Central nervous system

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Central nervous system


The central nervous system (CNS) consists of the brain and spinal cord and is composed of neurones, neuronal processes, supporting cells of the CNS (glial cells) and blood vessels. The CNS is invested with meninges and is suspended in fluid, the cerebrospinal fluid (CSF) which is produced by specialised choroid plexus structures.


Macroscopically, all parts of the CNS are made up of grey matter and white matter. Grey matter contains most of the neurone cell bodies and their dendritic processes while the white matter contains the axons. The lipid-rich myelin sheaths around the axons accounts for the white appearance of the white matter.


Central nervous tissue consists of a vast number of neurones and their processes embedded in a mass of support cells, collectively known as neuroglia, which form almost half of the total mass of the CNS. These are highly branched cells that occupy the spaces between neurones; they have intimate functional relationships with the neurones, providing both mechanical and metabolic support.


Four principal types of neuroglia are recognised:



Central nervous tissue proper lacks collagenous supporting tissue which is confined to the immediate surrounds of penetrating blood vessels and to the meninges that invest the outer surface of the brain. The CNS also contains little extracellular material.


While the basic organisation of grey and white matter remains consistent throughout the brain, in detail they form complex arrangements of the basic components to give a rich microscopic anatomy (histology) which is highly related to the macroscopic anatomy and function. In this chapter after description of the specialised supporting cells and tissues, including CSF production (choroid plexus), we will illustrate the microanatomy of some major structures in the CNS.





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FIG. 20.2 Astrocytes
(a) Diagram (b) Immunohistochemical method for glial fibrillary acidic protein (HP)
(c) EM ×12 000 (d) EM ×57 500
Astrocytes are identified by immunohistochemical staining for a protein called glial fibrillary acidic protein (GFAP) in micrograph (b); these are the most numerous glial cells in grey matter. They have long, branched processes which occupy much of the interneuronal spaces in the neuropil. In grey matter, many of the astrocyte processes end in terminal expansions adjacent to the non-synaptic regions of neurones. Other processes of the same astrocytes terminate upon the basement membranes of capillaries; these perivascular feet cover most of the surface of the capillary basement membranes and form part of the blood-brain barrier as illustrated in the diagram (a). Similar foot processes invest the basement membrane between the CNS and the innermost layer of the meninges, the pia mater (see Fig. 20.9), forming a relatively impermeable barrier called the glia limitans. Astrocytes mediate metabolic exchange between neurones and blood and regulate the composition of the intercellular environment of the CNS.
All astrocytes contain bundles of intermediate filaments and microtubules. The intermediate filaments are formed of GFAP, which is characteristic of astrocytes. The astrocytes of grey matter have numerous short, highly branched cytoplasmic processes and are described as protoplasmic astrocytes. These are demonstrated in micrograph (b) by using immunohistochemical staining for GFAP. By contrast, the astrocytes of white matter have relatively few and straight cytoplasmic processes rich in intermediate filaments and are known as fibrous astrocytes.
Micrograph (c) shows an astrocyte A lying adjacent to a nerve cell body N in the cerebral cortex. The astrocyte cytoplasm contains many ribosomes, a little rough endoplasmic reticulum and a few small mitochondria and lysosomes. The origins of several cytoplasmic extensions C can be identified. The cytoplasm appears moderately electron-dense due to its content of intermediate filaments IF, which can be seen at higher magnification in micrograph (d). Typical of CNS grey matter, the adjacent neuropil Np contains numerous neuronal and glial processes in various planes of section; some myelinated axons M are included in the field.



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FIG. 20.3 White matter CNS myelin and oligodendrocytes (illustrations (e) and (f) opposite)
(a) H&E (MP) (b) H&E (HP) (c) TS, solochrome cyanin (HP) (d) LS, solochrome cyanin (HP) (e) EM ×13 000 (f) Schematic diagram
White matter consists of nerve fibres (axons), often myelinated by oligodendrocytes, organised in tracts, with supporting astrocytes, microglia and vessels. Oligodendrocytes were so named because original heavy metal impregnation methods showed that they only had a small number of short, branched processes (Greek: oligos = few, dendron = tree). It is now known that oligodendrocytes are responsible for myelination of axons and the processes originally described are the short pedicles that connect the cell body to the myelin sheaths.
A single oligodendrocyte can contribute to the myelination of up to 50 axons from the same or different fibre tracts, as illustrated in the diagram (f). Conversely, any one axon will require the services of numerous different oligodendrocytes because of the limited length of the myelin segments (internodes) produced by each oligodendrocyte. The mechanism of myelin sheath formation is very similar to that of Schwann cells in peripheral nerve (see Fig. 7.6). Oligodendrocytes are the predominant type of neuroglia in white matter, as well as being abundant in grey matter.
Micrographs (a) and (b) show CNS white matter, with the oligodendrocyte nuclei ON often having an artefactual perinuclear halo and a few larger astrocyte nuclei A. Micrographs (c) is of a myelin stain, with the transverse ring-shaped profiles of blue-stained myelin each surrounding an axon, unstained and not visible in this preparation. Micrograph (d) is of longitudinal fibres using the same stain. Oligodendrocyte nuclei ON can be seen as rounded red-stained profiles.
Oligodendrocytes aggregate closely around nerve cell bodies in the grey matter, where they are thought to have a support function analogous to that of the satellite cells which surround nerve cell bodies in peripheral ganglia (see Fig. 7.20).
The electron micrograph (e) shows an oligodendrocyte O lying adjacent to a nerve cell body N, with a neuronal dendrite D at the upper right. The oligodendrocyte contains prominent rough endoplasmic reticulum, ribosomes and Golgi apparatus G. The commencement of a cytoplasmic process C is seen. The remainder of the image shows the complexity of the neuropil Np, comprising glial and neuronal processes including myelinated axons M.
Myelin sheath formation begins in the CNS of the human embryo at about 4 months gestation, with the formation of most sheaths at least commenced by about the age of 1 year. From this time, successive layers continue to be laid down, with final myelin sheath thickness being achieved by the time of physical maturity.






