[CANCER RESEARCH56, 5417-5422. December 1, 19961
Oligodendrocytes in the Adult Rat Spinal Cord Undergo
Radiation-induced Apoptosis1
Yu-Qmg
Li, Venita
Jay, and C. Shun
Wone
Division of Experimental Therapeutics (Y-Q. LI and Department of Radiation Oncology [C. S. WI, Ontario Cancer Institute and Princess Margaret Hospital, University of
Toronto, 610 University Avenue, Toronto, Ontario MSG 2M9; and Department of Pathology, Hospital for Sick Children and University of Toronto, Toronto, Ontario M5G 1X8
(V. J.J, Canada
ABSTRACT
Mitotic-linked death is generally regarded as the mode of radiation
induced cell death, particularly in late-responding normal tissues, such as
those found in the central nervous system. We have recently reported
evidence for radiation-induced apoptosis in the central nervous system
using the adult
vascular
rat spinal cord model. Glial cells, but not neurons
endothelial
cells, appeared
to undergo
apoptosis
within
or
24 h of
irradiation. To further characterize the apoptotic process and the type of
glial cells involved, a 2-cm segment of the adult rat cervical spinal cord
was irradiated
with single doses of 1-30 Gy and processed
for detailed
histological examination at 0, 4, 8, 12, 16, and 24 h after irradiation.
Apoptosis was assessed using standard morphological features under the
light and electron microscopes and an in situ end labeling assay. A dose
response for radiation-induced apoptosis was observed over a dose range
of 1—30
Gy, with the peak response at 8 h after irradiation. At 8 h after a
22-Gy irradiation, 96.1% of the apoptotic cells showed positive immuno
histochemical staining with Leu-7, a specific marker for oligodendrocytes;
only 4.4% of apoptotic cells were positive for Ricinus communis aggluti
nm-i (a marker for microglia), and none were positive for glial fibrillary
acidic protein (a marker for astrocytes). A significant decrease in the total
gilal cell density was observed at 24 h after irradiation with 22 (11%) or
30 Gy (14%) but not with 8 Gy. This was due primarily to a decrease in
the oligodendroglial density (24%, 22 Gy, P < 0.001; 19%, 30 Gy,
P = 0.001), because no decrease in the astroglial population was observed.
The duration of apoptosis was estimated to be —1h. We conclude that
there is a depletion of the oligodendroglial population in the adult rat
spinal cord within 24 h after irradiation and that the mode of this
radiation-induced cell death is apoptosis.
(2), suggesting that mitotic-linked death is not the exclusive mode of
radiation-induced cell death in the CNS.
Cell types in the CNS consist of neurons, glial cells and vascular
endothelial cells. The glial cells can generally be divided into oligo
dendrocytes, astrocytes, microglia, and ependymal cells. Oligoden
drocytes form and maintain the myelin sheaths that wrap around
axons in the CNS. The role of astrocytes is still emerging, and they
may participate in transmission of neuronal signals, and the formation
and maintenance of the blood-brain barrier. Microglia have phago
cytic properties and are regarded as macrophages in the CNS (3).
Ependyma are a population of cells lining the central canal of the
spinal cord and ventricles in the brain.
In our previous study, glial cells, but not neurons or vascular
endothelial cells, appeared to undergo radiation-induced apoptosis. In
this study, we have further characterized this process of radiation
induced programmed cell death in the adult rat spinal cord, and the
results and the evidence for oligodendrocytes being the target cells of
radiation-induced apoptosis in the adult rat spinal cord will be pre
sented in this paper.
MATERIALS
Animals.
AND METHODS
Adult female Fisher 344 rats, ages 9—10weeks, were used in this
study. The animals
were housed
given water ad libitum
anesthesia
using halothane
configuration.
