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The ultrastrcture of wallerian degeneration in the severed optic nerve of the newt (Triturus viridescens).

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The Ultrastructure of Wallerian Degeneration
in the Severed Optic Nerve of the Newt
(Trifurus viridexcens)
Department of Anatomy, School of Medicine, Case Western Reserve
University, CEeveland, Ohio 441 06
Wallerian degeneration in the severed newt's (Tritums viridesc e n s ) optic nerve is complete between the 10-14th post operative day (p.o.d.1.
Consequently, the newt optic nerve displays one of the most rapid degenerative
responses yet reported for the central nervous system of vertebrates. In most
cases it also exhibits the speed of degenerative phenomena in the vertebrate
peripheral nervous system.
The degeneration of unmyelinated axons is most rapid and is completed by
2-3 p.o.d., compared to myelinated axons, most of which degenerate between
2c10 p.0.d. Myeliii ring formation (vesicular transformation) is the principal
form of lamellar breakdown and occurs in a highly organized manner which
can be clearly staged.
The glial cell response to Wallerian degeneration is two-fold : cytoplasmic
hypertrophy and myelin-lytic. Glial hypertrophy subsides by the 10-14 p.0.d.
with the ingrowth of numerous regenerating nerve fibers. The myelin-lytic response accounts for most of the myelin destruction. Leukocyte-like and microglialike cells also participate in myelin breakdown but to a lesser degree.
Regenerative repair ( Wallerian degeneration and regeneration) in the central
nervous system of reptiles and higher vertebrates is a slow and often times ineffective process (reviews : Clemente, '64; Guth
and Windle, '70; Heyl, '72; Schneider, '72),
a fact well illustrated in studies after optic
nerve transection or enucleation (Crevel
and Verhaart, '63; Kruger and Maxwell,
'69; Luse and McCaman, '57). Much more
extensive regeneration occurs in the CNS
of lower vertebrates (amphibians and
fishes). For example, the optic nerves of
salamanders, frogs and toads, in both
larval and aduIt stages, and teleost fishes
are capable of morphological and functional regeneration with good visual recovery (Matthey, '25; Sperry, '51, ' 6 5 ) .
The simple design of the optic nerve of
lower vertebrates with its parallel array of
axons and its regenerative capacity makes
it a valuable experimental system for the
study of neurogenesis, synaptogenesis and
neuronal specificity, especially at the electron microscopy (EM) level (Edds et al.,
ANAT. REC., 181: 267-286.
In a previous paper we described the
ultrastructure of the newt optic nerve preparatory to an analysis of degeneration and
regeneration in the visual system (Turner
and Singer, '74). We reported that the
newt's optic nerve appears to be more primitive than that of higher vertebrates. Only
one type of glial cell (ependymo-glial) is
present and their cell bodies are located
in the core of the nerve. They send radiating processes to the surface to form a glia
limitans beneath the meninges. The processes and their branches enwrap the numerous fascicles of naked axons, and the
less numerous, scattered myelinated fibers.
The ependymo-glial cell subserves the
functions of astrocytes and oligodendrocytes, in the latter case forming myelin
wrappings. The present paper reports our
Received May 13, '74. Accepted July 31, '74.
1Supported by grants from the American Cancer
Society, the National Institutes of Health and the Multiple Sclerosis Society awarded to MS, and by an
NIH Postdoctoral Fellowship (GM54013-02) awarded
to JET
2 Present address: Department of Anatomy, Bowman
Gray School of Medicine of Wake Forest Universlty,
Winston-Salem, North Carolina 27103.
findings in a study of degeneration in the
newt's optic nerve.
Adult newts (TTiturus viridescens) were
chosen for this study because of their renowned hardiness and their visual regenerative capacities (Matthey, '25; Stone and
Zaur, '40; Sperry, '43, '45). The animals
were anesthetized in chloretone solution
and the left optic nerve was severed according to the method of Sperry ('43). The
optic nerve was exposed through the roof
of the mouth. A small incision in its dural
sheath was made with a fine pointed knife
several millimeters distal to the eyeball;
the nerve was pulled out through the slit,
cut and the two stumps tucked back into
the sheath. This procedure is rather bloodless and avoids damage to the retinal blood
supply (Sperry, '43) and therefore retinal
degeneration, a result affirmed in our observations. The major bulk of the ocular
muscles was excised to prevent movement
of the eyeball lest it hinder regeneration.
