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Proliferation of non-neuronal cells in spinal cords of irradiated immature rats following transection of the sciatic nerve.

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Proliferation of Non-neuronal Cells in Spinal Cords of
Irradiated, Immature Rats Following Transection
of the Sciatic Nerve
Department of Anatomy, UniveTsity of Arkansas Medical Center,
Little Rock, Arkansas 72201
Transection of a peripheral nerve not only elicits changes in the
injured neurons but also results in an increase in non-neuronal cells, considered
by most workers to be neuroglia, in the region of these neurons. Since studies in
this laboratory have shown that the neuroglial population of spinal cords of immature rats can be reduced markedly by ionizing radiation, the present investigation was undertaken to determine if this reaction would occur in the irradiated
spinal cord following transection of the sciatic nerve. In order to answer this
question the sciatic nerve was sectioned unilaterally at 17 days of age (14 days
post-irradiation ). Sham-irradiated littermates served as controls. Light microscopic examination showed an increase in non-neuronal cells throughout the
gray matter on the side of axotomy in spite of a decreased neuroglial population
in the 2,000 R and 3,000 R groups. These cells were scattered in the neuropil
or were adjacent to injured neuronal perikarya in the anterior horn. Qualitatively similar reactions occurred in the 500 R and 1,000 R groups and in shamirradiated controls. Whether the magnitude of response is the same in all groups
is currently under investigation, as are questions dealing with the origins of the
reactive cells.
Damage to peripheral nerves brings
about changes in non-neuronal cells in
areas occupied by nerve cell bodies whose
axons are injured. These changes consist
of reactions in the non-neuronal cells present at the time of injury as well as increases in the population of non-neuronal
cells. With regard to the existing cells,
Sjostrand ('66) reported hypertrophy of
astrocytes and microglia in hypoglossal
nuclei of rabbits in response to crushing
the right hypoglossal nerve. Hypertrophic
astrocytes and oligodendroglia were observed in hypoglossal nuclei following
hypoglossal axotomy in adult rats (Watson,
'72). An increased number of non-neuronal cells around injured neuronal perikarya has been observed by these investigators and others following axotomy or
crushing of the hypoglossal nerve (Sjostrand, '65, '71; Watson, '65; Adrian and
Smothermon, '70), the facial nerve (Cammermeyer, '65; Watson, '65; Blinzinger and
Kreutzberg, '68) and the sciatic nerve
(Friede and Johnstone, '67; Kerns and
ANAT. REC., 181: 799-812.
Hinsman, '73a). Watson ('65) identified
the cells making up this added population
as oligodendroglia, whereas Sjostrand ('65,
'71), Friede and Johnstone ('67) and Kerns
and Hinsman ('73a) classified them as microglia. In contrast, Adrian and Smothermon ('70) considered these cells not as
endogenous cells of the nervous system
but as leucocytes that had migrated into
the nervous system in response to trauma.
Although the identity and origin of the
proliferative cells is unsettled, the consensus based upon the studies cited above
as well as those by Fernando ('71, '73)
and Kerns and Hinsman ('73b) is that
they are neuroglia of endogenous origin.
Such origin raises a question regarding the
manner in which neural tissue can respond to injury when the neuroglial population is already reduced at the time the
injury is inflicted. Reduction in neuroglial
population can be achieved in rat lumbosacral spinal cord by exposure to ionizing
radiation when the animal is three days
Received July 15, '74. Accepted Sept. 17,'74.
old (Gilmore, '63b, '64). The type and
severity of these radiation-induced changes
can be modified, ranging from necrosis
when exposed to 4,000 R or more (Gilmore, '65; Gilmore and Arrington, '67)
to no discernible alterations when exposed
to 1,000 R or less (Rodgers, '65; Gilmore,
'73). In the intermediate dose range (2,000
R) it is possible to delay development of
the neuroglial population for several weeks
postnatally, after which the cells develop
and myelination ensues (Gilmore, '66).