The CNS3 is one of the major dose-limiting organs in clinical
radiotherapy. The lesion after radiation injury is characterized by
demyelination and necrosis in white matter, generally after a latent
period of several months to years after irradiation. Although the
histopathological features of these lesions have been well described,
their pathogenesis has remained unclear. Death of oligodendrocytes
and endothelial cells has been suggested to play an important role in
the development of white matter necrosis; however, the respective
roles of the target cells and their interaction remain to be elucidated
(1). Mitotic-linked or clonogenic cell death is regarded generally as
the mode of radiation-induced cell death in the CNS, and the long
latent time for the expression of radiation damage is consistent with
the slow turnover time of the target cells. We have recently reported
evidence for radiation-induced apoptosis in the adult rat spinal cord
Received4/29/96;accepted10/2/96.
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked advertisement in accordance with
18 U.S.C. Section 1734 solely to indicate this fact.
was supported
by the National
Cancer
Institute
of Canada
with
funds
from
the CanadianCancerSociety.
2 To
whom
requests
for
reprints
abbreviations
used
are:
formed
and kept in a fixed position
ofthe
cervical
be addressed.
Phone:
(416)
946-2125;
Fax:
CNS,
central
nervous
system;
GFAP,
glial
fibrillary
acidic protein; RCA-I, Ricinus communis agglutinin-l; DAB, 3,3'-diaminobenzidine;
dUTP, deoxyribonucleotide triphosphate; EM, electron microscopy.
Port films
prior
described
rodent diet, and
Cancer
Institute,
a
with a polystyrene
foam
spinal cord, C2 to T2, was irradiated
at 100
to confirm
to irradiation.
previously
Details
(4). Animals
accuracy
of field placement
of irradiation
were irradiated
and dosimetry
were
per
have
been
with single doses of 1, 2, 4,
6, 8, 16, 22, or 30 Gy, and sacrificed at 0, 4, 8, 12, 16 and 24 h for histological
analysis.
Three
animals
were irradiated
at each experimental
time point.
Histopathology
and Immunohistochemistry.
After transcardiac
perfu
sion with 10% formalin for 10 mm, the spinal cord was carefully dissected out
and cut transversely at the midpoint of C2-T2. The cord was then processed by
a standard paraffin embedding method. Sections were cut at 4 pm thickness
and stained with H&E or processed for immunohistochemical studies.
Anti-Leu-7
antibody,
which
binds
specifically
to oligodendrocytes
in hu
man and rodent central nervous tissue (5); GFAP, a marker for astrocytes (6);
and lectin RCA-l , a specific marker for human and rodent microglia (7), were
used to define
the glial cell type that undergoes
radiation-induced
apoptosis.
For immunohistochemical studies, the sections were deparaffinized, and en
dogenous peroxidase was blocked by H2O2in methanol. The method described
by Motoi et a!. (8) was used for Leu-7 immunohistology.
Sections were
incubated sequentially with normal goat serum, anti-Leu-7 antibody (Becton
Dickinson), biotinylated goat antimouse 1gM, and avidin-biotin peroxidase
complex. The reaction was visualized using DAB. For lectin RCA-l immu
nostaining,
biotinylated
biotin
should
(416) 946-4586; E-mail: [email protected].
3 The
of the Ontario
kV using two Picker Gemini 160 X-ray units employed in a parallel, opposed
INTRODUCTION
work
colony
laboratory animal colony accredited by the Canadian Council of Animal Care.
Irradiation. During irradiation, animals were immobilized by inhalation
jig. A 2-cm segment
I This
three per cage, fed a standard
in the animal
the sections
were incubated
lectin RCA-l
peroxidase
(Vector
complex,
sequentially
Laboratories,
and visualized
with DAB
nostaining,
sections were incubated
sequentially
biotin goat antimouse 1gM, streptavidin
peroxidase
slides were counterstained
Biochemical
evidence
with normal
Burlingame,
horse serum,
CA), and avidin
(9). For OFAP
immu
with anti-GFAP
antibody,
complex and DAB (6). All
with hematoxylin.
