Download Ultrastructure of the central nervous system: the basics

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]
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Folia Neuropathol., Supplement B/2004
Figure 1. Low-power micrograph of the brain; N — nerve cell, O — oligodendrocyte; C — capilary; ¥ 11,500.
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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.
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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.
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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
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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.
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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.
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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-
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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.
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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.
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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)
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