The capability of inducing this predictable
period of neuroglial hypoplasia provides a
model which can be used to evaluate
changes occurring in the central nervous
system following injury to a peripheral
nerve. Neuroglial hypoplasia in the lumbosacral spinal cord provides an opportunity
to study the reactions of non-neuronal cells
to transection of the sciatic nerve, since
i t is known that the motor axons in the
sciatic nerve of the rat have their nerve
cell bodies in spinal cord segments L4
through L6 (Kajzawa and Takahashi, '70).
In addition, this experiment is designed to
evaluate the manner in which the apparently normal but irradiated (1,000 R or
less) nervous system can react to an additional challenge. This information could
add some insight into the question of
whether or not the spinal cord receiving
1,000 R or less is indeed normal, or if it
has been altered in such a fashion that it
cannot respond to transection of the sciatic
nerve. Finally, one additional purpose is
to study the response of non-neuronal cells
to transection of the sciatic nerve in the
immature animal; all previous investigations cited above have dealt with the mature or nearly mature animal.
Litters of Charles River CD@ rats, each
containing a maximum of eight irradiated
and two non-irradiated animals, were used
for this study. All irradiations were carried
out on the third postnatal day in such a
manner that only a 5 mm length of lumbosacral spinal cord was exposed to ionizing
radiation (Gilmore, '63a). The x-rays were
derived from a Philips Contact Therapy
Apparatus operating at 50 KVP and 2 ma
(filter added, 0.25 mm Al; HVL, 0.16 mm
A single dose was administered at the
rate of 722 R per minute at a TSD of 8 cm.
The animals were divided into groups to
receive an exposure of 500 R, 1,000 R,
2,000 R, or 3,000 R. Sham-irradiated rats
served as controls.
Fourteen days following irradiation (17
days of age) the right sciatic nerve was
exposed and transected as closely as possible to its site of emergence from the
greater sciatic foramen. The surgery was
carried out under PenthraneB anesthesia.
The fourteen-day interval was selected because earlier studies had shown that the
vascular bed was normal or near normal
(Gilmore, '69a) at this interval after irradiation. The surgical wound was closed
and the animals were allowed to survive
until three days post-operative at which
time they were anesthetized (Nembutals)
and then perfused with phosphate-buffered
formalin (pH 7 ) via the abdominal aorta.
The period of three days following surgery
was selected because many investigators
including Sjostrand ('65), Watson ('65),
Friede and Johnstone ('67) and Kerns and
Hinsman ('73a) found that the maximum
increases in non-neuronal cells occur during the first two to five post-operative days.
In addition, littermates of the animals
undergoing surgery were perfused as described on the day of surgery in order to
provide information on the status of the
spinal cord at the time of transection. The
numbers of animals in each group are
shown in tabIe 1. After perfusion-fixation
the lumbosacral spinal cord and its surrounding vertebrae were removed and immersed in fixative for an additional period
of at least 24 hours. The spinal cords were
then dissected from the vertebrae, dehydrated, embedded in paraffin, sectioned
(8 p ) and mounted in a complete serial
or interrupted serial fashion. Light microscopic evaluation was then made on these
Numbers of animals used in study
Time of autopsy
Amount of
Day of
Three days
3,000 R
2,000 R
500 R
80 1
anterior horn exists 14 days post-irradiation when the sciatic nerve is transected,
as shown in figure 2A from an animal
killed on the day of surgery. Comparison
of this area on the intact side (fig. 2B)
with that on the transected side (fig. 2C)
shows that in spite of this radiationNon-irradiated animals
induced decrease in number of neuroglia,
By 17 days of age, when surgery is per- an increase in non-neuronal cells is evident
formed (fig. lA), the motor neurons of three days following axotomy. As in the
the anterior horn are mature in appear- non-irradiated animals (figs. 1A-C), the
ance. The Nissl material is well-defined, reactive cells are perineuronal, as well as
and the presence of more than one nu- scattered throughout the neuropil. The recleolus is a rarity. Neuroglia are abundant, active cells are present throughout the gray
but only a few are situated so that their matter and are readily apparent in the
nuclei immediately adjoin the neuronal substantia gelatinosa (figs. 7A,B).