of apoptosis
in situ was assessed
immunohistochemi
5417
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OLIGODENDROCYTESUNDERGOING APOPTOSIS
cally using the ApopTag (Oncor, Inc.) assay. This method detects the nucleo
in the spinal cord were selected
some-sized
was defined as the mean value in the dorsal, lateral, and ventral white matter,
and the density in gray matter was the mean of the values for dorsal and ventral
DNA fragments
by tailing
the 3-OH
ends of the fragments
with
digoxigenin-nucleotide using terminal deoxynucleotidyl transferase. After tis
sue sections were deparaffinized,
the protein was digested with proteinase K
(Sigma Chemical Co.). Endogenous peroxidase was inactivated by immersing
the sections in 0.3% hydrogen peroxide. DNA fragments were then tailed with
digoxigenin-nucleotide
by terminal deoxynucleotidyl
transferase
and incu
bated with an antidigoxigenin antibody conjugated with peroxidase. The re
action was visualized using DAB, and the sections were counterstained with
hematoxylin (Fig. 1A).
The spinal cord in a separate group of control animals (n = 3) and animals
irradiated to a dose of 30 Gy (n 6) was also processed for EM as detailed
previously (10) to provide additional morphological evidence for apoptosis and
to characterize the type of cells that underwent apoptosis.
Scoring of Apoptosis. The morphological criteria used in the scoring of
apoptosis
were as described
previously
(2, 11, 12). Briefly,
cells that showed
the following were considered apoptotic: (a) cell shrinkage or breakdown of
cell-cell interaction as the cell was isolated from its neighbors; and (b) nuclear
condensation
or fragmentation.
or unirradiated
rat were scored
for the incidence
of apoptosis.
For
immunohistochemistry, only apoptotic nuclei that were surrounded completely
by GFAP, Leu-7, or RCA-l
The cell density
in white mailer
gray matter. The spinal cord density
was the mean of white and gray matter cell
density.
and
Although
GFAP,
Leu-7,
lectin
RCA-l
are relatively
specific
markers of oligodendrocytes, astrocytes, and microglia respectively, with
respect to normal nonapoptotic cells, it was often difficult to identify whether
the immunoreactivity associated with the cytoplasm or nucleus was intrinsic to
the cell or was instead associated with processes from neighboring cells (13).
For apoptotic cells, this was not a problem, because apoptotic cells were
always shrunken and isolated or separated from the normal neighboring cells.
For normal cells, nuclear morphology was found to be reliable; H&E slides
were therefore used, and oligodendrocytes were distinguished from astrocytes
using their characteristic
morphological
features (14). Nine sections of the
spinal cord were assessed per dose group, i.e., three sections per animal. The
number of cells was counted by an observer (Y-Q. L.) blinded to the treatment.
Statistics. All data represent the means ±SE. Statistical significance was
determined by Student's t test.
Sections were examined under the light mi
croscope at X 1000 magnification. Apoptosis found in either gray or white
matter was recorded and added to obtain the total number of apoptotic cells in
the whole spinal cord section. Three histological sections of spinal cord per
irradiated
for the scoring.
immunoreactivity
were scored as positive for
GFAP, Leu-7, or RCA-l, respectively (Fig. 1, B—D).
Evaluation of Glial Cell Density. Because the number of apoptotic cells
returned to the baseline by 24 h after irradiation (2), the impact of radiation
induced apoptosis was evaluated by comparing the glial cell density at 24 h to
that in control. The total glial cell, oligodendroglial and astroglial densities in
the spinal cord were measured. Five representative areas, each of 0.094 mm2
RESULTS
No apoptotic cells were observed in the control unirradiated spinal
cord. Only glial cells, but not neurons and vascular endothelial cells,
were observed to be undergoing apoptosis within 24 h after single
doses of 1—30Gy. More apoptotic cells were found in white matter
than in gray matter. The number of apoptotic cells increased at 4 h,
reached a peak at 8 h, and then returned to the baseline by 24 h. The
time course of apoptotic cells observed appeared independent of dose
and is shown in Fig. 2.
