NEUROGLIA

 

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 the glial cells were classified 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.

 

 

ASTROCYTES

 

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 astrocytes 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 varietv 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. 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 cosin 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 cisternac of the rough endoplasmic reticulum are short, ribosomes few and Go Igi 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 mm 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 mm. Adjacent astrocytes are also joined by puncta adhaerentia, where the plasma membranes run parallel, separated by a gap of 25-30 mm.

 

 

Astrocytic filaments and glial fibrillary acidic protein (GFAP)

 

Astrocytic filaments are 10  mm in diameter and indeterminate in length. They are composed of four globular protein subunits, each measuring 2.5 mm, and linked by a cross-arm l.5 mm 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 mm, 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 burnt­out 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 69 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.

 

 

Functions of astrocytes

 

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.

 

Structural support. 

 

Virchow's original view of neuroglia  as cells which 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 which 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 which maintain and influence the overall morphology of mature cells.

 

Repair. 

 

The ability of astrocytes to produce abundant filaments is seen in various pathological conditions. Although the formation of glial scar tissue is important in 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 blood-brain barrier. 

 

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.

 

Isolation  of  neuronal  surfaces. 

 

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 which 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.

 

Neuronal development. 

 

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 mono layer is provided or the medium is conditioned by growing astrocyte cultures.  In vitro studies have also shown that glial cells can influence the prolection and branching patterns of nerve cells and may be instrumental in determining the neuronal polarity observed in vivo.

 

Electrophysiology  and  ion  transport. 

 

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.

 

Furthermore, release of potassium ions (K +) from astrocyte end-feet may play an important role 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 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 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.

 

Neurotransmitter  metabolism. 

 

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 (g-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.

 

Detoxification of ammonia. 

 

From the above description it follows that as astrocytes possess glutamine synthetase activity, they are also important in the detoxification of ammonia.

 

Phagocytosis. 

 

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.

 

Immune response. 

 

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 intefferon, produced by T lymphocytes, to express Ia antigens. Astrocytes and their precursors also respond, in culture, to glial maturation factor and produce interleukin 1; this, in turn, stimulates the proliferation of astro­cytes.

 

OLIGODENDROCYTES

 

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. 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 astrocvtes in that the ratio of filaments to micro-tubules 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.

 

Functions of oligodendrocytes

 

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 myelin­forming cell in the central nervous system is the oligodendrocyte, which first appears immediately before myelination begins. in the developing brain direct connection can be seen between oligo­dendrocytes and the myelin sheath as it forms. 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 sulphatides  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.

 

EPENDYMAL CELLS

 

The ependyma lines the ventricular system, the cerebral aqueduct and the central canal of the spinal cord. These epithelium-like cells are cuboidal or colurnnar and show 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 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 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 cyto skeleton is composed of 10 mm intermediate filaments, 4.6 mm microfilaments and occasional 24 mm 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 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 of 9 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

 

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.

 

 

Functions of ependymal cells

 

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 bQdies, 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.112 Ependymal cells may perform sensory and secretory functions in various animal species, but evidence for these activities in man is still not available.

 

 

MICROGLIAL CELLS

 

The profusion of names which 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. It has been established that microglial cells are present in the normal brain.

 

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.

 

 

Origin of microglia

 

The Origin of microglial cells has been the subpect 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's'4 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.517 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 rnembrane markers on entering the brain and then adapt to an entirely different environment.

 

Functions of microglia

 

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.sis 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. 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 phagocytic activity. This potential becomes manifest in pathological conditions.

 

 

BLOOD VESSELS AND THE BLOOD BRAIN BARRIER

 

Cerebral capillaries are fundamentally similar to those of other tissues, but there are important differences. There is a paucity of cytoplasmic vesicles in the endothelial cells, and the tight junctions between the endothelial cells differ from those of other tissue capillaries. In addition, astro­cytic foot processes surround each capillary. The lumen is lined by endothelial cells which display oval or elongated nuclei located in the thickened part of the capillary wall. The cytoplasm contains the usual set of organelles, of which mitochondria are the most abundant. Complex  membrane-bound structures, the Weibel-Palade bodies, are difficult to find. Cytoplasmic vesicles are scarce and fewer than in most other capillaries. The structure of the interendothelial connections is variable, but in general the adjacent cell membranes are parallel, and, towards the luminal end, the outer leaflets fuse to form tight junctions (zonulac occludentes).