2,000 R. Earlier studies in this laboraTransection of the right sciatic nerve on tory have shown that this amount of radiathe 17th postnatal day results in marked tion initially delays neuroglial development
changes in the spinal cord on the side of and myelinogenesis. By one month postaxotomy (fig. 1C). The most striking alter- irradiation, however, the spinal cord apation occurs in the anterior horn in the pears to have its normal complement of
area of motor neurons whose axons con- neuroglia and myelin (Gilmore, '66). By
tribute to formation of the sciatic nerve 14 days post-irradiation it is obvious that
(figs. lB,C). There is a hyperplasia of non- the neuroglial population in the anterior
neuronal cells with some of these cells horn of this group (fig. 3A) is markedly
being perineuronal in position as well as greater than in rats receiving 3,000 R (fig.
scattered diffusely in the neuropil. Their 2A). Figures 3B and 3C show that an innuclei are pleomorphic, although there is crease in non-neuronal cells occurs in rea definite tendency toward elongation. sponse to axotomy and that the previous
Apposition of these cells to the injured neu- exposure to x-rays has not altered this reronal perikarya (fig. 1C) is a relationship sponse. As in the previous groups an inbetween neuronal and non-neuronal ele- creased cellularity is present throughout
ments that is absent on the intact side the gray matter on the injured side, and
the presence of these in the posterior horn
(fig. 1B).
The increased cell population is not is evident when comparing figures 8A
limited to the anterior horn. Cells resem- and 8B.
1,000 R and 500 R. These two groups
bling those in figure 1C are present in both
intermediate gray matter and posterior are considered together since earlier studies
horn. Figure 6B shows numerous, darkly (Rodgers, '65; Gilmore, '73 ) demonstrated
stained cells in the substantia gelatinosa that these amounts of ionizing radiation
on the side of sciatic axotomy; this hyper- produced no significant changes in neurocellularity is absent from the same region glia and myelinogenesis in the immature
contralaterally (fig. 6A). Although only its spinal cord. The cords of these animals at
most medial part is illustrated (figs. 6A,B), the time of surgery (14 days post-irradiathe cells are found throughout the sub- tion) (figs. 4A, 5A) cannot be differentiated
from those of non-irradiated rats (fig. 1A).
stantia gelatinosa.
Increases in the number of non-neuronal
Irradiated animals
cells following sciatic transection are obvi3,000 R. The irradiated regions in ous in both groups (figs. 4A-C, 5A-C),
spinal cords of these rats can be delimited and the characteristics and positions of the
by a decreased number of neuroglia in additional cells are similar to those in the
both gray and white matter and an inhibi- previous groups. The reactive cells were
tion of myelinogenesis (Rodgers, '65). noted throughout the gray matter on the
This marked decrease in neuroglia in the side of sciatic axotomy, and figures 9 and
slides which were stained by the following
methods : hematoxylin and eosin, hematoxylin and periodic acid Schiff (PAS),
Einarson's gallocyanin, and luxol fast bluePAS.
10 demonstrate their presence in the substantia gelatinosa.
Transection of the sciatic nerve unilaterally in the pre-weanling, 17-day-old rat
results in non-neuronal hyperplasia in
those segments of the spinal cord giving
rise to this nerve. The cellular response
occurring at this particular developmental
stage when the spinal cord is still immature is similar in several ways to that reported in the anterior horns of young adult
rats following sciatic axotomy (Friede and
Johnstone, '67; Kerns and Hinsman, '73a).
First, the reactive cells are distributed
either randomly in the neuropil or in a
perineuronal position. Second, although
the animals in the present study were not
killed serially at progressive post-operative
intervals, the increase in number of cells
by three days after surgery was marked.
This finding correlates well with previously
reported data showing peaks in cellular
response three to four days after nerve
injury. Finally, the added cells are pleomorphic, tending to be elongate or irregular in shape as described by Friede and
Johnstone ('67).
The importance of considering the effects
of age of the animal upon reactions of
neurons to axotomy or other types of
axonal injury has been recognized since
late in the last century. Cammermeyer
('69) reviewed many of the earlier studies,
and more recent data have been provided
by LaVelle and associates (LaVelle and
LaVelle, '58a,b; LaVelle and Smoller, '60).
