Table of Contents
The term glia was introduced by Virchow to describe a second cell type whose function was to bind the nerve cells together in the brain. Later Cala1classified the glial cells into three categories: the first and second corresponded to the two varieties of astrocytes (protoplasmic and fibrous), whilst the third ‘element’ referred to a group of cells which was subsequently shown by del Rio Hortega to comprise both oligodendrocytes and microglial cells. Del Rio Hortega introduced the silver carbonate method, which not only distinguished and separated oligodendrocytes from microglia, but also indicated that they were of different derivation. Thus oligodendrocytes have a similar neuroepithelial Origin to astrocytes, whereas microglial cells originate from mesenchyme. Thus, the term neuroglia should not be applied to microglial cells. Neuroglia includes only astrocytes, oligodendrocytes, ependymal cells, and the choroid plexus epithelium derived from ependyma.
Of the neuroglial cells, astrocytes are the most varied morphologically and the most versatile functionally. It is traditional and convenient to distinguish protoplasmic and fibrous astrocytes. The difference between these two types is based upon the configuration and the number of processes rather than on the filament content of the cell body. the processes of fibrous astrocytes are fewer and longer and branch less frequently and at a more acute angle than those of protoplasmic astrocytes.
Protoplasmic astrocytes are found mainly in the grey matter whilst fibrous astrocvtes occur mainly in the white matter of the brain and spinal cord. Fibrous astrocytes, however, are also found in the outer cortical layer, around penetrating arterioles and, together with protoplasmic astrocytes, in the deep grey matter: the inferior olive contains a particularly large proportion of fibrous astrocytes. A variety of fibrous astrocytes, the so-called marginal glia, sends short, robust processes towards the pial surface, and contributes to the formation of the external glial limiting membrane (glia limitans) .
Subdivision of protoplasmic astrocytes in the grey matter, according to their localisation, into neuronal satellites, interneuronal astrocytes and vascular satellites serves little useful purpose. However, astrocytes of the cerebellar cortex, which are principally of the protoplasmic type, merit separate consideration on the basis of morphological variation and functional difference. The smooth astrocyte is found throughout the cerebellar cortex, the velate or lamellar type chiefly in the granular layer, and the Golgi epithelial cell in the Purkinje and molecular layer. Golgi epithelial cells are in fact the Bergmann glia, whose processes cover large areas of the cell body and dendrites of Purkinje cells. A further type in the cerebellar cortex is the feathered glia of Fafianas: these cells are more superficial than Bergmann cells and their short stout processes do not reach the pial surface. One form of astrocyte which has features of both the protoplasmic and fibrous astrocytes, but is identical with neither, occurs in the corpus callosum, basis pontis and spinal cord. This intermediate or mixed astrocyte has long fibril-containing processes identical to those of the fibrous type, whilst their shorter processes are more protoplasmic in structure.~ Astrocytic density varies greatly in various parts of the brain. The ratio of glial cells to neurons, for example, is 4:1 in the striatum and 100:1 in the globus pallidus, accounting for 20~~(, and ~ of the brain volume in these areas respectively.~5 These variations in astrocytic morphology in different parts of the central nervous system are well established, but information has only recently emerged from immunocytochemical and tissue culture work regarding the possible differences in astrocyte physiology and development in various topographical sites in the brain.
Although astrocytes can be identified in haematoxylin and eosin stained sections, the intricate pattern of cytoplasmic processes which render these cells star-shaped can be better demonstrated by the use of special stains . The oval or round vesicular nucleus usually lacks a prominent nucleolus and this feature proves particularly convenient in identifying cells in the cortex: neurons are distinguished by their conspicuous nucleoli. Astrocytic processes cover the external surfaces of the central nervous system, surround blood vessels and abut upon nerve cells. The external glia limitans is formed by stout astrocytic processes which, in turn, are covered by a basal lamina beneath the pia . Similarly, end-feet, the expansions of astrocytic processes, envelop blood vessels and form a limiting membrane around the adventitia of larger vessels. These astrocytic end-feet or foot processes also invest the capillaries, and this intimate relationship has provided the structural basis for the astroglial involvement in the blood-brain barrier. Astrocyte cell bodies can be in close apposition to neurons, and astrocytic processes are frequently seen covering the extensive surfaces of neuronal perikarva and dendrites.
