| DEGENERATION
/ REGENERATION |
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Degeneration
In either the central or peripheral nervous system (CNS or PNS,
respectively) of mammals, if an axon of a neuron is focally destroyed (e.g. cut,
crushed, frozen, rendered anoxic), that part of the axon disconnected from
the cell body, which would be considered 'distal' relative to the lesion,
invariably degenerates. That event is called anterograde or Wallerian
degeneration. The part of the axon that remains connected to the cell
body may degenerate a short distance (e.g. a millimeter or less), but
usually it survives at least in the short term. However, there are often
reorganizational changes in the cell body of the damaged neuron, denoted
by the term chromatolysis. Collectively, all these changes proximal
to the lesion site are referred to as retrograde degeneration;
some of them may represent reorganization rather than degeneration
changes. |
Intact
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Degeneration
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Regeneration
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| This light micrograph shows degenerating myelin that is being stained in
a peripheral nerve. It is a longitudinal section that shows clumps of degenerating
myelin darkly-stained. If this nerve has been focally cut or crushed
proximally, the axons would also be degenerating, although that can not be
known based only on this picture showing the Marchi stain, which darkens
only degenerating myelin. |
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| In this
photomicrograph fragments of myelin are either partially degenerated blobs
(blue-black) or more completely digested (red) in a peripheral neuropathy
known as Guillain-Barré syndrome that developed after infectious
mononucleosis. In this case, the axons probably are still intact and only
the myelin surrounding them is being destroyed, which is typical of a
primary demyelinating disorder. |
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| Alternatively,
there are histological stains that are specific for the degenerating
products of the axon itself. Examples
are the Nauta and the Fink-Heimer techniques.
An example is shown in the photomicrograph at right.
The preterminal and terminal parts of degenerating axons are
stained black. Intact neural
tissue is unstained and appears as a white or light brown/gold color. |
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| The
electron micrograph shows that presynaptic terminals that are in the
process of degenerating typically become electron dense. They are
the two small, solid black profiles that are just below and to the right
of the three dark ring-shaped structures that are normal myelinated
axons. A dendrite including a spine extending from it - on which one
of the degenerating terminals is ending - have been artificially colored a gold or
light brown. |
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Finally, results of anterograde degeneration may be evident
by the absence of staining.
In the picture to the right the unstained medullary pyramid (right)
of a
person who had a lesion some years earlier that had damaged neurons on one
side of their brain at some point rostral to the medullary pyramid on the
same side. The Weil and the
Weigert histological techniques stain normal myelin a blue/black or purple
color, as can be seen in the medullary pyramid on the other side of the
midline (left). Because the axons
that form the medullary pyramid have undergone anterograde degeneration on
the side of the lesion, there is no myelin there to be stained, and it is
also smaller in size.
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Chromatolytic changes
in the cell body of a neuron whose axon has been damaged include:
i. dispersion of Nissl bodies to the outer side of the nuclear membrane
and to the perimeter of the cytoplasm,
ii. movement of the nucleus to an eccentric position within the cell body,
and
iii. often a swollen appearance to the cell body.
Can you identify those characteristics in the images at the right which
show chromatolytic neurons (as well as normal ones) stained in different
ways? |
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