptic vesicles and form synaptic contact with the dendrites of the Purkinje cell. Action potential arriving at the varicosities depolarizes their membranes causing release of transmitter from synaptic vesicles. Red arrows represent transitions from light to EM Lower left EM of axodendritic synapses A cross-sectioned profile of a dendrite (D) forms synaptic relationships with two axon terminals, one (at) forming an asymmetric (excitatory) contact and the lower one (at) a symmetrical (inhibitory) contact. The synaptic vesicles of the first are mostly round, those of the second pleomorphic (many shapes) but mostly flattened. Neurocytology Page 10 of 20 Dr. Jean Peduzzi Nelson Figure 16. Ultrastructural characteristics of excitatory (upper image) and inhibitory (lower image) synapses. Upper figure. A: presynaptic profiles: Asterisk: synaptic vesicles B: postsynaptic profiles; red and green arrowheads: synaptic membrane thickenings denoting specialized areas of contact. Function As an action potential depolarizes the axon terminal, voltage-gated calcium channels in the presynaptic density open causing calcium to rush into the presynaptic terminal. This then causes some vesicles already “docked” to fuse with the presynaptic membrane and release neurotransmitter into the cleft (exocytosis). Selective ion (calcium, potassium, sodium, chloride) channel activation determines excitation or inhibition. Lower figure. the two green arrows at the center point at an adhesion dot (punctum adherens). Figure 17 Types of Synaptic Articulations in the CNS dendrite Soma (cell body) While axodendritic, axospinous and axosomatic synapses (1, 2, 3, 5 Figures 17) are very common in the CNS, axoaxonic (4, Figure 17) synapses are less common. Dendrodendritic and somatosomatic synapses are rare. Dendrodendritic and somatodendritic synapses (usually inhibitory, not shown) are usually formed between local circuit neurons and other neurons. 1. Axosomatic synapse (can be both excitatory and 1.,3.inhibitory). Axodendritic synapse (can be excitatory or inhibitory) 3. Axodendritic synapse (both excitatory & inhibitory) 2. 2. Axospinous Axospinous synapse (usually synapse (usually excitatory excitatory). 4. Axoaxonic 4. Axoaxonic synapse (usually synapse inhibitory). (usually inhibitory) 5. Axosomatic Neurocytology Page 11 of 20 Dr. Jean Peduzzi Nelson G. Classify the Interneurons (Local Circuit Neurons, LCN) 1. A neuron may have an axon that either projects out of the nucleus of origin (“projecting” axon, larger neuron) or remains Figure 18 within the nucleus of origin (“intrinsic” smaller neuron). The later intrinsic neurons are also known as interneurons or local circuit neurons (Figure 18 on left). 2. While some local circuit neurons have a well localized LCN intrinsic axon (e.g., Golgi cell or basket cell in the cerebellum), others have dendrites with axonal morphology. Such “axon-like” segments of the dendrite (or the cell body) contain aggregations of synaptic vesicles and sometimes form presynaptic contacts with a postsynaptic cell (dendrodendritic, somatodendritic and dendrosomatic synapses). Examples of this type of local circuit neurons are found in the thalamus, olfactory bulb, and superior colliculus. 3. Most of the above local circuit interneuron synapses have been found to be inhibitory. II. General characteristic of neuroglia and types of neuroglia in CNS and PNS Clinical correlation: Most brain tumors are of glial origin, medulloblastoma derived from stem cells (primitive neuroectodermal tumor, PNET) in the brain and more common in children. Cancer spreads through the CSF and frequently metastasize to other CNS sites. A. Summarize the Types of Neuroglia and Their Origin Glia (derived from Greek word glue) form an “interstitium” between neurons. May outnumber neurons by 1:10 and comprise 50% of brain volume. Retain ability to proliferate throughout life, Interact with neurons in their growth and survival. Selective markers reveal heterogeneity of form and involvement in disease processes. 