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Folia Neuropathol. Suppl. B/2004, pp. 1–9 Copyright © 2004 Via Medica ISSN 1641–4640 REVIEW ARTICLE Ultrastructure of the central nervous system: the basics Michał Karasek, Jacek Świętosławski, Anna Zielińska Department of Neuroendocrinology, Chair of Endocrinology, Medical University of Łódź, Poland Since the articles in this supplement describe ultrastructural changes in diseases of the nervous system, for better understanding of these papers dealing with pathology of the this system, basic elements of the ultrastructure of the central nervous system are presented in this survey. Description of the fine structure of cells and fibres of the central nervous system presented below is based on classical textbooks as well as on the authors’ personal experience. The details of ultrastructure of nerve cells (their nucleus and cytoplasm, including cell organelles: abundant granular endoplasmic reticulum, prominent Golgi apparatus, mitochondria, lysosomes, as well as other cell components including: neurofilaments, lipid droplets, multivesicular bodies, lipid droplets, lipofuscin granules, and in some cells melanin granules, and also cell processes), the interneuronal chemical synapses (specialized site of interneuronal communication) as well as all types of neuroglial cells (a class of non-neuronal supporting cells): astrocytes (fibrous and protoplasmic), oligodendrocytes, and microglia neuroglial cells are described. key words: central nervous system, ultrastructure, nerve cell, neurolial cell INTR ODUCTION INTRODUCTION THE NER VE CELLS NERVE The articles in this supplement describe the ultrastructural changes in diseases of the nervous system. The basic elements of the ultrastructure of the central nervous system are presented in this paper so that the articles on its pathology may be better understood. The description of the fine structure of cells and fibres of the central nervous system presented below is based on classical textbooks [1–5] as well as on the authors’ personal experience. All electron micrographs are the authors and have not previously been published. Nerve cells are generally polygonal in shape with one (in the case of unipolar neurons), two (bipolar neurons) or, most frequently, several (multipolar neurons) processes emerging from the cell body, which may vary in diameter from 5 to 135 mm. A single large rounded nucleus with fine chromatin granules widely dispersed and a single prominent nucleolus is usually centrally situated within the perikaryon (Fig. 1–4). The cytoplasm is characterised by an abundance of various cell organelles and filaments located more or less concentrically around the nucleus. A very characteristic feature of nerve cells is an abundance of granular endoplasmic reticulum, often arranged in aggregations containing numerous paralleloriented cisternae (Fig. 5, 6) corresponding to Nissl bodies observed in a light microscope. A prominent Golgi apparatus is present in all nerve cells in the form of Address for correspondence: Prof. Dr. Michał Karasek Department of Neuroendocrinology Chair of Endocrinology, Medical University of Łódź ul. Czechosłowacka 8/10, 92–216 Łódź, Poland tel./fax: (+48 42) 675 76 13 e-mail: [email protected] www.fn.viamedica.pl 1 Folia Neuropathol., Supplement B/2004 Figure 1. Low-power micrograph of the brain; N — nerve cell, O — oligodendrocyte; C — capilary; ¥ 11,500. 2 www.fn.viamedica.pl Michał Karasek et al., Ultrastructure of the central nervous system: the basics Figure 2. Nerve cell with pale nucleus and a variety of cell organelles; ¥ 11,500. Figure 3. Fragment of nerve cell with a process emerging from the cell body (asterisk); ¥ 29,000. www.fn.viamedica.pl 3 Folia Neuropathol., Supplement B/2004 Figure 4. The nucleus of the nerve cell with a prominent nucleolus; ¥ 10,000. Figure 6. Parallel-oriented cisternae of granular endoplasmic reticulum (GER) (corresponding to Nissl body) and lysosomes (Ly) in the nerve cell cytoplasm; ¥ 19,000. 