Neurons


Table of Contents


Nerve Cells

Nerve cells vary enormously in size and configuration. The cerebellum presents the best example of morphological extremes: the Purkinje cells and their impressive dendritic trees dwarf the neighbouring granule cells and their short dendrites.

The shape of the nerve cell body, the perikaryon, also varies throughout the central nervous system: pyramidal, polygonal, round, oval and fusiform forms can be distinguished. Neuronal cell processes fall into two categories: dendrites and axons. Whilst a neuron usually has only a single axon, the number of dendrites ranges from one to nearly one hundred. The length and arborisation of processes also display considerable individual variations. Silver impregnation techniques of classical neurohistology, electron microscopy, enzyme histochemistry and more recently immunocyto­chemistry have all contributed.. to the understanding of neuronal morphology and function. The previous morphological classifications of nerve cells are now gradually being replaced by classifications based on functions: neurons can be distinguished according to the neurotransmitters they use.

Nerve cell nuclei are round or oval, although the nuclear profile is occasionally indented. The nucluo- or karyoplasm is separated from the cytoplasm by the nuclear membrane which is com­posed of two laminae enclosing the perinuclear cisterna: the inner lamina is smooth, whilst the outer one, studded with ribosomes, is frequently continuous with the rough endoplasmic reticulum. Finely granular chromatin is evenly dispersed with some clumping at the nuclear membrane.

The nucleolus is prominent and this helps to distinguish neurons from astrocytes which contain only a small inconspicuous nucleolus. Electron microscopy reveals the nucleolus to be composed of granular and fibrillar constituents. Attached to the nucleolus is the nucleolar satellite or sex chromatin: a dense, coarsely fibrillated body that occurs only in females.

Nuclear inclusions of various types, filamen­tous, granulofibrillar, tubular and vesicular,’ often occur in normal neurons and can be occasion­ally discerned in the light microscope. Variations in nuclear morphology exist according to neuronal type, topography and functional activity. The large vesicular nuclei of the Purkin)e cells or the pyramidal cells of the cerebral cortex are in sharp contrast to the small, dense nuclei of the cerebellar granule cells.

Cytoplasmic organelles

Nissi substance

This cytoplasmic component is intensely baso­philic when stained with cresyl violet or methyl­ene blue. Electron microscopy shows that Nissl substance is composed of stacks of the rough-surfaced endoplasmic reticulum (RER) (Fig. 1.4). The cisternae are usually arranged in parallel arrays and their outer surface is covered by ribo­somes. The amount of RER varies from neuron to neuron and appears to be related to the length of the axon: it is particularly abundant in the anterior horn cells of the spinal cord and the pyramidal neurons of the cerebral cortex. It is present not only in the perikaryon but also in the dendrites, but it is absent from the axon hillock and from the axon itself. Since the RER is the site of protein synthesis, its amount is related to the metabolic

activity of the cell. The Nissl substance forms an integral component of the neuronal membrane system which is associated with the transfer of membrane molecules amongst organdies, the packaging and transport of exportable protein and energy metabolism.2 In addition to the membrane-bound particles of the RER, ribosomes are scattered throughout the cytoplasm either singly or in groups as polyribosomes or ribosomal rosettes.

The axon and dendrites

Although the precise function of the agranular reticulum is not known, its widespread distribution within the cytoplasm and its close spatial relationship to the cell membrane suggest a role in intracellular transport.

The internal reticular apparatus of Golgi appears as tortuous anastomosing strands in the light microscope. Located between the nucleus and the cell membrane or at the base of a dendrite, the Golgi apparatus is composed of arrays of closely packed, smooth-surfaced cisternae and numerous associated vacuoles and vesicles.

Its curved configuration allows a convex, external forming face and a concave, internal mature face to be distinguished. The internal face of the Golgi apparatus forms an integral part of the GERL system (Golgi apparatus, smooth endo­plasmic reticulum and lysosomes): a spatially closely related group of organelles that is devoted to the synthesising and condensing activity of the Golgi apparatus. This polarisation of the Golgi apparatus, although not evident in all neurons, is thought to serve the efficient transport of materials. The demonstration of concanavalin A binding sites on Golgi membranes strongly suggests that this organelle is concerned with glycoprotein metabolism. The formation of new plasma membranes during mitosis and the maintenance of normal membrane structure in resting cells are important functions of the Golgi apparatus.

The Golgi apparatus itself promotes the move­ment of membranes: vacuoles fuse to form a cisterna at the forming face, while vesicles are budding off at the mature face. The diameter of these vesicles ranges from 20-60 um and they are usually smooth-surfaced, round or elliptical. Coated or alveolate vesicles of 50-60 um diameter are distinguished by their regu]arly spaced arms or striae. These vesicles are concerned with the transport of hydrolytic enzymes to the lysosomal system. Larger alveolate vesicles, lOOnm in diameter, derived from the cell surface by invagin­ation or pinocytosis, ingest proteins which then are transferred to the lysosomes to be digested.

In addition, many neurons, particularly those of the supraoptic and paraventricular nuclei, contain dense-cored, neurosecretory granules up to -150 nm in diameter.

Multivesicular bodies are spherical structures which contain vesicles and an assortment of inclusions. Although frequently associated with the Golgi apparatus, they are not part of these organelles and occur throughout the perikaryon and cell processes. Multivesicular bodies sequester material transported to them by coated vesicles, and hydrolytic enzymes acquired from smaller coated vesicles convert them into lysosomes.