1) CNS Macroglia (all derived from neuroectoderm). a. Astrocyte (Fig.19)- fibrous (Fig.20A) or protoplasmic (Fig.20B) b. Oligodendrocyte (Fig. 19 & 20C) c. Ependyma/tanycyte (Fig.19, 20E & F) 2) CNS Microglia (derived from blood vessels or recent evidence from yolk sac ). (Fig.19 & 20D) 3) PNS: Schwann cell (derived from neural crest). 4). PNS: Satellite cell (derived from neural crest).  Neurocytology Page 12 of 20 Dr. Jean Peduzzi Nelson Fig. 19. Diagram illustrating ependymal cells (lines ventricle and central canal), astrocytes (covers surface of cell body and node of Ranvier), oligodendrocytes (form several segments of myelin on axons), microglia (surveillance of brain and spinal cord for infection) B. Describe the Astrocytes (As): General Morphology Fig. 20. Fibrous (A) and protoplasmic (B) astrocytes (As) exist both in white and gray matters, but protoplasmic predominate in gray matter and fibrous are more numerous in white matter. Both types have numerous branches, but the branches of fibrous are long and smooth, giving the cell a “star” appearance, while those of protoplasmic astrocytes are short and irregular giving the cells a “fuzzy” appearance. Processes of both types (Fig. 20A&20B) may terminate as cone-shaped expansions called “end feet”. Nearly continuous sheaths of glial end feet under the ventricular lining ependyma (i.e., inner glial limitans), the pia mater (i.e., outer glial limitans) and around the perimeter of blood vessels (i.e., perivascular glial sheath) serve to isolate the neuronal compartment from potentially harmful agents in the external and internal environments. Neurocytology Page 13 of 20 Dr. Jean Peduzzi Nelson Figure 21. Fibrous Astrocyte (low power, left); Astrocytic foot process on blood vessel (right) Figure 22. Protoplasmic astrocytes have large number of short processes. Figure 23 Astrocyte (arrow) & oligos (unlabeled, Electron micrographs (left, cell body: Dark rounded structures) in Nissl stains. right, high power of astrocyte process right) At light microscopic levels (e.g., Nissl stain, Figure 22, arrow) the nucleus is either round or indented. Dispersed intranuclear chromatin tends to form small clumps on the inner aspect of the nuclear envelop. At EM level both nucleus and cytoplasm have an electron-lucent matrix (Figure 23). Nuclear chromatin in again visualized primarily as small clumps on the inner aspect of the nuclear envelope (Fig. 23, left). The most prominent feature of the cytoplasm of the body and processes (Figure 23, right *) are intermediate filaments that contain glial fibrillary acidic protein (GFAP). Neurocytology Page 14 of 20 Dr. Jean Peduzzi Nelson Astrocyte Functions Support Radial glia in developing brain provide both support and a migration path for newly proliferated neurons (Figure 23). Structural integrity is provided by a complex cytoskeleton composed of macrofilaments (e.g., microtubules), intermediate filaments (e.g., vimentin, glial filaments) and microfilaments (e.g., actin). Figure 24 Radial Glia Compartmentalization of neuronal functional units Processes isolate (i.e., electrical events) synapses and synaptic complexes (e.g., synaptic glomerulus). Buffer potassium (acting as a potassium sink) from extracellular space immediately after depolarization and helps restore the transmembrane potential. BBB formation and maintenance (Figure 25) Tight junctions between adjacent endothelial cells of brain microvessels function as an effective blood brain barrier for macromolecules in blood. Adjacent glial endfeet are coupled by gap junctions. In addition, glia together with endothelium form and maintain their intervening basal lamina and possibly the endothelial tight junctions. Hatched area: astroglia processes End=endothelium; zo=zonula occludens; bl =basal lamina, Pc=pericyte; D=dendritic; S=synaptic terminals Neurocytology Page 15 of 20 Dr. Jean Peduzzi Nelson Repair processes by astrocytes after injury or disease As proliferate (i.e., hyperplasia) and/or increase in size (i.e., hypertrophy) in the formation of “scar” following injury or disease. This is a mechanism to “wall off” the non- injured neuronal compartment. Reacting