The dose-response relationship at 8 h after irradiation using the
A
,
@
.\
1@
@/
•‘
@1
.J@ -.
c::@
@1,
b_-I!.@*t.
‘s@
.@s
\ ,(
a'.,'
t.@4
, .
.,@
-
a—.
-.
;@.
,-‘-,-
@
.—
@
@-
-
k,,@.ç, .
.
I,
.b
Fig. I. A. ApopTag method for detection of apoptosis: 3-OH DNA fragments are evident as red staining in the apoptotic cell (arrow) with a lack of staining in several cells in the
background. B. a GFAP-negative perineuronal satellite oligodendrocyte undergoing apoptosis in gray matter (22 Gy. 8 h). GFAP-positive astrocytic processes are seen in the
background. The apoptotic oligodendrocyte
(arrow) shows nuclear condensation into a horseshoe-shaped
mass. C, an apoptotic cell with multiple fragments (arrow) in white matter
at 8 h after a single dose of 22 Gy. There is cell shrinkage, and the cytoplasm shows a granular staining pattern with Leu-7. D, an apoptotic cell with two condensed nuclear fragments
(arrow) in white matter is nonreactive for RCA-I lectin. A microglia with RCA-I positive staining is seen in the background (arrowhead). Original magnification, X 1000.
5418
Downloaded from cancerres.aacrjournals.org on June 18, 2017. © 1996 American Association for Cancer Research.
OLIGODENDROCYTES
UNDERGOING APOPTOSIS
90
C
0
0
0)
80
Cl)
70
0
0
C 60
0@
Cl)
Fig. 2. Time course of development of apoptosis in the
rat spinal cord after single doses of 1—30
Gy. Apoptosis was
assessed at 0, 4, 8, 16, and 24 h after irradiation. Each data
point represents the mean number of apoptotic cells ob
served in nine transverse sections of the spinal cord (three
per animal); bars, SE.
a)
0@
Cl)
a)
0
0
50
40
30
0
0
ci
20
‘4-
0
0
10
z
0
Time (h) after irradiation
ApopTag assay (Fig. 3) appeared similar to that using morphological
criteria on H&E. However, more apoptotic cells were observed after
single doses of 6, 8, and 22 Gy (P < 0.001) at 8 h in white matter and
the spinal cord section using ApopTag compared to H&E.
In gray matter, some apoptotic cells appeared to be juxtapositional
to neurons, and their location and size suggested that they were
perineuronal satellite oligodendrocytes undergoing apoptosis (Fig.
1B). Under EM, cells showing early apoptotic changes retained char
acteristic morphological features of oligodendrocytes with thin rims
of perinuclear cytoplasm where intermediate filaments were charac
tenstically absent.
Virtually all the apoptotic cells showed cytoplasmic staining for
Leu-7 (Fig. 1C), but were negative for GFAP (Fig. 1B) and lectin
RCA-l (Fig. 1D). Data on the three different immunohistochemical
staining assays at 8 h after a single dose of 22 Gy are summarized in
Table 1.
Changes in Glial Cell Density after Irradiation.
A decrease of
11 (P = 0.05 versus control) and 14% (P = 0.0001 versus control) in
the total glial cell density was observed at 24 h after a dose of 22 Gy
or 30 Gy, respectively, but not after 8 Gy (Table 2). No significant
change in the astrocyte density was observed after any of the three
doses. In contrast, there was a significant decrease in the oligoden
droglial density after a dose of 22 and 30 Gy, respectively. The
decrease appeared to be more pronounced in white matter (21 % after
a 30-Gy dose and 28% after a 22-Gy dose) compared to gray matter
(17% after a 30-Gy dose and 18% after a 22-Gy dose). No significant
change in the oligodendroglial density was observed in either gray or
white matter after 8 Gy, compared to controls.