4 Figure 5. Fragment of the cytoplasm of a nerve cell with Golgi apparatus (GA) and parallel-oriented cisternae of granular endoplasmic reticulum (GER); ¥ 17,000. multiple clusters composed of flattened sacs with frequently dilated ends surrounded by numerous small vesicles (Fig. 5, 7). Numerous rod-shaped mitochondria (Fig. 7), 0.1–1.0 mm in diameter, are scattered in the cell body and processes. Lysosomes (Fig. 6), usually 0.3–0.5 mm in diameter, although some may reach up to 2 mm, are numerous in all nerve cells. Neurofilaments, 7–10 nm in diameter and indefinitely long, are a typical component of all nerve cells, running through the perikaryon and penetrating the cell processes. These are often arranged in bundles (corresponding to neurofibrils in a light microscope). It is believed that neurofilaments belong to the cytoskeleton and are involved in axoplasmic and dendritic flow. Interspersed among the neurofilaments in the perikaryon as well as in the processes are neurotubules of 20–30 nm in diameter and many micrometres in length. Other cell components include lipid droplets, multivesicular bodies, lipofuscin granules and, in some cells, melanin granules. There are two kinds of nerve cell process, namely axons and dendrites. There is only one axon in the nerve cell (carrying nerve impulses away from the cell body) but several dendrites (receiving and conducting nerve impulses towards the cell body). Axons (Fig. 8, 9) are of variable length but they have a relatively constant diameter (1–20 mm), and may not www.fn.viamedica.pl Michał Karasek et al., Ultrastructure of the central nervous system: the basics branch profusely and the distal ends of these branches are enlarged. Dendrites branch widely and usually contain mitochondria, cisternae of the granular endoplasmic reticulum, free ribosomes and numerous neurofilaments and neurotubules. The Golgi apparatus can also be seen, especially in their proximal parts, before the first bifurcation. The densely packed conglomerate of intermingled and interconnected dendrites, axons, and neuroglial cells separating the nerve cells from each other is termed the neuropil (Fig. 10). Axons in the neuropil occur in the form of myelinated (Fig. 11) or unmyelinated fibres, whereas the great majority of dendrites do not posses a myelin sheath (Fig. 12), myelinated dendrites having only been identified in the olfactory bulb. THE INTERNEUR ON AL CHEMIC AL INTERNEURON ONAL CHEMICAL SYN APSES YNAPSES Figure 7. Golgi apparatus (GA) and mitochondria (M) in the nerve cell cytoplasm; ¥ 19,500. emit collateral branches along their course. They contain longitudinally-oriented cell organelles (long slender mitochondria, numerous neurotubules and neurofilaments). Shortly before termination axons commonly The cells of the nervous system are linked together to form functional conducting pathways. A specialised site of such interneuronal communication is referred to as a synapse. The synaptic contact between two nerve cells may occur between axon and dendrite (an axodendritic synapse) (Fig. 13), two axons (an axoaxonic synapse) (Fig. 14) or an axon and a cell body (an axosomatic synapse) (Fig. 15). A synapse is composed of a presynaptic element and a postsynaptic element separated by a synaptic cleft. The presynaptic element (terminal button) (Fig. 13–15) contains a cluster of mitochondria and numerous electron-lucent vesicles (containing most- Figure 8. Unmyelinated axon; ¥ 12,000. Figure 9. Myelinated axon; ¥ 25,000. www.fn.viamedica.pl 5 Folia Neuropathol., Supplement B/2004 Figure 10. Neuropil; ¥ 12,500. Figure 11. Myelinated axon; ¥ 12,500, insert: myelin sheath; ¥ 25,000. ly acetylocholine) of 40–60 nm in diameter or densecore vesicles (containing epinephrine or norepinephrine) of 40–60 nm in diameter, with a dense dot of 25 nm in diameter. These synaptic vesicles frequently aggregate very near the cell membrane of the presynaptic element (known as the presynaptic membrane). The presynaptic and postsynaptic membranes are separated by a narrow extracellular space of 15–30 nm in width, known as the synaptic cleft (Fig. 13–15), with some moderately dense material, sometimes made up of fine transverse filaments. These membranes are thickened and the underlying cytoplasm is often condensed (broken into groups in the presynaptic element and extending into the synaptic web in the postsynaptic element). The postsynaptic element (Fig. 13–15) contains mitochondria and often also parallel cisternae. 