astrocytes participate in limited “neuronophagia” (phagocytosis of neuronal parts (e.g., axon terminals and dendritic spines). This glial function is part of synaptic remodeling (or turnover) which is associated to injury states or age- related synaptic plasticity. Secretion of growth factors Insulin, IGF (insulin-like growth factor), NGF (nerve growth factor), BDNF (brain derived nerve factor), NT1-3 (neurotrophin 1,2,3) are present in glia and play a role in maintaining neurons alive. Different factors contribute selectively to viability of specific neuron phenotypes. Lack of the above factors has been associated to a variety of disease processes. Regulation of ionic milieu by astrocytes Potassium buffering (potassium “sink”) capacity mentioned above. Membrane water channels (aquaporins) in both endothelium and astroglia regulate water passage across the BBB. Dysregulation of the channels may result in cerebral edema (i.e., brain swelling). Excessive accumulation of water in the extracellular compartment (vasogenic edema) and in the neural compartment (cytotoxic edema) leads to increased intracranial pressure which is deleterious to neurons. Clinical Correlation: Astrocytomas (astroglia-derived tumors) are of different types (WHO grade I-IV) and malignancy, glioblastoma multiforme (Grade IV) being significantly malignant. C. Describe the morphology and function of Oligodendrocytes (Fig. 26) Most numerous cells among the glia. Different forms exist in both gray and white matter. From the Greek, oligo (few) dendro (branches), cyte (cell), the principal function of this cell is to form and maintain central myelin or sheath of axons. Therefore, the distal most portion of an oligodendrocyte process is continuous with the internodal flap of myelin. A single oligodendrocyte may form as many as 50 internodal flaps of myelin (this contrast the Schwann cell which forms only an internodal flap of myelin in PNS). At light microscopic levels (Nissl staining) the nucleus is small, round and contains dense clumps of heterochromatin (arrow). Oligos may be positioned adjacent to neurons (i.e., perineuronal) or in rows, along bundles of nerve fibers (i.e., interfascicular) Figures 26 Light microscopy Oligodendrocytes (arrows) Neurocytology Page 16 of 20 Dr. Jean Peduzzi Nelson Figure 27. Dense clumps of chromatin are seen in nucleus and the cytoplasm is “electron dense” due to the presence of numerous organelles as well as a proteinaceous matrix between the organelle While numerous microtubules are present no glial filaments can be found. D. Recall the morphology and function of Microglia Comprise approximately 5-10% of all neuroglia. Found both in gray and white matter, particularly in association to blood vessels. At light microscopic level (silver stains, Figure 28), the small cell body is elongated (rod shaped) with a few, short irregular process covered with thorn-like (“barb-wire-like”) appendages. Figure 28 Resting Microglia In inactive (resting) state the cell body is stationary but processes move to provide survey the area. When activated (in injury or disease), undergo rapid proliferation and migration towards pathologic foci where their processes thicken. Strong phagocytic property with engulfing and ingesting neuronal debris resulting from injury or disease. Microglia are part of innate immunity in the CNS. Some of these functions are shared by migrating monocytes from blood. Phagocytes engorged with neuronal debris are called “gitter cells” by pathologists. Neurocytology Page 17 of 20 Dr. Jean Peduzzi Nelson E. Describe the morphology and function of Ependymal and Tanycytes Ependyma: Form a continuous lining of the ventricular cavities and of the central canal of the spinal cord. Their shape varies from simple cuboidal (Figure 29A) to columnar (Figure 19E) depending on brain region. At their apical surface (facing the lumen of the ventricles) the cells possess numerous microvilli and motile cilia (Figure 29A). Membranes of adjacent cells form both gap and adhesion-type junctions (Figure 29B. Note no occluding or tight junctions in photo). Produce