After the operation, animals were kept in a
moist chamber for several days, then returned to aquaria at a constant water temperature of 252°C. The animals were fed
ground beef once a week.
The first several millimeters of the central stump of left (experimental) and right
(control) optic nerves were fixed for EM
study 2, 4, 6, 10, and 14 days after
transection. Each group consisted of three
animals. The vascular system was perfused
through the truncus arteriosus with 5-10
ml of the following solutions in order
( 1 ) amphibian Ringer's; ( 2 ) one-quarter
strength fixative; and ( 3 ) full strength
fixative. The perfusion apparatus consisted
of three 20 ml syringes connected by tubing and stopcocks feeding into a polyethylene tube which terminates in a glass
The fixative was Karnovsky's glutaraldehyde-paraformaldehyde preparation buffered to pH 7.4 with 0.1 M cacodylate
buffer (Karnovsky, '65). After 15 minutes
of fixation in situ the cranium was removed
to expose the brain, and the animals were
decapitated. Heads were immersed overnight in one-quarter strength fixative at
4 ° C . The following day optic nerves were
removed, washed in cacodylate buffer and
postfixed for two hours in 1% Os04 in
cacodylate buffer. After osmication the
optic nerves were washed in buffer, dehydrated in an ethanol series and embedded
in Epon.
Thick sections ( 1 f/.) and thin (600-900
A) were cut with glass knives on a PorterBlum ultramicrotome (MT2). The thick
sections were stained with toluidine blue in
borax buffer for examination in the light
microscope and the thin ones with uranyl
acetate and lead citrate (Millonig, '61).
Sections were photographed in a Zeiss
EM9A electron microscope. In order to
insure uniformity of results, ultrathin sections were taken from an area halfway between the two ends of the fixed portion of
the optic nerve.
Glial response
As mentioned earlier, the normal newt
optic nerve contains one type of glial cell
which we identified as a primitive ependyma1 (ependymo-glial) cell (Turner and
Singer, '74). Among other possible functions, this macroglial cell apparently subserves the functions of the astrocytes and
oligodendrocytes in the optic nerve of
higher vertebrates. For this reason we will
refer to it in this paper as an ependymoglial cell or just glial cell.
There is an immediate glial response to
the injury. It begins before 2 p.0.d. and is
initiated by movement of glial nuclei from
their central position in the nerve to a
more peripheral one and by a burgeoning
of cytoplasm which makes the glial processes more obvious (fig. 1 ) . There does not
appear to be an increase in the number of
glial cells at this time or at later periods.
By 4 p.0.d. the glial processes occupy a
great part of the degenerating nerve (fig.
4). The increase in the amount of glial
cytoplasm seems to peak at 6 p.0.d. (fig.
5 ) after which it appears to decline as regenerating fibers grow into the distal
stump. The first of these fibers are seen
about 4 p.0.d. (figs. 4, 5).
Another alteration in glial cytoplasm is
the appearance of dense core lysosome-like
vesicles and Golgi complexes (fig. 8 ) , not
prominent in the glial cytoplasm of normal
newt optic nerves (Turner and Singer,
'74). Similar lysosome-like material can
also be found within the degenerating
myelin sheath (figs. 3, 4, 8). There is a
gradual rise in lipid droplets within the
glial cytoplasm which reaches a peak by
6 p.0.d. and decreases between 6-14 p.0.d.
(fig. 1). At 6 p.0.d. the glial cytoplasm is
engorged with lipid droplets which can
readily be seen in thick sections by light
Conspicuous characteristics of the glial
cells during Wallerian degeneration are
signs of involvement in myelin and axon
destruction. Such signs are evidenced in
most sections. For example, figure 4 shows
myelinated axons in various initial stages
of degeneration. During degeneration the
myelin and its fragments appear to remain
enclosed within glial cell processes. As will
be noted later, the first sign of myelin degeneration is vesiculation of the adaxonal
cytoplasmic layer, followed by similar vesiculation in the perimyelin glial cytoplasm. As myelin degenerates, numerous
myelin figures in various terminal stages
of decomposition can be found within the
glial cytoplasm between 2-6 p.0.d. (figs.
1, 4, 8). We were unable to detect extra
glial myelin fragments although these may
also be present.
