Cells and connective tissues of the nervous system

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Cells and connective tissues of the nervous system

Supporting cells in CNS (neuroglia/glial cells/”nerve glue”):
Neurons in the CNS are supported by a number of nonexcitable cells (called neuroglia) as the NS lacks basic connective tissue; cells are smaller than neurons and are more numerous (>10x) - only occupy 50% of the CNS volumes. Function is to provide mechanical support, insulation and nourishment to neurons.

Neurons are postmitotic, but neuroglia are not  tumours are more likely to originate from the neuroglia.

a) Astrocytes:
small cell body with branches in all directions – ‘star shaped’ – projections cling onto neurons and capillaries to anchor and support them; most numerous of the neuroglia cells.
have may other functions (may depend on location) - electrical insulators (protect axon from neighbours); may function as antigen presenting cells; collect leaked K+ ions and neurotransmitters; secrete growth factors; secrete cytokines (possible role in healing of injury); from ‘glial scars’ in empty space following death or damage
two major types – protoplasmic astrocytes (Type 1; in gray matter; clearer cytoplasm; have short thick processes) and fibrous astrocytes (Type 2; in white matter; have large number of glial filaments and long thin processes)
both types have vascular end processes that are in contact with and surrounding blood vessels
develop from the prenatal radial glial cells

b) Microglia:
small ovoid cell with short processes scattered throughout CNS
not embryological related to other neuroglial cells – come from outside nervous system (develop from monocytes); migrate into nervous system during foetal development
special type of macrophage that remains inactive/resting state until migrates to sites of lesions  become activated as phagocytes
increase in number at sites of damage or neuron degeneration

c) Ependymal cells:
various shapes (columnar to cuboidal) – some are ciliated
line internal CNS cavities as a single layer of cells
three types – ependymocytes (line ventricles of brain and central canal of spinal cord); tanycytes (line floor of third ventricle); choroid epithelial cells (line surfaces of choroid plexuses; produce and secrete CSF)
movement of cilia help circulate CSF within cavities of brain and spinal cord

d) Oligodendrocytes:
small cell bodies, have less, but more delicate projections than astrocytes (up to 15, resemble an ‘octopus’)
line up along neuron fibres – wrap around them
responsible for formation of myelin sheath in CNS (Schwann cells do this in PNS)
function to insulate nerve fibre from surrounding extracellular fluid and speed up impulse conduction
develop from the t embryological radial glial cells
formation of myelin sheath begins at about 16th week

Supporting cells in PNS:
a) Satellite cells:
• surround neuron cell bodies
• play role in controlling chemical environment

b) Schwann cells (neurolemmocytes):
• flattened cells that surrounds and form myelin sheaths around peripheral nerve fibres
• winds repeatedly around nerve fibre
• functionally similar to oligodendrocytes (each oligodendrocyte myelinates up to 50 neurons, but Schwann cells only myelinate one)

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