Electron microscopy has revealed that the vesicular astrocyte nucleus contains evenly dispersed, fine chromatin which is occasionally clumped at the nuclear membrane. The nuclear profiles can be somewhat irregular with indentations, and the nucleolus, when present, is small. The cytoplasm is of low electron-density and contains the usual assortment of organelles. The cisternae of the rough endoplasmic reticulum are short, ribosomes few and GoIgi complexes not well developed. Mitochondria are present both in the perikaryon and the cell processes, and the larger, more unusual forms are likely to correspond to the gliosomes detected by light microscopy. Glycogen granules are numerous in well-fixed tissues and appear to be concentrated in areas of high synaptic density and near neuronal perikarya. Lysosomes are also seen as electron-dense bodies limited by a single unit membrane. Astrocytic filaments, 10 nm in diameter, belong to the class of intermediate filaments of the cytoskeleton and are conspicuous in the perikaryon and in cell processes. The filaments are present in protoplasmic astrocytes, usually in the form of cytoplasmic bundles, but they are more abundant in fibrous astrocytes, in which they occur throughout the cell body and extend into the larger processes.
Microtubules, although present, are not plentiful in mature astrocytes. Astrocytes often form specialised cell contacts of the gap junction type (so-called nexus) in which the outer leaflets of the apposed plasma membranes are separated by an interval of 2-3 nm. Adjacent astrocytes are also joined by puncta adhaerentia, where the plasma membranes run parallel, separated by a gap of 25-30nm.
Astrocytic filaments are 10 nm in diameter and indeterminate in length. They are composed of four globular protein subunits, each measuring 2.5nm, and linked by a cross-arm l.5nm in thickness. These units are stacked upon each other to produce the characteristic configuration: in transverse section, a hollow central core is surrounded by a dense wall of 2-5 rim, whilst in longitudinal section two parallel dense lines enclose the lumen.
The filaments are composed of glial fibrillary acidic protein (GFAP) which was first isolated in a water-soluble form from glial scar tissue of burnt out plaques of multiple sclerosis and from hydrocephalic brains. GFAP has a molecular weight in the range of 50000 daltons and is well characterised chemically. Immunocytochemistry indicates that GFAP may exist in two structural states, diffuse in the cytoplasm or localised to filaments, and these, in turn, may correspond to the water-soluble and water-insoluble forms, respectively Metabolic studies of cytoskeletal proteins in cultured astrocytes have revealed that GFAP is amongst the most actively synthesised proteins with a relatively fast turnover, and both the level of synthesis and accumulation of GFAP can be experimentally manipulated. It is present in normal, reactive and neoplastic astrocytes and has become the most reliable immunocytochemical marker by which the identity of astrocytes can be positively established both in diagnostic work and for research. In addition to astrocytes, GFAP can be demonstrated by immunocytochemistry in other cells which contain glial filaments: in reactive ependymal cells, tanycytes, reactive Muller cells of the retina, in the pituicytes of the neurohypophysis and in the glial cells of the enteric nervous system. It is also present transiently during development in the myelin-forming oligodendrocytes of the human fetal spinal cord, and in ependymal cells of human fetuses between the 15th and 40th week of gestation. The function of GFAP has yet to be determined, but experimental evidence suggests an important role in fibrillogenesis which, in turn, is associated with astrocytic differentiation.
The concept of astrocytic function has changed profoundly during the last 20 years: cells which once were thought to be mere physical supporting elements have been shown to have a wide range of activity in the developing, normal and diseased central nervous system.
Virchow’s original view of neuroglia as cells that somehow held the neurons together has been supported by successive observations of astrocytic morphology. First, astrocytic processes cover the outside surfaces of the central nervous system, forming the glia limitans, and envelop intraparenchymatous blood vessels, constituting a sleeve of end-feet . Second, astrocytic filaments provide mature astrocytes with a cytoskeleton which stabilises the cell configuration and endows processes with considerable strength. These processes may form bundles that interweave with nerve fibres, particularly in the white matter. Finally, astrocytic processes are often joined together by specialised cell contacts which increase their strength and cohesion. Although microtubules play a role in process formation in differentiating astrocytes, filaments are the organelles that maintain and influence the overall morphology of mature cells.
The ability of astrocytes to produce abundant filaments is seen in various pathological conditions. Although the formation of glial scar tissue is important in the repair of the central nervous system following injury, fibrillary gliosis may occur physiologically in certain areas including the olivary nuclei, the floor of the fourth ventricle, and around both the cerebral aqueduct and the central canal of the spinal cord.
The structural proximity of astrocytic foot processes to capillaries has provided a morphological basis for the view that astrocytes contribute to the maintenance of the blood-brain barrier (see p.30).