6 Figure 12. Group of dendrites; ¥ 27,500. THE NEUROGLIAL CELLS In addition to neurons, neuroglial cells, a class of non-neuronal supporting cells, are a constant component of the central nervous system. Beside the ependymal cells which line the ventricular cavities and spinal canal, there are three types of neuroglial cell: astrocytes (fibrous and protoplasmic), oligodendrocytes, and microglia. In addition to their mechanical supportive function, neuroglial cells insulate and protect the neurons, participate in the nutritive processes of the neurons, are involved in myelin sheath formation, and play a role in degenerative and regenerative processes. Fibrous astrocytes, with a cell body of 10–12 mm in diameter (Fig. 16), are found mainly in white matter and are characterised by the presence of long thin processes and irregularly-shaped nuclei, poor in chromatin and sometimes with a folded nuclear membrane. The cyto- www.fn.viamedica.pl Michał Karasek et al., Ultrastructure of the central nervous system: the basics Figure 13. Axodendritic synapse; A — presynaptic element, B — synaptic cleft, C — postsynaptic element; ¥ 210,000. Figure 14. Axoaxonic synapses; A — presynaptic element, B — synaptic cleft, C — postsynaptic element; ¥ 93,000. Figure 15. Axosomatic synapse; A — presynaptic element, B — synaptic cleft, C — cell body; ¥ 50,000. www.fn.viamedica.pl 7 Folia Neuropathol., Supplement B/2004 Figure 16. Fragment of fibrous astrocyte; ¥ 11,000. plasm contains a few mitochondria, a relatively small number of short cisternae of the granular endoplasmic reticulum, some free ribosomes, a small Golgi apparatus, confined to a few sacs, centrioles and glycogen particles. The numerous filaments of 8–9 nm in diameter that occur in the perikaryon and extend into the processes form a very conspicuous component of fibrous astrocyte but there are only a few microtubules. Protoplasmic astrocytes, with a large polygonal cell body of 15–25 mm in diameter, occur mainly in the grey matter. Their numerous processes are shorter, thicker and more branched than those in the fibrous astrocytes. Besides some sparse organelles (a few mitochondria, short cisternae of granular endoplasmic reticulum, a small Golgi apparatus and centrioles) and glycogen particles, they also contain filaments which are, however, less abundant than those in the fibrous astrocytes and often lie in bundles. Oligodendrocytes, 6–8 mm in diameter (Fig. 17, 18), occurring in both the white and grey matter, are smaller than the astrocytes and have few delicate processes. These cells are often found along nerve fibres or the surrounding nerve cells. Their nuclei are relatively large and rich in heterochromatin. The thin cytoplasmic rim contains small mitochondria, short cisternae of granular endoplasmic reticulum, a moderately developed Golgi apparatus, free ribosomes and only a few microfilaments and glycogen particles. In contrast to the astro- Figure 17. Oligodendrocyte (O) adjacent to nerve cell (N); ¥ 11,000. 8 www.fn.viamedica.pl Michał Karasek et al., Ultrastructure of the central nervous system: the basics Figure 18. Oligodendrocyte; ¥ 12,500. cytes, the oligodendrocytes contain numerous microtubules of about 25 nm in diameter. Small stellate microglial cells with short slender extensions that give off numerous spine-like projections are scattered throughout the central nervous system, being located predominantly along the capillaries. The oval nucleus contains an appreciable amount of chromatin. The dense cytoplasm, forming a thin rim around the nucleus, contains a prominent Golgi apparatus, long narrow cisternae of endoplasmic reticulum, numerous lysosomes and phagosomes and some mitochondria. REFERENCES 1. Cajal SRY, Swanson N, Swanson LW (1995) Histology of the Nervous System of Man and Vertebrates. Oxford Univ Press, Oxford. 2. Karasek M, Pawlikowski M (1979) Atlas Ultrastruktury Tkanek i Narządów. PZWL, Warszawa. 3. Krstic RV (1984) Illustrated Encyclopedia of Human Histology. Springer, Berlin. 4. Peters A, Palay SL, Webster HdeF (1991) The Fine Structure of the Nervous System. Neurons and their Supporting Cells, 3rd ed. Oxford Univ Press, New York/Oxford. 5. Snell RS (1984) Clinical and Functional Histology for Medical Students. Little, Brown and Co., Boston/Toronto. ACNOWLEDGEMENT This work was supported by Medical University of Łódź (project 503-584-1) www.fn.viamedica.pl 9