 

The endothelial cytoplasm is richly endowed with  enzymes,  including  adenosine  tri­phosphatase, nicotinamide adenine dinucleotide, monoamine oxidase, acid and alkaline phos­phatases,  various  dehydrogenases,  DOPA decarboxylase and g-glutamyl transpeptidase. The wealth of these enzyme systems reflects the unique role played by the cerebral endothelium in the blood-brain barrier. Moreover, differences in the intensity of various hydrolytic enzymes at the luminal and abluminal cell membrane strongly indicate the polarity of endothelial function in the control of the blood-brain interface. Outside the endothelium lies a continuous basal lamina or basement membrane approximately 40-50 mm thick and composed of an admixture of substances, including type IV collagen, heparan sulphate proteoglycan, laminin and entactin. Astrocytic foot processes abut onto capillaries, forming a complete envelope in most cases: occasionally other cells may have direct contact with the basal lamina. Pericytes are completely surrounded by a duplication of the basement membrane,  and are frequently seen extending their processes around the capillary; their cytoplasm contains many lysosome-like bodies. The origin and function of pericytes remain to be established, although the view that they could give rise to microglial cells has gained some support.

 

Arterioles and small arteries differ from capillaries not only in their larger size, but also by the presence of smooth muscle cells in their walls. Outside the endothelium, one or two layers of smooth muscle cells are transversely orientated and sandwiched between thick basal laminae. Venules resemble large capillaries and the transition between the two types of vessel is difficult to identify.

 

Histological demonstration of blood vessels  Van Gieson's technique alone or combined with an elastin stain gives good results in demonstrating connective tissue components of vessel walls. The overall pattern of vascularisation is well demon­strated by reticulin stains. Of the enzymes, the allraline phosphatase reaction reliably identifies endothelial cells both by light and electron micro­scopy. The immunocytochemical demonstration of factor VII I-related antigen is now routinely used to define endothelial cells.  Mono­clonal antibodies against endothelial anglo­tensin-converting enzyme are also available.

 

 

The blood-brain barrier

 

The concept of a blood-brain barrier was first based upon the observation that intravenously injected vital dyes, like Evans (azovan) blue and trypan blue, entered and stained various organs, but not the brain. Later, ultrastructural studies with electron-dense tracers, such as horseradish peroxidase or lanthanum, demonstrated that tracers do not penetrate the interendothelial cell junctions in the brain, neither are they carried across  the  endothelial  cell  by  vesicular transport.  The morphological basis for the blood-brain barrier appears to reside in two features of endothelial cells: the presence of tight junctions and the paucity of cytoplasmic vesicles. The oversimplified single-membrane model, however, is no longer accepted; intercellular tight junctions, intracellular enzyme systems and the two endothelial cell membranes all contribute to the barrier effect. It has been demonstrated in vitro that astrocytic foot processes, unique to cerebral capillaries, could contribute to the barrier. Astrocytes are essential for the expression of the endothelial enzymes which play a role in transport mechanisms  and for the induction of barrier properties in vitro.

 

The blood-brain barrier does not pertain in all parts of the mammalian brain: a few, relatively small and usually periventricular structures are freely permeable to vital dyes and electron-dense tracers. These structures include the area postrema, median eminence, subcornmissural organ, pineal gland, subfornical organ, supraoptic crest and neurohypophysis. The blood vessels in these areas have ultrastructural, enzymatic and permeability features which are different from those in other areas of the brain.

 

It has been realised that the blood-brain barrier is more a regulatory interface between the blood flow and the cerebral parenchyma than a simple rigid physical barrier. The passage of a particular substance across the blood-brain barrier may depend upon various factors, including its lipid solubility, electrical charge, molecular size, dissociation constant,  affinity for a carrier molecule and the nature of the substance in relation to the capacity of the blood-brain barrier for active transport.

 

The function of the blood-brain barrier is threefold. First, it prevents or hinders the entry of most water-soluble substances into the brain; the permeation rates are usually determined by lipid solubility. Secondly, the blood-brain barrier promotes the transport of certain materials, including some hexoses and several amino acids, by stereo-specific carrier transport systems which are present in the cerebral endothelium. Thirdly, the blood-brain barrier plays an important role in the volume regulation of the central nervous system. This is achieved by two mechanisms which limit the bulk flow of water across the blood-brain barrier: these are the low hydraulic conductivity of capillaries and the high osmotic activity of the major solutes.

 

 

ThE CHOROID PLEXUS

 

The choroid plexus is composed of a vascular fold of the pia mater and an epithelial layer derived from the ependymal lining. There are four choroid plexuses, one in the medial wall of each lateral ventricle and one each in the roofs of the third and fourth ventricles. They have clearly defined attachments to the ventricular wall and their free edges are invaginated into the ventricles. The surface area of the choroid plexus is greatly increased by the many fronds, which in turn consist of tiny villous processes. The arteries supplying the choroid plexus branch out into capillaries, one for each villus, which then join to form a vein.