The relationship between age and reaction
of non-neuronal cells, however, has not
been considered since the studies thus far
have been carried out on mature or nearly
mature animals. In the absence of data
dealing specifically with this question, this
investigator examined the illustrations in
the papers by LaVelle and associates and
by Torvik and Soreide ('72). These studies
concern retrograde changes in neuronal
perikarya of immature animals but do
not show the perineuronal hyperplasia reported here. The disparity, however, may
be due to differences in species and neuronal aggregates studied. LaVelle and associates studied the facial nucleus in immature hamsters and Torvik and Soreide
('72) the same nucleus in immature rabbits, whereas this investigator dealt with
the sciatic nerve in the immature rat.
Again the differences between species and
particular nuclei which must be considered
in evaluating retrograde reactions of neurons (Cammermeyer, '69) need also to be
recalled when evaluating non-neuronal responses to nerve injury.
Irradiation of the spinal cord prior to
transection of the sciatic nerve does not
inhibit the reaction seen in non-neuronal
cells in non-irradiated controls. With regard to larger amounts of radiation (3,000
R and 2,000 R ) known to decrease the
neuroglial population (Rodgers, '65; Gilmore, '66), these data indicate that the
spinal cord is capable of reacting to the
added challenge of sciatic axotomy. The
design of the present investigation cannot
provide information as to the site(s) and
time(s) of origin of the reactive cells. An
earlier study in this laboratory showed that
4,000 R did not completely inhibit incorporation of 3H-thymidineinto non-neuronal
cells of the gray matter (Gilmore, '69b).
In fact, in that study cells associated with
the vasculature of gray matter but not
readily identifiable with the light microscope readily incorporated 3H-thymidine.
Because the results of another study (Gilmore, '69a) indicated that the vascular bed
was not increasing, it was hypothesized
that the labeled cells were perivascular elements. If they were pericytes, and if such
give rise to microglia (Maxwell and
Kruger, '65; Wendell-Smith et al., '66; Mori
and Leblond, '69 and others), non-neuronal hyperplasia could be anticipated in
the present study where less than 4,000 R
was administered to the spinal cord. In
addition to this origin it is also possible
that the cells could have been derived from
neuroglia present in the involved segments
of the spinal cord at the time of axotomy.
Observations in this and previous studies
(Gilmore, '63a, '66, '69b) place some neuroglia in the gray matter at the time of
axotomy irrespective of the amount of radiation administered. Autoradiographic studies are now in progress in this laboratory
in order to obtain more information concerning the reactive cells and their time of
development. In addition, these data will
be evaluated quantitatively for any corre-
lation between the amount of radiation and
the magnitude of response following sciatic
Sjostrand ('68) investigated the effects
of ionizing radiation on the reactive cell
population in the hypoglossal nucleus of
the rabbit after crushing the hypoglossal
nerve. The brains of these animals were
irradiated with 300 R to 3,000 R of x-rays
one day after injury. This procedure produced an inhibition of DNA synthesis, evidenced autoradiographically by lack of incorporation of 3H-thymidine and marked
reduction in microglial hyperplasia. In contrast, irradiation prior to transection of the
sciatic nerve in the present investigation
did not prevent or alter significantly the
pattern of non-neuronal hyperplasia. This
alteration of non-neuronal hyperplasia by
irradiation one day following injury supports the consensus by many investigators
that the reactive cells are endogenous in
origin, not exogenous as suggested by
Adrian and Smothermon ('70).
A previously unreported finding is the
increased number of non-neuronal cells in
the substantia gelatinosa. Kerns and Hinsman ('73a) observed labeled and/or mitotic cells in the intermediate gray as well
as in the anterior gray after transection of
the sciatic nerve. Their observations, however, did not extend into the posterior horn.
The presence of these cells throughout the
substantia gelatinosa as shown here, as
well as in the intermediate gray, suggests
that these reactive cells are related to the
proximal processes of primary afferent
neurons (Scheibel and Scheibel, '69).
Kerns and Hinsman ('73a) considered but
rejected involvement of such sensory elements, attributing the diffuse distribution
of reactive cells to involvement of the
motor neurons. Additional evidence supporting involvement of afferent neurons is
provided by the increased enzymatic activity noted in the posterior horn of the feline
cervical spinal cord following unilateral
brachial plexectomy (Barron and Tuncbay,
'64; Means and Barron, '72). Additional
evidence must be obtained before a valid
interpretation of these data can be made.