Myelinated fibers
The rate of degeneration varies per fiber,
but within 2 days ( 2 p.0.d.) the various
degenerative stages are apparent in the
nerve (figs. 1-3). One of the earliest signs
of myelin degeneration (stage 1 ) is the
appearance of numerous vesicles within
the adaxonal glial cytoplasm (figs. 1-2).
At the same time there is a reduction in
the number of myelin lamellae, and these
events appear to be inseparable. For example, figure 1 shows an intact myelinated
axon with 12 lamellae which is the characteristic number for internodal myelin of
the optic nerve (Turner and Singer, '74);
however, an adjacent myelin sheath showing adaxonal vesiculation contains only 5
lamellae, which is well below the normal
average (also figs. 2, 3, 4, 9). During stage
one, the myelinated axons are still intact
and show little sign of impending degeneration (fig. 1). In addition, there may be
some early signs of external vesiculation
in stage one.
The second stage is characterized by
axonal degeneration and an increased
spread of vesiculation to the glial cytoplasm surrounding the myelin sheath externally. The vesiculation in the outer glial
cytoplasm may initally be confined to cytoplasm within the enlarged outer mesaxon.
External vesiculation is accompanied by
further loss in myelin lamellae (fig. 3).
The vesicles themselves are clear to moderately dense and vary in size, the larger
ones are clear and sometimes hexagonal
while the smaller ones are more dense and
rounded (fig. 3). The external vesicles
resemble the adaxonal ones. The adaxonal
glial cytoplasm is greatly enlarged at this
time. The axon exhibits loss of microtubules and mitochondria and there is notable
reduction in its size as well as signs of
axon membrane (axolemmal) decomposition (fig. 3 ) . Most of the neurofilaments
remain intact and are concentrated as the
axon decreases in diameter (fig. 4 ) . The
axon is then rapidly destroyed often leaving a circular clear space within the degenerating sheath. At this stage, there is no
evidence that degeneration of the myelin
sheath in any of its parts occurred outside
of the glial cell. Instead, the reactivity of
glial cytoplasm immediately surrounding
the myelin suggests that the glia participate actively in myelin destruction. In
later stages the reactive zones of glial cells
and the enclosed degenerating axons and
sheaths are replaced by the burgeoning
cytoplasm of the glial processes (fig. 4).
The third stage continues the myelin
degeneration until lamellae are no longer
recognizable and only debris is found
within the original myelin areas (fig. 3).
The last of the myelin lamellae undergo
degeneration either by vesiculation, as described above, or by shearing into strands
(figs. 1, 4). In the latter case, electron
dense condensates are observed in the
strand, especially at its ends. Finally, the
only remnants of the myelin sheath which
remain at the end of this stage are areas
consisting of mixed electron dense and
electron lucent vesicles (fig. 3 ) . Some electron dense vesicles resemble lysosomes and
multivesicular bodies.
The final stage of myelin degeneration
is characterized by disappearance of the
myelin debris sometimes leaving semivacu-
olated spaces marking the original location
of the myelinated nerve fiber (fig. 4 ) .
Some of the spaces contain remnants of
formed structures and vesicles; others are
rather clear. The spaces appear to be digestion chambers (i.e. phagocytic vacuoles)
within glial processes which complete the
final breakdown of the sheath (also fig. 8).
By 4 p.0.d. most of the myelinated fibers
are in late stage two and early stage three
of degeneration by 6 p.0.d. only an occasional degenerating myelin figure remains.
Myelin figures often enclosing electron
dense bodies are seen 6-10 p.0.d. within
the cytoplasm of glial as well as in occasional microglia-like cells (fig. 7). For further qualification of the term "microgliallike" cells see section marked phagocytes
below. By 14 p.0.d. the nerve is essentially
free of these figures or any other signs of
myelinated fibers. At this time there are
many regenerating fibers (fig. 6).
cleus and its cytoplasmic matrix is much
denser than that of glial cells. Also, the
phagocytes contain numerous lysosomelike vesicles as well as numerous lighter
ones, but lack the characteristic microtubules and microfilaments found in glial
cytoplasm. Leukocyte-like cells apparently
participate in the early phases of degeneration by engulfing some myelin and axonal
fragments and forming phagocytic-like
vacuoles within its cytoplasm (fig. 9).
A second type of phagocyte is first seen
by 6 p.o.d., reaches a peak between 6-10
p.0.d. and diminishes between 10-14 p.0.d.
Its number is very small compared with
leukocyte-like cells which appear earlier.