 

The epithelium consists of a single layer of cuboidal cells mounted on a basement membrane; in a few areas, however, pseudostratification or even true stratification may occur. Choroid plexus epithelium  can  be  identified  immunocyto­chemically by the presence of carbonic anhydrase 11(c) in the cytoplasm;  this enzyme probably plays a role in the production of cerebrospinal fluid (CSF). The round or oval nucleus of the cell is usually centrally located in the cytoplasm, which has the usual complement of organelles. Mitochondria are particularly numerous and are mainly in the apical portion of the cell: they provide the energy necessary for the active transport carried out during production of the CSF. Smooth and coated vesicles of various sizes are seen throughout the cytoplasm and these take part in the transport of materials. The apical surface of the epithelial cell is greatly increased in area by masses of microvilli and occasional cilia which tend to be grouped together. The lateral plasma membranes are bound together by complex connections: by tight junctions (zonulac occludentes) at the apical end, by zonulac adhaerentes, and by intricate infoldings at the basal end. Occasional phagocytes, the epiplexus or Kolmer cells, are seen on the surface of the choroid plexus epithelium; they may play a role in keeping this surface free of debris.

 

The fibrovascular core of the choroid plexus supporting the epithelium contains arachnoid cells; whorls of collagen fibres in the fibrous stroma become calcified with advancing age. Blood vessels of various sizes include small arteries, arterioles,  capillaries  and  venous  sinuses. Capillaries in the villi are fenestrated and their endothelial lining is very thin.

 

 

Functions of the choroid plexus

 

The main function of the choroid plexus is the production of the CSF, although a proportion of the CSF, estimated to be 10-20%, is derived from extrachoroidal sources. In man, approximately 500-700 ml is produced every day; of this only 140 ml can be accommodated at any one time: 25-30 ml in the ventricles and the rest in the sub­arachnoid space. Although it has been disputed whether the CSF is the result of passive dialysis or active secretion, evidence now favours the latter mechanism. Factors which are involved in the formation of CSF include pressure, serum osmolality, temperature, age, innervation of the plexus and prostaglandins.

 

THE MENINGES

 

The meninges covering the central nervous system are composed of three layers: the dura mater, the arachnoid mater and the pia mater. The arachnoid and pia jointly form the leptomeninges.

 

The dura mater (pachymeninx) is a tough, dense membrane which surrounds the brain. It has two extensions: the falx, between the two cerebral hemispheres,  and  the  tentorium  cerebelli, separating the contents of the posterior fossa from the rest of the brain. The cranial dura is closely attached to the skull; its two layers, the periosteal and meningeal dura, are fused and separate only to form the venous sinuses. In the spinal canal, however, the dura is separated from the vertebral periosteum by the epidural space, which contains fibro-fatty tissue and an epidural venous plexus.

The dura is formed by dense, interlacing bundles of collagen in which flattened fibroblasts are embedded. The central part contains more cells and occasional blood vessels. Its outer surface is covered by thin, overlapping cell processes and its inner border also has a covering of flattened cells. The subdural space is artefactual, since the dura and arachnoid are closely apposed in life with no appreciable gap between them.

 

The arachnoid mater has a variable thickness, in places being formed by several cell layers. Its outer, dural aspect is smoother than the inner, pial -aspect from which trabeculae emerge to bridge the subarachnoid space. The arachnoid cells are joined together by specialised contacts, including tight junctions, which ensure an effective physiological barrier impermeable to CSF.

 

The cells of the pia mater are similar to those of the arachnoid, but the pia itself is thinner than the arachnoid. Pial cells form a complete layer joined by desmosomes and gap junctions. The subpial space separates the pia from the glia limitans of the underlying neural tissue and the pia mater separates the subarachnoid space from the perivascular (Virchow-Robin) spaces of the brains.

 

Arachnoid villi are diverticula of the arachnoid mater and the subarachnoid space which extend into veins and venous sinuses of the dura. Arachnoid granulations are larger than villi and are visible to the naked eye, whereas villi are microscopical structures. Each villus or gran­ulation is coated on its venous aspect by endo­thelial cells and is bathed by venous blood. As the villus or granulation penetrates the dura, it forms a narrow neck which then expands to form a central core composed of channels and collagenous trabeculac. Towards the apex of the granulations there is a cap of arachnoid cells with wide channels running through to the coating endothelium. These structures are a major pathway for the drainage of cerebrospinal fluid, which percolates through the cores of the villi or granulations and is transported across the endothelium into the blood.

 

 

THE SUBEPENDYMAL PLATE

 

The subependymal plate has long been recognised as a layer of primitive cells beneath the ependymal lining of the lateral ventricles in the adult human. It is the remnant of the embryonal matrix (the subventricular zone).  Studies of the sub­ependymal plate in various animal species have revealed that in the fetus it gives rise to both neurons and glia, whilst after birth it is a source of glial cells oflly.  The cells of the sub­ependymal plate display ultrastructural features common to primitive cells: high nuclear-cytoplasmic ratio, dominance of free ribosomes over membrane-bound ribosomes and a general scarcity of organelles.  Mitotic activity persists into later adult life in various species, including primates.  Unfortunately, information on the human subependymal extrapolation from may be misleading.