a small amount of CSF in ventricles. Clinical correlation: Ependymoma are ependyma-derived tumors originating from the ependyma lining the ventricles or possibly radial glia but later invading the underlining brain parenchyma. Figure 29. Ventricular Ependyma in Electron Micrographs Choroidal Ependyma Form part of the choroid plexus. Shape of choroidal ependyma is mostly cuboidal (Figure 30). Tight (occluding) junctions are found at apex of adjacent cells (arrow, Figure 30). This results in a CSF-Brain barrier. Blood vessels of choroid plexus are fenestrated, thus “leaky” to water and ions from blood. Detection of ATPase-dependent pumps have been detected in both apical and basilar membranes of choroidal ependyma, these pumps being critical in the formation and final composition of CSF. Produces most of the CSF in ventricles and subarachnoid space. Figure 30 Choroidal Ependyma (EM) Neurocytology Page 18 of 20 Dr. Jean Peduzzi Nelson Tanycytes (Figure 19F) Specialized ependyma with long processes extending from the base of the cell body to blood vessels in the adjacent neuropil or to the pial surface. Found in small patches in circumventricular organs (e.g., median eminence, area postrema, pituitary gland, etc.) where there is no BBB. May absorb and transport releasing hormones from CSF to circulation F. Contrast the differences between Oligodendrocytes and Schwann cells in myelination Schwann cells are the myelin producing cell in the PNS. Node covered with basal lamina. Schwann cells maintain only one internodal myelin segment in mature myelinated fibers. In addition to forming myelin, it also forms the sheath for unmyelinated axons. A single Schwann cell may form the sheath for as many as 20 unmyelinated axons (usually C fibers for pain conduction and autonomic axons). In developing PNS axons, Schwann cells first align along an axon. Elongation and spiraling of the membranes around the axon as well as compaction of the cytoplasm ultimately results in opposing membranes with a distinct periodicity. Oligodendrocytes myelinate several internodal segments. Node is covered with astrocyte process. Figure 31 A. CNS node 1. Astrocyte 2. Oligodendrocyte 3. Axon 4. Node of Ranvier 5. Tight junction 6. Compacted myelin A. Peripheral node 1. Basal lamina 2. Schwann cell 3. Axon 4. Node of Ranvier 5. Tight junction 6. Compacted myelin Neurocytology Page 19 of 20 Dr. Jean Peduzzi Nelson Figure 32. Proteins in Myelin: There are overall similarities between CNS and PNS, but the proteins in these membranes are largely distinct. Major structural proteins in CNS: proteolipid protein (PLP) and myelin basic proteins (MBP). PNS: Protein zero (P0), PMP22 and MBP. Central Nervous System Peripheral Nervous System Figure 33 EM Peripheral node of Ranvier (below left) Schwann cell and myelinated axons (below right) Figure 34 EM Central Myelin Neurocytology Page 20 of 20 Dr. Jean Peduzzi Nelson Myelin in CNS and peripheral myelin Myelin results from compaction of the Oligodendrocytes membranes (CNS) or Schwann cell membranes (PNS). The axon insulating properties of myelin allows for “saltatory” impulse conduction along nerve fibers. In both CNS and PNS, spaces between internodal segments of myelin are called nodes of Ranvier. However, in the CNS, nodes are covered by astrocytic processes, while in PNS nodes are covered both by cytoplasmic extensions of the adjacent Schwann cells (SC) and by a basal lamina outside the Schwann cell membrane. This last feature plays a role in the regeneration of PNS axons following nerve injury. CNS and PNS myelin have different molecular compositions and immunological properties Clinical correlation: Demyelination is involved in multiple sclerosis where the myelin sheath is diseased from an autoimmune response to one of the proteins in central myelin. It is more prevalent in females than in males, 3:1 ratio). Formation of sclerotic plaques occur randomly throughout the CNS, particularly in the heavily myelinated tracts of the white matter. Anti-inflammatory drugs, monoclonal antibodies and interferon beta are common therapies.