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FIG. 20.6 Choroid plexus
(a) H&E (LP) (b) H&E (HP)
The choroid plexus is a vascular structure arising from the wall of each of the four ventricles of the brain and responsible for the production of cerebrospinal fluid (CSF). CSF drains from the interconnected ventricular cavities via three channels connecting the fourth ventricle with the subarachnoid space which surrounds the CNS. CSF is produced at a constant rate and is reabsorbed from the subarachnoid space into the superior sagittal venous sinus, via finger-like projections called arachnoid villi. Thus the CNS is suspended in a constantly circulating fluid medium which acts as a support and shock absorber.
Each choroid plexus consists of a branching system of blood vessels which run in fronds composed of collagenous tissue and covered by a cuboidal or columnar epithelium. The choroid plexus is therefore a villous structure. Micrograph (a) shows the choroid plexus CP within a ventricle of the brain V.
Micrograph (b) shows detail of one of the choroid plexus processes. The capillaries and vessels of the choroid plexus are large, thin-walled and sometimes fenestrated lying in a fibrous core. The epithelial cells rest on a basal lamina. At the ultrastructural level, long bulbous microvilli project from the luminal surfaces of the choroid plexus epithelial cells and their cytoplasm contains numerous mitochondria, features which suggest that the elaboration of CSF is an active process.
The mode of CSF secretion involves active secretion of sodium ions by choroid epithelial cells into the CSF, followed by passive movement of water from the local vessels. Continuous tight junctions (zonula occludens) between epithelial cells contribute to a blood-CSF barrier, preventing ingress of almost all other molecules.





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FIG. 20.8 Meninges
(a) Diagram (b) Dura mater, H&E (MP) (c) H&E (LP)
The brain and spinal cord are invested by three layers of supporting tissue, collectively called the meninges. The surface of the nervous tissue is covered by a delicate layer called the pia mater, containing collagen fibres, fine elastin fibres and occasional fibroblasts, separated from the astrocytic processes of underlying CNS parenchyma by a basement membrane. The basement membrane is completely invested by astrocytic processes, the two layers forming the impermeable glia limitans.
Overlying the pia mater is a thicker fibrous layer, the arachnoid mater, which derives its name from the presence of cobweb-like strands which connect it to the underlying pia; since the pia and arachnoid are continuous, they may be considered as a unit, the pia-arachnoid, also called the leptomeninges. The space between the pia and arachnoid is called the subarachnoid space and, in places, forms large cisterns. This subarachnoid space is connected with the ventricular system by three foramina in the fourth ventricle (in the brainstem), and CSF circulates continuously from the ventricles into the subarachnoid space.
The subarachnoid space is lined by flattened arachnoidal cells. The outer surface of the arachnoid mater is also lined by flat cells. As shown in the diagram, arteries and veins passing to and from the CNS pass in the subarachnoid space loosely attached to the pia mater and invested by subarachnoid meningothelium. As the larger vessels extend into the nervous tissue, they are surrounded by a delicate sleeve of pia mater. Between the penetrating vessels and the pia there is a perivascular space. In humans, the pia blends with the adventitia of the vessel as it penetrates the brain, thus separating the perivascular space from the subarachnoid space. This pia component is not present around the capillaries of the CNS.
External to the arachnoid mater is a dense fibroelastic layer called the dura mater D, micrograph (b), which is lined on its internal surface by flat cells. The dura is closely applied to but not connected with the arachnoid layer and there is the potential for a space, the subdural space, to develop between the two layers. In the cranium, the dura mater merges with the periosteum of the skull but also extends into the brain space as several folds. A large fold, the falx, extends along the midline from top of the skull into the space between the cerebral hemispheres while a horizontal fold, the tentorium, is attached to the posterior skull and extends into the space between the cerebral hemispheres and cerebellum. These folds help support the brain and contain venous sinuses, forming part of the brain’s venous return system.
Around the spinal cord, dura is suspended from the periosteum of the spinal canal by denticulate ligaments, the intervening epidural space being filled with loose fibrofatty tissue and a venous plexus.
The pia and arachnoid layers of the brain meninges are illustrated in micrograph (c). Pia mater P is attached to the surface of the brain and continues into the suclus S and around the penetrating vessels. The arachnoid mater A appears to be a completely separate layer and bridges the sulcus. Meningeal vessels lie in the subarachnoid space.

Aug 22, 2016 | Posted by in HISTOLOGY | Comments Off on Central nervous system
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