Supported by USPHS grant NB-04761
from the National Institute of Neurological
Diseases and Stroke.
This investigator gratefully acknowledges the capable technical assistance
of Mr. Napoleon Phillips throughout all
phases of this study.
Adrian, E. K., Jr., and R. D. Smothermon 1970
Leucocytic infiltration into the hypoglossal nucleus following injury to the hypoglossal nerve.
Anat. Rec., 166: 99-116.
Barron, K. D., and T. 0. Tuncbay 1964 Phosphatase histochemistry of feline cervical spinal
cord after brachial plexectomy: Hydrolysis of
beta-glycerophosphate, thiamine pyrophosphate
and nucleoside diphosphates. J. Neuropath. expNeur., 23: 368-386.
Blinzinger, K., and G. Kreutzberg 1968 Displacement of synaptic terminals from regenerating motoneurons by microglial cells. Zeit. f.
Zellforsch., 85: 145-157.
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retrograde changes in the facial nucleus of
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retrograde degeneration of the hypoglossal
nerve. Brain Res., 27: 365-368.
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Portion of anterior horn i n spinal cord of non-irradiated r a t at the
time of sciatic nerve transection (17 days of age) ( A ) . Three days
following surgery the population of non-neuronal cells on the transected side ( C ) is markedly increased over that of the intact side ( B ) .
Note that the reactive cells tend to be elongate and occur both in the
neuropil and apposed to neurons. Hematoxylin and eosin. x 320.
Shirley Ann Gilmore
A marked decrease in neuroglia a t two weeks following irradiation
when the sciatic nerve is transected ( A ) is characteristic of the spinal
cord following exposure to 3,000 R on the third postnatal day. In
spite of this previous exposure to x-rays, there is an increase i n nonneuronal cells by three days following sciatic axotomy ( C ) . These
reactive cells, particularly evident in perikaryal positions, are absent
from the same neuronal group of the opposite, intact anterior horn
( B ) . Hematoxylin and eosin. x 320.
This neuronal group in the anterior horn of a 17-day-old rat ( A )
irradiated with 2,000 R a t three days of age has a greater number
of neuroglia than when exposed to 3,000 R (fig. 2 A ) but less than
normal (fig. 1A). Unilateral sciatic transection at this time results in
an increase in non-neuronal cells by the third post-operative day ( C )
when contrasted to the non-neuronal population of the opposite, intact
side (B). Hematoxylin and eosin. x 320.
Shirley Ann Gilmore
The spinal cord of a 17-day-old animal receiving 1,000 R when
three days of age ( A ) does not appear to be markedly different from
that of the non-irradiated animal (fig. 1 A ) at the time of sciatic
transection. A marked increase in non-neuronal cells occurs by three
days following transection ( C ) when the injured neurons are readily
differentiated from their counterparts on the intact side ( B ) by the
apposition of these reactive cells. Hematoxylin and eosin. x 320.
Portion of irradiated (500 R ) anterior horn of 17-day-old rat ( A ) .
Transection of the sciatic nerve at this time results in a n increase
in non-neuronal cells that is readily visible three days post-operative
in the region of the injured neurons (C). These reactive cells are
absent from the same region on the opposite, intact side of the spinal
cord ( B ) . Hematoxylin and eosin. x 320.
Shirley Ann Gilmore
Non-irradiated animal.
Irradiated rat having received 3,000 R when three days of age.
Irradiated rat having received 2,000 R when three days of age.
Irradiated rat having received 1,000 R when three days of age.
Irradiated rat having received 500 R when three days of age.
Medial portions of substantia gelatinosa on intact ( A ) and injured ( B ) sides of spinal
cords of immature rats three days following unilateral transection of the sciatic nerve when
17 days of age. Note the increased number of small, darkly stained, usually elongate cells
on the transected side ( B ) in comparison with the intact side ( A ) . Hematoxylin and eosin.
x 320.
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spina, sciatic, nerve, following, irradiated, non, cells, cord, transection, proliferation, neuronal, immature, rats
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