[CANCER RESEARCH 29, 1527--1534, August 1969]
A Comparison of Cell Proliferation Parameters in Solid and
Ascites Ehrlich Tumors
I. F. Tannock
~
Biophysics Department, Institute of Cancer Research, Belmont, Sutton, Surrey, England
SUMMARY
Results of labeled mitoses and repeated labeling experiments
are presented for the Ehrlich ascites tumor at 2, 6, and 10 days
after implantation of 1.7 X 106 cells, and for the solid Ehrlich
tumor in the weight range 0.4-1.8 gm. Computer methods
were used to fit curves to the experimental points.
In the ascitic form of the tumor all phases of the cell cycle
increased with tumor age, and the DNA synthesis period was
48 hours in 10-day tumors. The rate of cell loss also increased
in older tumors, but the growth fraction remained close to
unity.
Solid Ehrlich tumors had a doubling time of about 10 days
in the weight range investigated. They had a median cell cycle
time of 17 hours and a growth fraction of 0.8; there was no
evidence for ceUs with abnormally long synthesis times. The
rdatively slow growth of solid tumors was caused by a high
rate of cell death.
A general method is outlined for calculating the rate of resorption of dead tissue from necrotic tumors. Solid Ehrlich
tumors were about 60% necrotic, and the halving time for
resorption of necrotic material was about 36 hours.
INTRODUCTION
Thymidine labeling technics in combination with autoradiography provide a powerful method for studying the cell kinetics of both normal and malignant tissues. Determination of the
fraction of labeled mitoses at intervals after a single injection
of tritiated thymidine (21) has yielded estimates of cell cycle
phase durations in several tumors (7,8, 12, 15, 19, 24, 26, 27,
29), and a method by which the results of a labeled mitoses
experiment may be computer analyzed to obtain a distribution
of cell cycle times has been proposed by Barrett (1). Estimates
of labeling index after repeated thymidine injections may provide an estimate of growth fraction (18, 24, 26, 27), and these
technics form the basis of the present study. Also, the rate of
cell loss has been deduced by comparing the rate of cell production with the rate of tumor growth (11, 22, 23); and the
rate of dead cell resorption from solid tumors has been esti-
mated by comparing the rate of cell death with the rate at
which the necrotic part of the tumor increases its volume.
Recent applications of thymidine technics have indicated
regional variations in the rate of cell production within solid
tumors (14, 20, 26, 27). Studies of cell proliferation in relation to the vascular system have indicated that kinetic parameters may be sensitive to metabolite concentrations, and in a
transplanted mouse mammary tumor the growth fraction appeared to depend on the tissue oxygen tension (26, 27). Solid
and ascites tumors have rather different nutrient environments, and a comparison of kinetic parameters in solid and
ascites tumors derived from the same type of cell may therefore give some insight into the factors that influence tumor
cell proliferation. An investigation of this type on a mouse
fibrosarcoma cell line has been reported by Frindel and her
colleagues (7, 8); they showed that deceleration of ascites
growth was caused largely by an increase in mean cell cycle
time, while changes in the growth rate of the solid tumor
resulted from an increase in the rate of cell death and a decrease in the growth fraction. Lengthening of mean cell cycle
time with age has also been reported in Ehrlich ascites tumors
(15, 29), but there have been few investigations of the kinetics
of Ehrlich cells in solid tumors. The present communication
describes the application of thymidine technics to obtain kinetic parameters for a subline of the Ehrlich tumor in both solid
and ascites forms; it attempts to relate these results with the
cellular environment within each type of tumor.
MATERIALS AND METHODS
The Ehrlich tumor described in this paper was a hyperdiploid subline grown in female C57 mice. It was maintained
by routinely injecting 0.1 ml of ascites fluid at roughly 10-day
intervals. In the experiments described below, 1.7 X 106 cells
in 0.1 ml of isotonic saline, containing heparin, were injected
into the peritoneum to generate ascites tumors, or subcutaneously into each flank to generate solid tumors (two per animal). In each experiment the animals were randomized after
injection.