It is distinguished by a small, more elongated, darker and less homogenous nucleus, with patches of heterochromatin dispersed about its periphery; its perikaryon
consists of a thin rim of cytoplasm with a
few projections (fig. 7). The density of its
cytoplasmic matrix is about that of the surUnrnyelinated axons
rounding glial processes; however, it does
By 2 p.0.d. most of the unmyelinated not contain the microfilaments and microaxons have disappeared, their spaces occu- tubules characteristic of glial cytoplasm
pied by expanded glial cytoplasm (fig. 1). (fig. 7). Its cytoplasm contains numerous
Many of the remaining unmyelinated dense core lysosome-like vesicles and nuaxons are swollen and relatively agranular merous small clear vesicles. Within the
in appearance (fig, 1). Between 2-4 p.0.d. cytoplasm myelin figures, presumably exdegeneration of unmyelinated axons is ogenous in origin, are present (fig. 7). This
complete and by 4 p.0.d. the only unmy- cell exhibits the characteristics of microelinated fibers in the degenerating nerve glial or brain macrophages described in the
are a few regenerating neurites (fig. 4 ) . normal (Lentz, '71 : Stensaas and Stensaas,
New ingrowing nerve fibers were recog- '68; Mori and Leblond, '69) and degenernized by their small diameter (0.1-0.2 p ) ating (Schultz and Pease, '59; Blinzinger
and by the predominancy of microtubules and Kreutzberg, '68; Hollander et al., '69;
(4-10) but few, if any, microflaments, Westman, '69; Matthews and Kruger, '73)
and a paucity of mitochondria (Peters and CNS of other vertebrate species. Like the
Vaughn, '67). The exact nature of decom- leukocyte-like cells, they may serve to supposition of the unmyelinated fibers was not plement the histolitic action of the glial
ascertained; presumably they are phago- cells, picking up scattered fragments.
cytized and destroyed by the glial processes
It is possible that the phagocytes dewhich enwrap them.
scribed above are only two stages in the
life cycle of one cell type. However, until
Non-glial phagocytes
we have further evidence they will be
Phagocytes are found in the degenerat- treated as separate cell types. Also, it has
ing nerve parenchyma, reaching a peak been conclusively shown that the first type
on or before 2 p.o.d., and quickly diminish of phagocyte (leukocyte-like) invades the
until few can be found by 4 p.0.d. They nerve from the surrounding perivascular
are not present in great numbers and ap- spaces (Turner and Singer, unpublished).
pear to be polymorphonuclear leukocytes
(Matthews and Martin, '71). They are
clearly distinguished from the glial cells
The ependymo-glial cell appears to be
(fig. 9 ) . The phagocyte has a lobed nu- the most active agent of myelin destruc-
tion in the newt optic nerve. This contrasts with the findings that principal glial
cell types (astrocytes and oligodendrocytes) in the degenerating CNS of higher
vertebrates take little part in phagocytic
phenomena (Bignami and Ralston, '69 ;
Bunge et al., '60; Gonatas et al., '64;
Kruger and Maxwell, '69; Lampert and
Cressman, '66; Skoff and Vaughn, '71;
Vaughn, '65; Vaughn et al., '70; Vaughn
and Pease, '70). However, a recent report
by Nathaniel and Nathaniel ('73) ascribes
more of a phagocytic role to astrocytes in
crushed rat dorsal roots located within the
spinal cord. Usually, myelin degeneration
in the CNS of higher vertebrates has to
await the arrival or activation of extraneuronal phagocytes, resident microglia or
multipotential glia (Bignami and Ralston,
'69; Bunge et al., '60; Konigsmark and Sidman, '63; Gonatas et al., '64; Matthews and
Kruger, '73; Oehmichen et al., '73; Skoff
and Vaughn, '71; Vaughn, '65; Vaughn et
al., '70). Again it must be brought to mind
that the ependymo-glial cells are the only
glia in the newt's optic nerve, and that glial
processes form the myelin lamellae; no
oligodendrocytes or other glial cells are
present (Turner and Singer, '74). The
ependymo-glial cell shows an early reaction to nerve injury. The nuclei move to a
more peripheral position without increase
in cell numbers. This agrees with the findings from degenerating rat optic nerves in
which numbers of astrocytes and oligodendrocytes remain constant but their cytoplasm increases to compensate for loss of
nervous tissue (Vaughn and Pease, '70;
Skoff, '70). The peripheral processes increase in size and the burgeoning ependymo-glial cytoplasm resembles a glial scar
(gliosis) which is reported to form in other
degenerating CNS structures of higher
vertebrates (Bignami and Ralston, '69;
Kruger and Maxwell. '69; Lampert and
Cressman, '66; Vaughn, '65; Vaughn and
Pease, '70). However, there is a gradual
withdrawal of glial cytoplasm to its original state. The glial hypertrophy response,
up to this last stage, is typical of other degenerating CNS structures; however, the
removal of the scar is quite unusual. Perhaps it can be explained by the fact that
prior to the peak of scar formation a few
ingrowing "pioneer" nerve fibers establish
themselves between the expanding glid
processes and "spearhead a path through
which other fibers follow. We could speculate that an interaction between the ingrowing nerve fibers and glial membranes
signals a displacement of the glial scar;
however, we have no proof that this is true.