Lysosomes

Primary lysosomes are round or oval, uniformly dense bodies up to 1 pm in diameter, bound by a unit membrane. They contain various hydrolytic enzymes, including acid phosphatase which serves as a marker enzyme for their identification.3 Secondary lysosomes are larger, more irregular and usually contain various ingested materials.

Lipofuscin granules

Although visible by light microscopy, lipofuscin can be better appreciated in the electron micro­scope: a single membrane encloses a dense granule and one or two peripheral vacuoles. Lipofuscin or lipochrome is frequently referred to as ‘wear-and-tear’ pigment since the number of granules within the neurons increases with age. These granules are end products of intracellular peroxidation and polymerisation of unsaturated fatty acids. Their origin is controversial, but according to the most widely accepted concept, they are denv~d from lysosomes. The precise function of lipofuscin is unknown. Degenerated cytoplasmic constituents contribute to its formation, which requires intracellular oxidants and antioxidants. Lipofuscin is thus the by­product of metabolic activity; it is stored in lyso­somes and is only disposed of by expulsion during mitosis or by cell death.5 Since mature nerve cells do not divide, lipofuscin granules accumulate during life. Appearing as light brown granules in haematoxylin and eosin stained sections, lipo­fuscin can be demonstrated by a variety of fat stains including Sudan black B, Sudan III and Nile blue sulphate. It is acid-fast and PAS positive and reduces ferric ferricyanide to Prussian blue (Schmorl’s reaction).

Melanin

This pigment is present in the leptomeningeal cells and in the pigmented nuclei of the brainstem. Melanin-containing cells in the leptomeninges are most plentiful over the ventral aspect of the brain-stem; this is the area in which the rare primary malignant melanomas most often develop. Two major accumulations of pigmented neurons occur in the brainstem: in the substantia nigra of the midbrain and in the much smaller locus ceruleus of the pontine tegmentum. The cells of the substantia nigra and locus ceruleus are dopaminergic and noradrenergic respectively. Other brainstem nuclei, including the motor nucleus of the vagus, also contain melanin in small amounts.

Mitochondria

These organelles vary considerably in shape and size, depending upon their location within the neurons. They are apparently distributed at random in the perikaryon and also occur in den­drites and axons, including the presynaptic terminals. Mitochondria have a smooth outer membrane and an irregular inner membrane. The inner membrane forms complex folds (cristae); the inner compartment of the mitochondrion is filled with a moderately dense matrix . The inner surfaces of the cristae are rendered uneven by many minute projections, each ending in a knob. The inner and outer membranes differ in composition, structure and function; the electron transport mechanism and many hydrogenases are located in the inner membrane, whereas the monoamine oxidase system is found in the outer membrane.1

The cytoskeIeton

The neuronal cytoskeleton is composed of three elements: neurofilaments, microtubules and microfilaments.

Neurofilaments

These filaments are 10 nm in diameter and of indeterminate length. They belong to the class of intermediate filaments, their diameter being in between that of the micro-filaments (4-6nm) and that of microtubules (24 nm). Of the five types of intermediate filaments found in animal cells, two occur in the central nervous system: neurofilaments in nerve cells and glial filaments in astrocytes. Although the various types of intermediate filaments appear to be similar morphologically, they are composed of different proteins.

In cross-section, neurofilaments display a central hollow core) surrounded by a wall of 3 nm in thickness. The basic unit of the wall is composed of four globular subunits, each 3.5 nm in diameter, which are linked together by connecting arms of 2~5 nm in thickness. These units are stacked one upon another and rotated in the transverse plane by 45~ to each other. In the longitudinal section, neurofilaments appear as two dense, parallel lines which enclose the central hollow core. They are dispersed throughout the perikaryon, but tend to accumulate at the base of large processes, into which they extend, forming parallel arrays.

Neurofilaments isolated from the nervous system are formed by three polypeptides with the approximate molecular weights of 70, 150 and 200 kilodaltons.7 Recent investigation has revealed that immunohistochemical differences may exist between neurofilaments in the perikarya, den­drites and axons.

Although the morphology and biochemistry of neurofilaments have been established,9 very little is known about their functions. They are intimately related to microtubules, the other component of the neuronal cytoskeleton, and with them, they could play a role not only in stabilising nerve cells but also in axonal transport.10 Neurofilaments are also major intrinsic deter­minants of axonal diameter in large myelinated nerve fibres: the expression of a single set of neuron-specific genes encoding neurofilaments directly determines axonal calibre.”

Microtubules

Microtubules are of indeter­minate length and 24nm in diameter. Their walls are 6nm thick and composed of 13 globular subunits, each representing a constituent proto­filament.’0 Microtubules are intermingled with neurofilaments both in the perikaryon and in cell processes: the ratio of neurofilaments to microtubules decreases as the axon becomes smaller, and in thin unmyelinated axons usually, only microtubules are present. Tubulin, the major protein component of microtubules.

In an adult brain, this protein comprises 10-30″ of the soluble protein’0 and more recently a group of microtubule-associated proteins has also been recognised. Many functions have been attributed to microtubules: skeletal support, maintenance of axonal flow, transport of various substances and of cytoplasmic vesicles, cellular contraction and a role in mitosis.’ Microtubules also play a role in the maintenance and function of the Golgi apparatus.”

Microfilaments

These are not obvious in nerve cells: they have a diameter of 4-6nm and are composed of actin, a globular protein with a mol­ecular weight of 42000.” Microfilaments, in addition to their stabilising function, also play a role in axonal transport.

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