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Glial Cells
Most neurons are surrounded by glial cells (neuroglia), the other cell
type found in the nervous tissue. Glial cells are the supportive cells of
the nervous system and are 10 times more numerous than neurons. The
most well defined role for neuroglia is to provide structure to the
delicate nervous tissue. They fill the space between neurons, serving as
mortar or “glue” and thus hold nervous tissue together. Unlike neurons,
glial cells retain the ability to divide throughout one’s lifetime. When
neurons are injured, neuroglia is stimulated to divide and form glial
scars. Glial cells have different shapes and sizes and their processes are
indistinguishable in contrast to the distinct axon and dendrites found in
neurons.
There are 6 types of glial cells, 4 types are found in the CNS and 2 types
in
the
PNS.
The
CNS
neuroglia
is:
astrocytes; oligodendrocytes; microglia, and ependymal cells.
The 2 types of glia found only in the peripheral nervous system (PNS)
are satellite cells and Schwann cells.
CNS Glial Cells
Astrocytes are star-shaped neuroglia and are the most numerous cells
in the central nervous system. They make up half of all cells in the brain.
Astrocytes provide a structurally supportive framework for neurons with
their processes wrapping most non-synaptic regions of neurons in gray
matter and covering the entire outer surface of the brain to form the
glial – pia (connective tissue meninx) interface. Astrocytes help form the
protective blood-brain barrier by encircling CNS capillary endothelial
cells and stimulating the cells to form tight-junctions. They help to
maintain the concentration of chemicals in the extracellular space and
remove excess signaling molecules. Astrocytes also react to neural tissue
damage by forming scar tissue in the damaged space.
Oligodendrocytes are glial cells of the CNS that wrap and insulate
axons and give the CNS white matter its characteristic glossy, white
appearance. Oligodendrocytes have a large soma with up to 15
processes. The processes reach out to axons of nearby neurons and wrap
around them (like wrapping tape around a pencil) forming a high
resistance sheath called myelin. Myelin insulates a small region of the
axon (prevents ions from leaking out into the extracellular fluid), which
facilitates signal propagation down the axon towards the synaptic
terminal. A single oligodendrocyte’s processes will wrap axons of
numerous different neurons. Processes from many different
oligodendrocytes contribute to the myelin sheath of a single neuron’s
axon.
Microglia are small highly mobile, phagocytic neuroglia that protect
nervous tissue from pathogen infection, remove debris and waste, and
may play a role in remodeling of the synapse that occurs during
development and with learning. About 10-15% of CNS glial cells are
microglia. Microglia are derived from monocytes and thus are more
closely related to white blood cells than to the other glial cells. Since
cells of the immune system cannot penetrate the blood brain barrier,
microglia serve as brain macrophages, destroying foreign invaders,
promoting inflammation and destroying cancer cells and cells infected
with virus. Clusters of microglia in nervous tissue provide pathologists
with evidence of recent injury.
Ependymal cells are cuboidal-shaped glial cells that are joined
together to form a continuous sheet lining the fluid-filled ventricles and
central canal of the brain and spinal cord. Ependymal cells produce and
secrete cerebrospinal fluid (CSF), the fluid that bathes the tissues of the
CNS. The basal side of the cell has rootlets that anchor the cells to the
underlying tissue. The apical surface is marked by cilia, which helps
circulate the CSF.
PNS Glial Cells
The remaining two glial cells, Schwann cells and satellite cells are found
solely in the peripheral nervous system.
Schwann cells are analogous in function to oligodendrocytes (found in
the CNS). They insulate the axons of peripheral nerves in one of two
ways. A Schwann cell can wind its way round and round the axon (up to
100 times), while squeezing its cytoplasm out of the way (much like a
toothpaste tube could be wrapped around a pencil), forming a myelin
sheath. Like myelinating a single fiber in the CNS, which requires many
oligodendrocytes, a complete myelin sheath in the PNS requires many
Schwann cells. Schwann cells can also envelop PNS axons without
forming a myelin sheath. Instead of wrapping a single axon many times,
the Schwann cell forms an envelope around a bundle of unmyelinated
axons.
Additionally, Schwann cells can also assist in the regeneration of a
damaged peripheral nerve. If a peripheral nerve is damaged, it may
regenerate if its soma is undamaged and the neurilemma (the plasma
membrane of the Schwann cell) enveloping it is intact.
Satellite
cells
are
found
surrounding
neural
somas
in
peripheral ganglia (collections of cell bodies located outside the CNS).
Satellite cells resemble CNS astrocytes and are thought to have similar
functions, providing structural support and regulating the chemical
environment.