Duration of Apoptosis. Integrating the area of the apoptotic re
sponse over the 24-h period after irradiation (Fig. 2) gave an apoptotic
yield of 461 and 645 cell-h after a single dose of 22 and 30 Gy,
respectively (Fig. 4). In the control unirradiated spinal cord, there was
a total of4708 ±218 glial cells per transverse spinal cord section, and
an 11 and 14% reduction in the total glial cell population after a dose
of 22 and 30 Gy, respectively, suggested that about 518—659 cells
died within 24 h after irradiation. This would mean that the lifetime of
C
0
8
U)
@0
8
a
C
@0.
U)
a
0
0
0
0.
8.
a
0
0
z
C
0
a
U)
@0
0
0
a
C
0.
U)
U)
8
a
+
a
‘a
8.
‘C
0
a
z
Dose (Gy)
Fig. 3. Apoptotic yield at a peak time of 8 h after single doses of 0, 1, 2, 4, 6, 8, 16,
22, and 30 Gy obtained by H&E (A) and after single doses of 0, 1, 2, 4, 6, 8, and 22 Gy
by ApopTag assay (B). Radiation-induced apoptosis in white matter ($). gray matter (•),
and whole spinal cord section (A). Each data point represents the mean number of
apoptotic cells observed in nine transverse sections of the spinal cord (three per animal);
bars, SE.
5419
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OLIGODENDROCYTESUNDERGOING APOPTO5IS
Table 1 Number of apoptotic cells per transverse spinal cord section in the adult rat spinal cord at 8 h after irradiation to a dose of 22 Gy
using three d@fferent immunostaining
assaysa
Leu-7GFAPRCA-lPositives'NegativePositives'NegativePositive1'NegativeWhite
±2.8
I 3.3 ±I .0
51.4 ±3.1
matter
Gray matter
Total38.1
±0.5
0.5 ±0.2
2.1 ±0.5
(96.1%)1.6
a Mean
± SE of nine
b Defined
(3.9%)0
(0%)26.3
as apoptotic
nuclei
completely
surrounded
by GFAP,
Leu-7,
or RCA-I
In our previous study (2), a significant increase in the number of
apoptotic cells was observed within 24 h in the adult rat spinal cord after
a single dose of 8 or 22 Gy. Evidence for apoptosis was obtained using
standard morphological criteria under light microscopy and EM, and in
situ end labeling with ApopTag. More apoptotic cells were observed in
white compared to gray matter. The time course of radiation-induced
apoptosis after a single dose of 22 Gy obtained using morphological
was
similar
to that using
the in situ end
labeling
assay.
Only glial cells, but not neurons and vascular endotheial cells, were
observed to be undergoing apoptosis (2). The number of apoptotic cells
reached a peak at 8 h and returned to the baseline level by 24 h. The
present results confirmed the presence of a strong dose-response relation
ship for radiation-induced apoptosis over the dose range of 1—30Gy.
Furthermore, the profile of the apoptotic yield after irradiation appeared
independent of the irradiation doses.
Four lines of evidence supported the notion that oligodendrocytes
were the target cells for radiation-induced apoptosis in the rat spinal
cord: (a) under light microscopy and EM, a number of apoptotic cells
in the gray matter were juxtapositional to neurons (Fig. 1B). The
location and size of these cells suggested that they were perineuronal
satellite oligodendrocytes; (b) the morphological features of cells
undergoing early apoptotic change on EM were consistent with those
of oligodendrocytes. Intermediate filaments were notably absent in the
cytoplasm of the apoptotic cells, suggesting that they were not astro
cytes (2); (c) virtually all the apoptotic cells were positively stained
with anti-Leu-7, a specific marker for oligodendrocytes, but none
showed GFAP or RCA-l immunoreactivity; and (d) a reduction of
19—24%in the oligodendroglial population was observed within 24 h
after a dose of 30 and 22 Gy, respectively. In contrast, there was no
change in the astroglial population after these doses. In white matter,
mm2White
(Gy)Total
sectioncControl
49.6 ±9.2
(95.6%)
22
NS
NS
68.5 ±2.6
NS70.3
55.3 ±1.6
P = 0.000173.7
3068.0
a Mean
± SE
of
27
b Mean
± SE
of
I 8 representative
representative
C Mean
± SE of 45
representative
P >
0.05
mailer1'Cord
±1.1
72.0 ±2.1
NS
68.6 ±2.5
71.3 ±2.2
NS―
59.0 ±4.3
8
i.e.,
where more apoptotic cells were observed (21—28%white versus
17—18%gray matter), there was a greater reduction in the oligoden
droglial density compared to that in gray matter.