Measurement of Tumor Growth
1present address: Dept. of Experimental Radiotherapy, M. D. Anderson Hospital, Texas Medical Center, Houston, Texas 77025.
Received November 14, 1968; accepted March 27, 1968.
Growth curves were obtained for solid and ascites tumors
after implantation from the same donor animal, and are shown
in Chart 1.
AUGUST 1969
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1527
I. F. Tannock
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ASCITES
TUMORS
-
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SOLID
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TUMORS
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Chart 1. Growth curves for solid and ascites tumors implanted from the same batch of cells. For ascites tumors the mean and range of cell
numbers for three animals killed at each interval is shown; for solid tumors the mean tumor weight and its standard error were calculated from
measurements on 12 tumors.
Twelve solid tumors grew from implants in six mice. At
intervals after implantation, each animal was lightly anesthetized with ether, and the largest and smallest superficial
tumor diameters (including skin thickness) were measured
with vernier calipers. The product of these two caliper measurements was used to estimate tumor weight from a calibration curve (24, 27). The mean tumor weight and its standard
error were calculated at each interval and are plotted in Chart
1. The tumor weight doubling time increased from 2 days for
tumors weighing about 0.1 gm, to a relatively constant value
of about 10 days in the weight range 0.5-2.0 gm.
Ascites tumors grew from implants in 30 mice, and three
animals were killed with ether at each of ten intervals after
implantation. Immediately after death, cells were flushed from
the peritoneum of each animal into a known volume (10 or 20
ml) of heparinized saline by repeated injection and withdrawal
of 1 - 2 ml volumes. Ehrlich cells were recognized and counted
in a hemocytometer, and the mean and range of cell numbers
for each set of three animals are shown in Chart 1. There was a
short delay in growth immediately after implantation of cells;
thereafter the doubling time increased gradually from about
12 hours (2-day tumor) to about 6 days (10-day tumor).
Histology and Autoradiography
Tritiated thymidine (Radiochemical Centre, Amersham,
England, TRK 61; specific activity: 25 c/mmole) was diluted
1528
in physiologic saline to 100/~c/ml and injected intrapedtoneally. In the labeled mitoses experiments, animals were kiUed at
intervals after a single thymidine injection; for repeated labelhag 4- or 6-hourly thymidine injections were used, and one or
more animals were killed between 30 minutes and I hour after
each injection. All animals were killed by cervical dislocation
under ether anesthesia.
Ascites cells were withdrawn into heparinized isotonic saline
immediately after death, centrifuged at 1500 rpm, and resuspended in a small volume of saline. Smears were made on clean
slides within 30 minutes of death of the animal Solid tumors
were excised, weighed, and bisected following death of the
host; after fixation in neutral formol saline for at least 24
hours, 4-/~ paraffin sections were cut. Smears and sections were
stained by the Feulgen reaction.
For preparation of autoradiographs, slides were dipped in
Ilford K5 emulsion, exposed, and developed in Kodak D19b
developer. Slides were left unmounted because grain fading has
been observed in mounted slides. The tritium doses and exposure times given in Table 1 led to good resolution between
labeled cells and background. Grain count distributions were
plotted for two slides at intermediate sampling intervals in
each experiment, and a labeling criterion was established from
them (usually 4 or 5 grains per cell). Counts of 100 mitoses or
1000 cells were made in labeled mitoses and repeated labeling
experiments respectively. An exception was made for labeled
mitoses results on the solid Ehrlich tumor. Because of someCANCER RESEARCH VOL. 29
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Research.
Solid and A scites Ehrlich Tumors
Table 1
Labeled mitoses experiment
Injection dose
(Uc)
2-day ascites
6-day ascites
10-day ascites
Solid tumors
10
50
50
50
Exposure
5 days
1 month
2 months
1 month
Repeated labeling experiment
Injection dose
(Uc)
Exposure
10
20
20
20
5 days
1 month
2 months
1 month
Details of autoradiography.
what poorer autoradiographic resolution, the number of grains
(G) per mitosis was recorded (approximately, ff G > 10 grains)
for 100 mitotic cells in each tumor, and the fraction of labeled
mitoses was determined graphically from the resulting grain
count distribution. Random numerical codes were assigned to
each batch of slides to avoid bias when counting autoradiographs. Microscope fields were chosen from all parts of both
smears and sections; this prevented selective counting of large
or small cells in the smears, or of cells in well or poorly nourished tumor regions in the sections. Abnormal and degenerating mitoses were often observed, and these were excluded
from mitotic counts.