Accompanying increases in glial cytoplasm is the appearance of myelin vesiculation in the adaxonal and perimyelin regions. This type of myelin degeneration
has often been reported in other CNS structures (Bunge et al., '60; Gledhill et al.,
'73; Harrison et al., '72; Kruger and Maxwell, '69; Lampert, '65) as well as in peripheral nerves (Thomas and Sheldon; '64;
Masurovsky et al., '67; Nathaniel and
Nathaniel, '73). The vesicular transformation of myelin lamellae and its subsequent
digestion occur in a sequential manner
with remarkable orderliness and can thus
be staged. The intimate relationship of
glial processes to the myelin sheaths
(Turner and Singer, '74) coupled with the
speed and intensity of the glial cell degenerative response may, in part, explain the
remarkable orderliness in the reduction of
myelin lamellae as well as subsequent degenerative phenomena. The myelin remains enclosed within the glial cells during stages 1 and 2 and its initial destruction during stages 3 and 4 is very intense
and involves breakdown and still later
sequestration of the products in glial lysosome-like vesicles. During this period of
breakdown and before these digestive vacuoles appear, the relation between the degenerating myelin and the glial processes
may be variously interpreted. It may be
that the myelin remnants are released
from their intraglial position early or later
(for example, in stages 3 and 4 ) and then
secondarily phagocytized by other adjacent
glial processes and rnicroglial cells. Yet,
we have not seen myelin fragments in an
extracellular position; and so, such a
phase, if i t does exist, is possibly transitory
and its detection elusive. We tend to favor
the possibility which seems more consistent
with our findings and our previous experiences on the role of the Schwann cell in
Wallerian degeneration in the newt's peripheral nerve (Singer and Steinberg, '72),
namely that most of the myelin and its enclosed axon, enwrapped by the ependymo-
glial processes, are destroyed in situ by the
glia; and that only occasional particles
may escape into surrounding extracellular
spaces. More convincingly, our preliminary
ultrastructural studies indicate that acid
phosphatase activity appears only in the
adaxonal glial cytoplasm at 2 p.0.d. and is
localized in the adaxonal vesicles mentioned earlier in this paper.
Indeed, in stages 3 and 4, there are
many circular ghosts or semi-vacuolated
spaces within the glial cytoplasm which remain for a short period and are essentially
outlines of previous sheaths. Apparently,
chemical degradation of myelin, initiated
in stages 1 and 2, is very intense in stages
3-4 which is similar to the situation in the
PNS (Johnson et al., '50; Majno and
Karnovsly, '58; Nobuck and Reilly, '56).
The end result of myelin decomposition in
the newt's optic nerve is the formation of
numerous lipid droplets, in this case found
mostly in glial cytoplasm, which has also
been reported in other demyelinating
events (Bignami and Ralston, '69; Bunge
et al., '60; Gledhill et al., '73; Gonatas et
al., '64; Lampert and Kies, '67).
There is a marked resemblance in the
activity of the glial cell in optic nerve degeneration to that of the Schwann cell in
peripheral nerve degeneration. In the latter
case, Singer and Steinberg ('72) reported
burgeoning of Schwann cytoplasm as the
initial response accompanied by breakdown of the apposed adaxonal myelin and
axolemmal membranes forming a highly
reactive zone. The reactive zone was interpreted as Schwann cytoplasm carrying
lytic agents intermingled with the axoplasm (also Nathaniel and Pease, '63a,
'63b; Schlaupfer and Hager, '64; Wechsler
and Hager, '62). In subsequent stages, the
myelin is broken into ovoids along the line
of the clefts; but the myelin remains enclosed within Schwann cytoplasm where it
is digested. Thus, Wallerian degeneration
of peripheral myelin resembles that of central myelin in that the cell of myelin origin,
in one case the Schwann cell and in the
second the ependymo-glial cell, preserves
its morphological association with the rnyelin and at the same time destroys it.