A significant reduction in the total glial cell and oligodendroglial
density was observed at 24 h after a dose of 22 and 30 Gy, but not
after 8 Gy. The lack of a significant reduction observed after a lower
dose of 8 Gy was probably because the number of total glial cells or
oligodendrocytes that died was too small for the difference to be
detected. This was consistent with the dose-response relationship for
the number of apoptotic cells observed.
In the control unirradiated adult rat spinal cord, there were
2217 ± 127 oligodendrocytes per transverse spinal cord section.
These cells were almost equally distributed between white
(1 113 ±61) and gray matter (1 104 ±68). The number of cells
observed to be undergoing apoptosis was small. At the peak time of
8 h afteradoseof 30Gy,only76.1±4.0apoptoticoligodendrocytes
were observed per spinal cord section, suggesting that only 3% of
oligodendrocytes were undergoing apoptosis. This small number,
however, resulted in a 19% reduction in the oligodendroglial density
after 24 h. Based on the apoptotic yield at the different time points
after irradiation, we estimated the duration of apoptosis to be —1h.
This remarkably rapid clearance of apoptotic cells is presumably one
main reason why radiation-induced apoptosis was unrecognized for so
long in the adult CNS.
In the developing rat optic nerve, the duration of apoptosis in the
oligodendrocytes was estimated to be about I h by Barres et al. (15).
The present value of 1 h for the duration of radiation-induced apop
tosis, therefore, agrees well with that observed in the developing rat
optic nerve. For cells dying during normal development in the nem
atode Caenorrhabditis elegans, a similar clearance time has also been
observed directly (16). In the present study, the decay in the number
of apoptotic cells observed after the peak response time of 8 h
appeared to be exponential, and decay half-times (t@,@)
of 4.4, 3.8 and
3.3 h were obtained after single doses of 8, 22 and 30 Gy respectively.
Using the equation,
.
duratlon =
ti,2
ln 2
of oligodendrocytes/0.094
no. of glial cells/0.094 mm2No.
matter―Gray
±1.2
significance,
2.3 ±0.7
Table 2Total glia! cell and oligodendrocyte densities in the adult rat spinal cordafter
irradiationIrradiation
d No
12.3 ±6.4
(4.4%)37.3
(100%)2.3
±3.0
0
immunoreactivity.
DISCUSSION
(H&E)
±0.7
sections.
the cells in apoptosis was only about 0.9—1.0h. Alternatively, there
were 2217 ±127 oligodendrocytes in the whole spinal cord section,
and the mean reduction of 24 and 19% in oligodendroglial population
after 22 and 30 Gy, respectively, translated to 421—532oligodendro
cytes dying of apoptosis, and hence a value for the duration of
apoptosis of about 1.1—1.2h.
criteria
±1.2
11.4 ±1. I
37.7 ±1.2
0
0
sectiot(White
±0.9
71.5 ±1.9
NS
62.8 ±3.2
P = 0.05
60.6 ±1.3
P = 0.000131.6
matter―Gray
±1.6
32.3 ±1.4
NS
22.8 ±1.5
P = 0.001
maner―Cord
±1.2
33.4 ±2.1
NS
30.0 ±1.8
P = 0.008
±1.2
32.8 ±1.5
NS
25.7 ±1.4
P = 0.007
25.0 ±1.0
30.4±1.9
27.2±1.1
P = 0.00436.7
P = 0.0233.7
P = 0.001
areas.
areas.
areas.