Astrocytes establish intimate spatial relationships with nerve cells. They are occasionally satellites to neurons, but more importantly, their processes regularly cover receptive surfaces of the nerve cells, including the perikarya and dendrites. Moreover, astrocytic processes often abound in areas of intense synaptic activity; the most striking example of this phenomenon is to be found in the thalamus, where the synaptic glomeruli are wrapped in astrocytic sheets which form capsules several layers thick. Astrocytic processes are thus not disposed at random, but they conform to a pattern that ensures that receptive neuronal surfaces are protected from non-specific afferent influences. Astrocytes also play a role in synaptic remodelling in the mature, normal brain by removing degenerating synapses.
Evidence for astrocyte involvement in neuronal development originates from both in vivo and in vitro observations. Radial astroglial fibres appear to guide immature, migrating neurons and to form a template for the growth of nerve cells. Neurons of the central nervous system are difficult to grow in primary cultures unless an astrocyte monolayer is provided or the medium is conditioned by growing astrocyte cultures. In vitro studies have also shown that glial cells can influence the protection and branching patterns of nerve cells and may be instrumental in determining the neuronal polarity observed in vivo.
Recent investigations have revealed that astrocytes are not inactive cells: they respond to K accumulation or release, with changes in intracellular K + concentration. Astrocytes can act both as spatial buffers, merely redistributing extracellularly accumulated K+, and as active accumulators of K+. These mechanisms complement each other and may enable astrocytes to monitor the extracellular ionic milieu and consequently to control neuronal function.53 Furthermore, the release of potassium ions (K +) from astrocyte end-feet may play an important r8le in regulating regional cerebral blood flow in response to changes in neuronal activity. It has been demonstrated in primary astrocyte cultures, which are presently the best experimental system for ion transport studies, that not only K+, but also Na+ and Cl- can enter and leave these cells, suggesting that astrocytes contain significant ion transport pathways. Under controlled conditions in culture, astrocytes show both spontaneous action potentials and action potentials induced by the current. These responses indicate the presence of voltage-dependent Ga2 -channels which may be important in the regulation of excitability within the central nervous system. Furthermore, both glutamic and aspartic acid directly depolarise rat brain astrocytes in primary cultures, suggesting that the electrophysiological effects of excitatory amino acids in situ may not be exclusively a neuronal property.
Creatine kinase (isoenzyme BB) has been shown by immunocytochemistry to be present in human astrocytes. The function of this enzyme in the brain may be related to the increase in respiration and the fall in both ATP and creatine phosphate levels which result from exposure to high potassium concentrations or from electrical stimulation.
Astrocytes in culture possess receptors for neurotransmitters, and exposure to noradrenaline results in increased levels of intracellular cyclic adenosine mono-phosphate (cAMP). Adrenergic x- and fl-receptors have also been identified on astrocytes, and dopamine, for example, enhances cAMP by activation of fl-receptors.Astrocytic processes surrounding synapses could control the levels of transmitters by taking up these compounds, but it remains to be determined whether this uptake is involved in the further metabolism or inactivation of the neurotransmitter. Primary cultures of astrocytes indeed take up dopamine, noradrenaline, serotonin, GABA (Gamma-aminobutyric acid) and glutamate and have some of the enzyme systems required for their metabolism. Astrocytes in culture also respond to histamine and its agonists, but the functional role of histamine receptors remains to be elucidated. However, the wide range of receptors on astrocytes allows these cells to play an important and varied role in the central nervous system. The role of astrocytes in the metabolism of glutamate, a putative excitatory neurotransmitter, is well documented. Glutamate is released by neurons at synapses and is taken up by astrocytes in which glutamine synthetase catalyses the reversible formation of glutamine from glutamate and ammonia. Glutamine is freely diffusible and reaches nerve cells in which glutaminase will produce glutamate and thus complete this metabolic pathway. The small glutamate pool in the brain is, therefore, compartmentalised in astrocytes. Immunohistochemical studies have demonstrated that glutamine synthetase is confined to astrocytes, and the amount present in various brain areas correlates well with sites of presumed glutaminergic activity.
From the above description it follows that as astrocytes possess glutamine synthetase activity, they are also important in the detoxification of ammonia.
There is now little doubt that astrocytes have pinocytotic and phagocytic functions and by removing various substances, including plasma, particulate material and cell debris from the extracellular space, they contribute to the maintenance of a controlled internal environment.