The most striking difference between
Wallerian degeneration in the newt's optic
nerve and that in CNS structures of higher
vertebrates is the time taken to complete
the phenomenon. Kruger and Maxwell
('69) found some intact myelin in the alligator optic nerve 20 months after enucleation. Crevel and Verhaart ('63) report that
myelin degeneration took several months
in the enucleated cat optic nerve. Luse and
McCaman ('57) report that only the early
signs of myelin phagocytosis are apparent
100 p.0.d. in enucleated rabbit optic
nerves. Also, Lampert and Cressman ('66)
report that in the severed rat spinal cord
complete myelin degeneration takes place
only in the peritraumatic region, but collapsed intact myelin is found at 52 p.0.d.
in other affected spinal cord areas. In contrast, myelin degeneration in the newt
optic nerve is rapid and complete within
10-14 p.0.d.
Unmyelinated axons are the first nervous structures to degenerate completely
(by 2-3 p.0.d.) and their loss is rapid compared with the myelinated fibers, which is
in agreement with the speed of degeneration reported for unmyelinated peripheral
nerves (Dyck and Hopkins, '72; Brat et al.,
'72). Also, Kruger and Maxwell ('60) report that all unmyelinated axons in the
degenerating alligator optic nerve are gone
by one week post enucleation.
Although the newt optic nerve is a CNS
structure (Cajal, '06, ' 2 8 ) , i t appears more
attuned to the PNS time sequence for completion of myelin degeneration. For example, Luse and McCaman ('57) found
marked phagocytosis by 16 p.o.d., and by
100 p.0.d. little myelin debris remained in
the severed rabbit tibia1 nerve. However,
newt optic nerve myelin decomposition
follows most closely the time sequence for
degeneration reported by Nathaniel and
Pease ('63) in severed rat dorsal roots.
Consequently, the newt's optic nerve, during Wallerian degeneration, displays one
of the most rapid and efficient CNS degenerative systems yet reported €or vertebrates
and in some cases i t equals or surpasses
the efficiency of degenerative phenomena
in the vertebrate PNS.
Non-glial cell types were also observed
in the degenerating nerve, namely leukocyte-like and microglia-like cells. The
quick entrance and exit of phagocytic
leukocytes is an expected event, for the
initial lesion severed numerous capillaries
which course along the sudace of the
nerve, and is in agreement with other
studies in which allergic inflammatory
phenomena or lesions resulted in a leukocytic invasion of affected CNS areas
(Schultz and Pease, '59; Konigsmark and
Sidman, '63; Lampert and Carpenter, '65;
Astrom et al., '68; Bunge et al., '69). Then
there is the appearance of a cell resembling microglia between 6-10 p.0.d. and its
disappearance by 14 p.0.d. which agrees
with the findings of Schultz and Pease
('59) and Hollander et al., ('69). Because
of their low population density and transient appearance of these non-glial cells,
we do not believe them to be principal participants in Wallerian degeneration of the
newt optic nerve as they apparently are in
other degenerating systems (Hollander et
al., ' 69; Konigsmark and Sidman, '63;
Matthews and Kruger, '73; Oehmichen et
al., '73; Vaughn, '65; Westman, '69).
Therefore, for reasons stated earlier, the
ependymo-glial cell appears to be the most
active, consistent agent of myelin destruction; and thus, responsible for rapid optic
nerve degeneration.
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Optic nerve two days after transection showing the initial stages of
myelin degeneration. The first myelinated axon ( M A ) appears to be
intact while a second one (DM1) shows the first signs of myelin degeneration (stage one). Note the numerous vesicles ( V ) within the
adaxonal glial cytoplasm of DMI with only a few external to it. Also
note that the axon of DMI is still intact and shows little sign of
degeneration. A second but infrequent type of myelin degeneration
characterized by a shearing of the outermost myelin lamellae into
short strands ( S ) is shown just below DMI. Glial cell ( E ) cytoplasm
( EC ) has begun t o invade those areas originally occupied by unmyelinated axons ( D U ) which can be seen in various stages of degeneration (arrows). Note that myelin, in later stages of degeneration, is
found i n phagocytic vacuoles within glial cytoplasm (DM2). A lipid
droplet ( L D ) is located within the glial cell cytoplasm. Bar equals 1 p.