(compared
with
control).
5420
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OLIGODENDROCYTESUNDERGOING APOPTOSIS
gested to be more important than bcl-2 in the maintenance of cellular
viability in the adult CNS. Additional study is required to determine
whether radiation-induced apoptosis in oligodendrocytes is related to
the differential expression of bcl-2, bcl-x, or related gene products in
oligodendrocytes versus other glial cells or neurons in the adult CNS.
A depletion of the oligodendroglial population after irradiation may
stimulate cell division by the surviving oligodendroglial cells or
progenitor cells to maintain homeostasis in terms of cell number. It is
possible that this is the signal that leads to subsequent mitosis-linked
death in the oligodendrocytes. However, the compensation for the
oligodendroglial population would be slow, given that glial cells have
a slow turnover time (25). Radiation may also cause some oligoden
droglial cells to be arrested in the cell cycle without ever having
entered mitosis (26, 27); i.e., the cells may be reproductively dead but
would continue to be metabolically alive for weeks to months (28) or
even longer. Whatever the mechanism, the challenge is to determine
how or whether this early apoptotic death in oligodendrocytes is
linked to the downstream events such as demyelination and white
matter necrosis.
In conclusion, the present study suggests that there is a depletion of
the oligodendroglial population in the adult rat spinal cord within 24 h
after irradiation due to radiation-induced apoptosis.
-C
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a)
>
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0.
Cl)
-C
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REFERENCES
Dose(Gy)
I. Van der Kogel, A. J. Central neuron system radiation injury in small animal models.
in: P. H. Gutin, S. A. Leibel, and 0. E. Sheline (eds.), Radiation Injury to the Nervous
Fig. 4. Dose response for apoptotic yield over a 24-h period induced by single doses
of 0, 1, 2, 4, 6, 8, 16, 22, and 30 Gy. Each data point represents the integration of the area
covered by curves of radiation-induced apoptosis scored at 0, 4, 8, 16, and 24 h after
System, pp. 91—111.
New York: Raven Press, 1991.
2. Li, Y-Q., Guo, Y. P., Jay, V., Stewart, P. A., and Wong, C. S. Time course of
radiation-induced apoptosis in the adult rat spinal cord. Radiother. Oncol., 39: 35—42,
irradiation (see Fig. 2).
1996.
3. Travis, J. Glia: the brain's other cells. Science (Washington
DC), 266: 970—972,
I994.
4. Wong,
the duration of radiation-induced apoptosis derived from these half
times were 6.3, 5.5, and 4.7 h for single doses of 8, 22, and 30 Gy,
respectively, in the rat spinal cord. Potten (17) reported a half-life of
5 h for the decay
of apoptotic
cells
in the small
intestine
after
small
doses of radiation. In the rat liver, the duration of apoptosis induced
after withdrawal of hepatomitogens, calculated again based on decay
half-times, was about 3 h (18). In the adult rat spinal cord, a higher
dose of irradiation was associated with a more rapid clearance of
apoptotic cells, hence an apparent decrease in the duration of apop
tosis. The duration of apoptosis estimated based on quantitation of cell
death or observed directly is, therefore, smaller than that calculated
from the half-time of decay of apoptosis. When the actual duration of
apoptosis is very brief, it is likely that the latter method is simply a
measure of the random scattering of apoptotic cells that developed
after the peak response, and hence, this method overestimates the
actual duration of the apoptotic process.
Apoptosis is an important mode of cell death during development in
the CNS (19). It is believed that this is one of the mechanisms, in
addition to growth arrest and cell proliferation, by which cell home
ostasis is regulated. It is unclear why oligodendrocytes, but not
astrocytes, microglia, neurons or vascular endothelial cells, are the
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Oligodendrocytes in the Adult Rat Spinal Cord Undergo
Radiation-induced Apoptosis
Yu-Qing Li, Venita Jay and C. Shun Wong
Cancer Res 1996;56:5417-5422.
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