Recent experimental evidence suggests that astrocytes may have a physiological function essential for the generation of immune responses within the brain: they respond to lymphocyte-derived growth factors and secrete immunoregulatory molecules in culture.~ They can be activated to release prostaglandin E and interleukins. Rat astrocytes appear to be able to present antigen to T lymphocytes in a specific manner which is restricted by the major histocompatibility complex. Astrocytes are stimulated in the presence of interferon, produced by T lymphocytes, to express Ia antigens. Astrocytes and their precursors also respond, in culture, to glial maturation factor and produce interleukin a1; this, in turn, stimulates the proliferation of astrocytes.
The configuration of astrocytes, complete with their cytoplasmic processes, can be demonstrated by a range of specialised histological stains. The older techniques include Mallory’s phosphotungstic acid haematoxylin (PTAH), Holzer’s crystal violet, Cajal’s gold chloride sublimate and del Rio Hortega’s silver carbonate impregnation.
The most widely used method is Mallory’s PTAH but, unfortunately, this technique also stains myelin sheaths, smooth muscle fibres and extracellular proteins. The intimate relationship between astrocytes and capillaries is best visualised by Cajal’s method: the close apposition of foot processes to the capillary wall is most convincingly shown.
Immunohistochemical techniques, developed more recently, offer superior specificity and sensitivity over conventional staining methods, and are increasingly used in diagnostic and research work. Astrocytic antigens can be demonstrated by both immunoperoxidase and immunofluorescence techniques; and whilst the former is preferred for paraffin sections, the latter is the method of choice for tissue culture preparations. GFAP is now most widely used to identify astrocytes, whilst glutamine synthetase, confined to astrocytes in the central nervous system, offers an alternative marker.
Oligodendrocytes can be subdivided according to their location: satellite cells are adjacent to neurons in the grey matter and interfascicular oligodendrocytes occur between nerve fibres in the white matter. In the grey matter, oligodendrocytes are also associated with nerve fibres or are adjacent to blood vessels. Perineuronal oligodendrocytes are functionally similar to those in the white matter. Immunocytochemistry for myelin basic protein and myelin-associated glycoprotein reveals a similar staining pattern for all oligodendrocytes in the normal brain, during remyelination and following trauma. Based on ultrastructural features, particularly on cytoplasmic density, three subtypes can be distinguished: light, intermediate and dark.102 This separation is arbitrary and these forms represent developmental stages through which oligodendrocytes evolve during postnatal life. Light oligodendrocytes appear to be mitotically the most active and they become smaller and darker as they mature.
In sections stained with haematoxylin and eosin, oligodendrocytes can be recognised by their round or oval nuclei which are surrounded by a rim of cytoplasm. Silver impregnation reveals only a few processes, which are usually long and delicate and radiate from the polygonal or spherical perikaryon. By electron microscopy, oligodendrocytes are, in general, darker cells than astrocytes, with an intrinsic density of the cytoplasmic matrix created by tiny granules which occupy all the space between organelles. The chromatin in the round) oval or, occasionally, irregular nucleus often forms clumps, which is another difference from the evenly dispersed chromatin of astrocytes. Oligodendrocyte cytoplasm contains abundant rough endoplasmic reticulum, many free ribosomes and a well-developed Golgi apparatus. Mitochondria, lysosomes and heterogeneous inclusion bodies are present. Unlike in astrocytes, however, glycogen granules are not seen in oligodendrocytes. There is a striking difference between the cytoskeletons of oligodendrocytes and astrocytes in that the ratio of filaments to microtubules is reversed: oligodendrocytes have many microtubules and only an occasional filament. The microtubules are dispersed at random in the cell body, whilst in processes, they are arranged in parallel bundles. The following features distinguish the oligodendrocyte from the astrocyte: greater nuclear and cytoplasmic density, the lack of filaments and glycogen granules, and the abundance of microtubules.
Two major functions are usually attributed to oligodendrocytes: the formation and maintenance of myelin, and the nutrition of neurons. There is now little doubt that the myclinforming cell in the central nervous system is the oligodendrocyte, which first appears immediately before myelination begins. in the developing brain, a direct connection can be seen between oligodendrocytes and the myelin sheath as it forms.103 Biochemical studies have shown that oligodendrocytes, particularly neuronal satellites, can contribute to the nutrition of nerve cells. The metabolic activities of neurons and oligodendrocytes can complement each other in a symbiotic fashion, and these two cell types form a functional unit.’~ Furthermore, mature oligodendrocytes, in vitro, proliferate only in the presence of neurons, indicating that axons are mitogenic for these cells. Tissue culture studies have also shed new light on the functions of oligodendrocytes. Isolated cells synthesise both galactocerebrosides and sulphatides1~ and contain various enzymes including 2′ :3′-cyclic-nucleotide 3′-phosphodiesterase, carbonic anhydrase, glycerol phosphate dehydrogenase and glucose-6-phosphate dehydrogenase, indicating a high rate of metabolic activity. Oligodendrocytes respond to a variety of lymphokines and other growth factors and thus may be involved in immunological reactions.