Other types of distribution may be incorporated into the program if required (e.g., normal, normal distribution of reciprocals), but computed curves are insensitive to the shape of the
assumed distribution. Means and standard deviations for GI, S,
and G 2 were entered in the program, and computed curves
were compared with experimental points; the chosen parameters were then adjusted to obtain a curve that was a good fit
to the data. This method ensures that the areas under successive waves of a labeled mitoses curve are equal, and also tabulates the distribution of cell cycle times corresponding to each
computed curve. Computed distributions of cell cycle time for
Ehrlich tumors are shown in Chart 5, and cell cycle phases are
listed in Table 2.
The labeled mitoses curve computed for 2-day ascites tumors
was a good fit to the points up to 24 hours after thymidine
injection, and cell cycle parameters derived from it are therefore characteristic of 2-3-day tumors. This corresponds to a
100
2-DAY
o9o
9
50
Morphology of Sofid Tumors
Solid Ehrlich tumors all had large necrotic centers around
which there was a shell of viable tissue and a fibrous capsule.
The relative volumes of viable and necrotic regions were measured over a range of tumor sizes, using Chalkley's method (3).
This method uses a graticule which is marked with 25 random
points. Equatorial tumor sections were scanned in a regular
manner, and for each field the number of points in coincidence with viable and with necrotic tissue was recorded. The
total score for each region was thus proportional to its area in
the section. The proportion of necrosis by volume (fN) was
estimated by assuming spherical symmetry within the tumor;
f/v is then equal to the proportion of necrosis in an equatorial
section raised to the power ~. For tumors in the weight range
0.2-1.8 gm the necrotic proportion was relatively constant
(mean fN = 0.6).
1
g
100 1
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RESULTS
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The results of labeled 'mitoses and repeated labeling experiments on ascites tumors are shown in Charts 2 and 3 respectively; each point was obtained from a separate animal. Results
for solid tumors are shown in Chart 4. All tumors were in the
weight range 0.4-1.8 gm; this corresponds approximately to
the period of exponential growth with a doubling time of 10
days (Chart 1).
Analysis of Labeled Mitoses Results
A computer method was used to fit labeled mitoses curves t o
the experimental points (1). The program assumes log normal
distributions of phase times for the G], S, and G 2 phases of
the cycle, and mitosis is divided equally between G l and G 2.
10 - DAY
50
9
.
0 0
t
20
hours
1
40
after
thymidine
I
60
I
80
injection
Chart 2. Results o f labeled mitoses experiments and computed curves
for Ehrlich ascites tumors.
AUGUST 1969
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Research.
1529
I. F. Tannock
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Analysis of Repeated Labeling Results: Growth Fraction
-
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t
t
The following method of calculating growth fraction is an
extension of that used by other authors (15, 24). For each
tumor, an age distribution diagram (12, 24) was constructed
for proliferating cells by assuming the distribution of phase
times indicated by the computer-determined labeled mitoses
curve. The expected labeling index of proliferating cells was
determined from this diagram; it is the ratio of the number of
~
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Q
12
5(]
obtained from it (Chart 5) are relatively inaccurate. The mean
length of the S period estimated from the first peak was 48
hours, and this is much longer than is generally found in mammalian cell systems.
The labeled mitoses curve for solid tumors (Chart 4) is consistent with a median ceil cycle time of about 17 hours and a
median S period of about 10 hours. Phase durations in the
solid tumors were therefore much shorter than in ascites tumors growing at the same rate (Table 2). The computed distribution of cell cycle times is shown in Chart 5; there appear to
be few cells in solid tumors with cycle times longer than 30
hours.
