Cells Of The Nervous System II: Neuroglial Cells
There are four main classes of
neuroglial cells within the central nervous
system (CNS): oligodendrocytes,
astrocytes, microglia and ependymal cells, all of which have different
functions. In contrast, in the peripheral nervous system (PNS), Schwann cells
are the glial cells involved in myelination and facilitating axonal
regeneration.
Astrocytes are small stellate cells that are found
throughout the CNS and classified
either morphologically or ontogenetically. They have many important functions
within the CNS and are not simply passive support elements.
•
They form
a structural and supporting framework for neuronal cells and capillaries by
virtue of their cytoplasmic processes, which end in close apposition not only to neurones but also to capillaries.
In this respect they form the glia
limitans – where the astrocytic foot processes cover the basal laminae around
blood vessels and at the pia mater.
•
They
maintain the integrity of the blood–brain barrier (BBB), by
promoting the formation of high-resistance junctions between brain capillary
endothelial cells (see Chapter 5).
•
They are
capable of taking up, storing and releasing some neurotransmitters (e.g.
glutamate, γ-aminobutyric acid [GABA]) and thus may be an important adjunct in
chemical neurotransmission within the CNS.
•
They can
remove and disperse excessive ion concentration in the extracellular fluid,
especially K+.
• They
participate in neuronal guidance during development (see Chapter 1), and may be
involved in the response to injury (see Chapter 49), and adult neurogenesis.
•
They may
have a role in presenting antigen to the immune system in situations where the
CNS and BBB are damaged (see Chapter 62).
• The most
common clinical disorder involving astrocytes is their abnormal proliferation
in tumours called astrocytomas. These tumours produce effects by
compressing adjacent CNS tissue and this presents as an evolving neurological
deficit (with or without epileptic seizures) depending on its site of origin.
In adults, the tumours most commonly arise in the white matter of the cerebral
hemispheres.
Microglial cells are the macrophages of the brain, and are found
throughout the white and grey matter of the CNS. They are phagocytic in nature
and are important in mediating immune responses within the CNS (see Chapter
62). They have a role in inflammation seen in some neurodegenerative disorders
of the CNS, such as Parkinson’s disease (see Chapters 42 and 60), where there
is great interest in whether they can be both neurotrophic as well as
neurotoxic (see Chapter 62).
Ependymal cells are important in facilitating the movement of
cerebrospinal fluid (CSF) as well as interacting with astrocytes to form a
barrier separating the ventricles and the CSF from the neuronal
environment. They also line the
central canal in the spinal cord
(see Chapter 5). These ependymal cells are termed ependymocytes to distinguish
them from those ependymal cells that are involved in the formation of CSF (the
choroid plexus) and those that transport substances from the CSF to the blood
(tanycytes).
Tumours of the ependyma (ependymomas
or choroid plexus papillomas) occur either in the
ventricles, where they tend to produce hydrocephalus (see Chapter
5), or in the spinal cord, where they cause local destruction of the neural structures.
Oligodendrocytes are responsible for the myelination of CNS
neurones, and are therefore found in
large numbers in the white matter. Each oligodendrocyte forms internodal myelin
for 3–50 fibres and also surrounds many other fibres without forming myelin
sheaths. In addition, they have a number of molecules associated with them that
are inhibitory to axonal growth, and thus contribute to the failure of axonal
regeneration in the CNS (see Chapter 49).
Clinical disorders of
oligodendrocyte function cause central demyelination which is seen in a number
of conditions including multiple sclerosis (see Chapter 62),
while abnormal proliferation of oligodendrocytes produces a slow-growing tumour
(an oligoden-droglioma) which tends to present with epileptic
seizures (see Chapter 61).
Schwann cells are found only in the PNS and are responsible
for the myelination of peripheral nerves by a process that involves the
wrapping of the cell around the axon. Thus, the final myelin sheath is composed
of multiple layers of Schwann cell membrane in which the cytoplasm has been
extruded. Unlike oligodendrocytes, one Schwann cell envelops one axon and
provides myelin for one internode. In addition, Schwann cells are important in
the regeneration of damaged peripheral axons, in contrast to the largely
inhibitory functions of the central neuroglial cells (see Chapter 48).
A number of genetic and
inflammatory neuropathies are associated with the loss of peripheral myelin (as
opposed to the loss of axons), which results in peripheral nerve dysfunction (demyelinat-
ing neuropathies; see Chapters 17 and 63). In addition, benign tumours
of Schwann cells can occur (schwannomas), especially in certain
genetic conditions such as neurofibromatosis type I, where there
is the loss of the tumour suppressor gene, neurofibromin.
These tumours are typically
asymptomatic but if they arise in areas of limited space they can produce
symptoms by compression of the neighbouring neural structures; e.g. at the
cerebellopontine angle in the brainstem or spinal root (see Chapters 8, 9, 54 and
55).
Finally, there is a group of rare
disorders, typically inherited, that cause a central abnormality of
myelination, which together are
called leucodystrophies.