Oligodendrocytes are usually recognised without difficulty in sections stained with haematoxylin and eosin , but silver impregnation techniques to demonstrate cell processes are capricious even in the most experienced hands.” Of the immunocytochemical markers, carbonic anhydrase (isoenzyme II or C) appears to be the most promising, whilst the surface marker) galactocerebroside, can be demonstrated in cultured cells and myelin basic protein is present in immature oligodendrocytes during myelination.
The ependyma lines the ventricular system, the cerebral aqueduct and the central canal of the spinal cord. These epithelium-like cells are cuboidal or columnar and show the characteristic polarity of cellular organisation. A round or oval nucleus is located in the basal part of the cell, whilst most organdIes occupy the apical portion. The cell membrane is specialised according to the surface. Thus the lateral membranes of adjacent cells run parallel and are joined together by specialised junctions; the cell membrane at the base of the cell has an irregular contour and rests upon glial fibres; the apical surface is studded with cilia. Ependyma lining the ventricular system varies in its morphology. In general, cells covering the white matter are more flattened and have fewer cilia than those covering the grey matter.
Electron microscopy shows that the nucleus contains evenly distributed, fine chromatin and a small eccentric nucleolus. The well developed Golgi apparatus is supranuclear, and mitochondria tend to crowd the apical portion of the cell. The rough endoplasmic reticulum consists of only a few, short cisternae, but free ribosomes are numerous. Multivesicular bodies, lysosomes and vesicles of various sizes are also present. The cytoskeleton is composed of 10 nm intermediate filaments, 4.6 nm microfilaments and occasional 24 nm microtubules. Intermediate filaments are similar to those found in astrocytes, but they do not appear to contain GFAP in normal, mature ependyma. However, ependymal cells in human fetuses contain GFAP transiently between the 15th and 40th week of gestation. The nature of filaments in mature ependymal cells remains an enigma: they may be antigenically different from astrocytic filaments or the binding of GFAP within the filament may have resulted in the loss of affinity for the specific antiserum.
Cilia spring from the basal bodies in the apical cytoplasm which can occasionally be seen as blepharoplasts with the light microscope in good PTAII preparations. Each cilium is composed of9 pairs of microtubu]es surrounding a central pair. Between the cilia, microvilli and simple cytoplasmic protrusions increase the apical surface area of the cell. The lateral plasma membranes of adjacent ependymal cells often interdigitate, forming gap junctions (nexus), extensive zonulac adhaerentes and occasional tight junctions (zonulae occiudentes) toward the apices.
Tanycytes are modified ependymal cells distinguished by their long, radially orientated and unbranching basal processes which usually reach subependymal capillaries These cells, which owe their name to their elongated shape, have a somatic portion, which lies in the ependyma and contains the nucleus, a neck portion, which is located in the periventricular neuropil, and a tapering tail. The apical surfaces of tanycytes have more thin cytoplasmic projections and fewer cilia than other ependymal cells and, by electron microscopy, their cytoplasm is somewhat darker and contains fewer filaments and more microtubules.
The cilia of ependymal cells beat rapidly during life and this movement may contribute to the circulation of the cerebrospinal fluid (CSF). The structural organisation of ependyma, with its interdigitations and specialised junctions at the lateral cell surfaces, is suggestive of a supportive function, similar to that performed by astrocytes. As ependyma forms a barrier between the ventricular CSF and the parenchyma of the central nervous system, it is ideally situated to influence the transport of substances. When electron-dense markers, such as horseradish peroxidase and ferritin, are injected into the ventricles they penetrate into the brain between ependymal cells. In addition, these materials are taken up by the ependyma and transported in vesicles and multivesicular bodies, suggesting that other substances in the CSF may follow a similar pathway. As tanycytes connect the ventricular surface and capillaries, it is thought that they transport material from the CSF to the brain and into the vascular circulation, but their major function is structural. Ependymal cells may perform sensory and secretory functions in various animal species,1 but evidence for these activities in man is still not available.
