Transcriptional Characterization of Glioma Neural Stem Cells Diva ...

1.1 Glial Cells in the Central Nervous System Introduction
and producing "calcium signals”. These signals can spread rapidly and thus
influence many neurons almost simultaneously. Also, since neurotransmitter
release depends on extracellular Ca 2+ concentrations, glial cells can contribute
to coordination of synaptic activity [10,17,73,516]. Finally, glial activation
and inflammation has been implicated in neurodegenerative diseases such as
Alzheimer’s disease, Parkinson’s disease and multiple sclerosis [269]. Glial cells
are usually divided into three categories: astrocytes, oligodendrocytes and mi-
croglial cells, each carrying functional as well as structural differences. Whilst
astrocytes have many processes of various shapes and appear to serve a home-
ostatic function, oligodendrocytes tend to have fewer and shorter processes,
their main function being production of myelin sheaths for axon insulation.
Microglial cells are much smaller than other glial cells and serve to the brain
similar functions that macrophages 1 serve to the rest of the body [73].
In addition to the three main kinds described above, there are other spe-
cialised forms of glial cells. Schwann cells are present only in the peripheral
nervous system and form myelin sheaths that influence axonal thickness, ax-
onal transport and neurofilament content [73]. Ependyma cells are epithelial
cylindrical cells that line the surface of the ventricles 2 of the CNS and are
responsible for the production, transport and absorption of cerebrospinal fluid
(CSF). These cells have recently become candidates for the location of the
neural stem cell niche [81,328]. Müller cells are glial cells that reside in the
vertebrate retina and have recently been observed to undergo dedifferentiation
in vitro into multi-potent progenitor cells in fish, chicken and mouse, to then
differentiate into a number of retinal cell types, although in vivo evidence is
not conclusive [144]. They have also been shown to act as a light collector in
the mammalian eye [53,298]. Bergman cells are astrocytes that reside in the
cerebellum and are responsible for the migration, dendritogenesis, synaptoge-
nesis and maturation of Purkinje neurons 3 [534]. Finally, pituicytes are glial
cells that reside in the posterior of the pituitary gland, where they participate
in the control of secretory events [434].
1 Macrophages are white blood cells that protect the body from harmful bacteria, particles
and dying cells by ingesting them in a process called phagocytosis.
2 The ventricular system of the brain is composed of four communicating ventricles filled
with cerebrospinal fluid that bathes and cushions the brain and communicates with the
central canal of the spinal chord.
3 Purkinje cells are very large neurons in the cerebellum that release the inhibitory neurotransmitter
gamma-aminobutyric acid (GABA) and are responsible for transmission of all
motor coordination signals.
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