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.