The profusion of names that exist for microglial cells is a good indicator of the controversy surrounding the Origin and morphological heterogeneity of microglia. Hortega or Robertson-Hortega cell, cerebral histiocyte, phagocyte, rod cell and mesoglia are synonyms for the microglial cell.
Microglial cells are ubiquitous in the central nervous system, although they are somewhat more numerous in the cortex than in the white matter. Most appear to be distributed at random, but some are preferentially located near neurons and blood vessels. The nucleus is triangular or elongated and the chromatin pattern is less vesicular than in astrocytes, although not as dense as in oligodendrocytes. Very little cytoplasm is visible in haematoxylin and eosin-stained sections, but immunocytochemical techniques and silver impregnation demonstrate the cell processes, which are occasionally very long, but less numerous than astrocytic processes. Electron microscopy reveals the usual complement of organelles in microglial cytoplasm. Thus, a few, long cisternae of the rough endoplasmic reticulum, some ribosomes, an active Golgi apparatus, mitochondria, microtubules and a few filaments are seen. The most prominent feature of the cytoplasm is the presence of dense inclusion bodies, mainly lysosomes. The nucleus contains coarsely clumped chromatin and is often located at one end of the cell, surrounded by a thin rim of cytoplasm, whilst the organdIes occupy the opposite pole.
The Origin of microglial cells has been the subject of long controversy and the problem has yet to be convincingly settled. Various hypotheses have been proposed and include origins from pial, neuroepithelial, pericytic and monocytic cells. A mesodermal or pial origin of microglia suggests that primitive mesodermal cells accumulate beneath the pia and penetrate the brain parenchyma as diverse forms of amoeboid microglia which, during differentiation, retract their pseudopodia and develop branching processes to become mature microglial cells. The opposing neuroepithelial theory maintains that microglial cells originate from the primitive cells of the subependymal plate either directly or indirectly through amoeboid microglial cells. Microglioblasts or amoeboid microglia are, in fact, glioblasts derived from the matrix cells and thus, according to this theory, microglial cells would share a common progenitor with astrocytes and oligodendrocytes.
Pericytes are cells closely apposed to the capillary endothelium and enclosed by the basement membrane. Morphological similarities and the phagocytic capacity of pericytes have led to the view that they could be a source of microglia; these cells could migrate into the brain through disruptions in the basement membrane. However, pericytes have been shown to remain anchored to the vascular wall in pathological conditions and do not become transformed into actively phagocytosing microglial cells.
The theory that microglial cells originate from monocytes has attracted many supporters, who maintain that monocytes from the blood enter the brain, in which, conforming to a different environment, they acquire the features of resting microglial cells. That blood monocytes can permeate the brain has been shown using carbon-labelled cells; thus, the sequential appearance of carbon particles in monocytes, amoeboid microglia and microglia strongly suggests that monocytes become microglial cells. However, the infiltration of blood monocytes into the brains of experimental animals is age-dependent, and the blood-brain barrier may also play a role in preventing monocytes from entering the brain in large numbers in mature animals. Moreover, immunological studies have produced evidence against the monocytic origin of microglia; microglial cells do not possess monocytic membrane antigens and mononuclear phagocytic markers. However, monocytes may lose their membrane markers on entering the brain and then adapt to an entirely different macrophage microglia ii similarities
In conclusion, the original concept of a pial origin for microglial cells has not been refuted; on the contrary, it has gained new support.123 However, the possibility of a dual origin, both pial and monocytic, cannot be excluded. It is likely that occasional monocytes can enter the normal brain, and this has been demonstrated in experimental animals and in the adult normal human brain; A the relationship of these cells to resting microglial cells has yet to be determined.
The function of resting microglia in the normal brain is far from clear. These cells are thought to maintain a close, functional relationship with neurons, axons and myelin sheaths, to regulate the ion and fluid balance of the extracellular space and to transport substances. The presence of Fc and complement receptors and of HLA class II complex on microglial cells suggests that they could play a role in the immunological defence of the nervous system. Microglial cells have the ability to engulf and ingest various substances, including particulate material, parts of other cells or even whole cells. They are rich in lysosomal hydrolytic enzymes, which enable them to perform the phagocytic activity.
Although silver impregnation demonstrates cell processes and gives the most comprehensive picture of microglial cells, the technique only works in expert hands. Enzyme histochemistry for acid phosphatase and non-specific esterase stains lysosomes and thus indicates phagocytic activity.