James E. Turner and Marcus Singer
Optic nerve two days after transection showing a longitudinal section of degenerating myelinated axon (DA). Note that the degenerating axon (DA) is displaced to one side by the numerous vesicles
( V ) found within the adaxonal glial cytoplasm. Also note the surrounding burgeoning glial cytoplasm (EC). Bar equals 1 p.
3 Optic nerve two days after transection showing a continuation of the
stages of myelin degeneration (DMI-DMI) begun in figure 1. Late
stage two of degeneration is demonstrated in DMI and is characterized
by increased external vesiculation. The axon (DMI) may also show
some early signs of decreased diameter, decreased cytoplasmic organelles and increased agranularity. Also external vesiculation ( V ) is
prominent about DMz. The late second stage of degeneration is seen
in DMz, the axon of which has completely degenerated leaving a n
empty space (arrow). Also the intact myelin lamellae are fewer in
DMz. DMa represents stage three in the myelin breakdown sequence
which is characterized by the lack of intact myelin lamellae and the
presence of only vesicles, some containing dense lysosome-like material (L), within the circular area originally formed by the intact
myelin sheath. Also note two late stage degenerating myelin figures
in the glial cytoplasm (DM4 and DMs). Bar equals 1 p.
James E. Turner and Marcus Singer
Optic nerve four days after transection. All degenerating unmyelinated axons have been removed and replaced by increasing amounts
of burgeoning glial cytoplasm (EC). The only unmyelinated fibers are
those of new ingrowing neurites ( N F ) . Very few intact myelinated
axons (MA) can be found for most of them are in various stages
of degeneration (DA and DM). The top two myelinated axons (DMI)
are in late stages of degeneration and appear almost as semivacuolated areas. A dense core lysosome-like vesicle ( L ) is seen within
the adaxonal glial cytoplasm of a degenerating myelinated axon
( D M ) . Here again are seen signs of a shearing phenomena of the
outermost myelin lamellae ( S ) . Bar equals 1 ,u.
Optic nerve six days after transection showing in more detaiI how
the burgeoning glial cytoplasm (EC) h a s begun to occupy most of
the degenerating nerve, forming a glial scar-like structure. A new
regenerating nerve fiber ( N F ) can be seen growing between the
tortuous folding of the glial membranes. Bar equals 1 F .
James E. Turner and Marcus Singer
Optic nerve ten days after transection, showing that Wallerian degeneration is almost complete a t this time. Note that regenerating nerve
fibers ( N F ) have begun to replace most of the burgeoning glial cytoplasm ( E C ) . Bar equals 1 p.
Cross section of optic nerve ten days after transection showing a
microglial-like cell (MG). The cytoplasmic matrix density is the same
as that of the surrounding glial processes (EP); however, its cytoplasm does not contain the characteristic microfilaments and microtubules found in the glial cytoplasm. The most conspicuous feature
of the microglial-like cell is the presence of numerous circular associations of myelin lamellae (DM) in what appears to be various stages
of degeneration. Also noted in the cytoplasm are numerous small,
round mitochondria ( M ) , a Golgi complex ( G ) , several dense core
lysosome-like vesicles ( L ) and numerous small clear vesicles (arrow).
Bar equals 1 p .
James E. Turner and Marcus Singer
Optic nerve two days after transection showing a type of degeneration characteristic of the late stages of myelin digestion as is occurs
within the glial cytoplasm ( E C ) . Once engulfed, the degenerating
myelin figures, in phagocytic-like lysosomic vacuoles, are condensed
until nothing remains (arrows). Bar equals 1 ,u.
Optic nerve two days after transection, showing three degenerating
myelinated fibers ( D M ) within a phagocytic leukocyte. Note the presence of numerous lysosomes ( L ) within the leukocyte cytoplasm.
Degeneration within the leukocyte appears to follow the sequence
described i n figures 1-4. Bar equals 1 p.
James E. Turner and Marcur; Singer
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optics, wallerian, ultrastrcture, degeneration, nerve, newt, viridescens, severed, triturus
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