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EHRLICH
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l
10
20
30
40
hours
after
first
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Chart 3. Results of repeated labeling experiments on Ehrlich ascites
tumors. Curves represented by solid lines were calculated by assuminga
growth fraction of 100%; curves represented by dashed lines were
calculated by assuming growth fractions equal to 90% (2-day tumor),
80% (6-day tumor), 80% (middle curve), and 60% (lower curve) (10-day
tumor).
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period of exponential growth (Chart 1), and within the limits
of experimental error the median ceil cych time was equal to
the tumor doubling time (12 hours). By 6 - 9 days after implantation, all phases of the cell cycle had increased in duration (Table 2), and the median ceil cycle time was about 44
hours. This was somewhat less than the measured tumor doubling time (about 60 hours). The labeled mitoses experiment
for 10- to 13-day tumors was terminated before the appearance
of a second peak (Chart 2); in consequence the length of the
G 1 period (Table 2) and the distribution of cell cycle times
1530
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l
8
l
16
I
I
24
32
I
40
I
48
hours
Chart 4. Upper: Results of a labeled mitoses experiment and a
computed curve for the solid Ehrlich tumor. Lower: Results of a
repeated labeling experiment on the solid Ehrlich tumor. Repeated
labeling curves were calculated by assuming a growth fraction of 80%
with random cell loss (solid line) or with 50% cell loss in mitosis (dashed
line).
CANCER RESEARCH VOL. 29
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Solid and A scites Ehrlich Tumors
Table 2
Doubling time (TD)
Labeling index (%)
G l (hr)
S (hr)
G2 (hr)
Median cell cycle time,
(Te) (hr)
Growth fraction (%)
Cell loss factor (~) (%)
2-day ascites
6-day ascites
10-day ascites
Solid tumors
12 hr
69
1.7 + 0.5
9.2 + 1.2
1.5 + 0.5
12
2.5 days
42
18 +-4
22 -+ 2
5+3
44
6 days
25
40 + 40a
48 + 16
6+2
83a
10 days
49
4.0 +- 3.0
10.5 + 3.0
3.0 + 2.0
17
90-100
0
90-100
25
60-100 a
20-40 a
80
90
Experimentally determined parameters for ascites and solid Ehrlich tumors.
aThese estimates are relatively inaccurate as they are derived from a labeled mitoses curve with only
one peak.
cells in the S phase to those in the whole cycle and was calculated from area measurements. A rough estimate of growth
fraction (GF) was obtained from the formula:
GF
measured labeling index
expected labeling index of proliferating cells
=
40(
30(
TUMORS
200
100
I f this estimate of growth fraction was less than unity, the age
distribution diagram was redrawn to include nonproliferating
cells (Chart 6). A repeated labeling curve was calculated from
the age distribution diagram by area measurements: the right
hand boundary of the S phase was allowed to move into mitosis (assuming constant G2) and then, as a vertical line, through
the age diagrams of both proliferating and nonproliferating
cells. This repeated labeling curve was compared with the exper/mental points, and if necessary the fit was improved by
assuming a different value of growth fraction a n d repeating the
procedure. All such repeated labeling curves tend eventually to
100% labeling because labeled cells are continually leaving mitosis to enter the nonproliferating c o m p a r t m e n t ; it is the rate
of approach to this assymptote that depends on growth fraction~
Repeated labeling curves calculated for ascites tumors by
assuming a growth fraction of unity are shown as solid lines in
Chart 3. Dashed lines assume lower values Of growth fraction.
For 2-day and 6-day ascites tumors, the growth fraction is
close to 100%. For 10-day tumors a repeated labeling curve
generated by assuming a growth fraction of 80% is a reason-
o
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o
20
o
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60
40
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i
Nonproliferating
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100-
~.
o o
20
40
60
80
100
120
;1
0
I
0
cell
cycle
time
8
16
24
32
(hours)
Chart 5. Computed distributions of ceU cycle time for solid and
ascites Ehrlich tumors. The distribution for the 10-day ascites tumor is
shown as a dashed line; it was obtained from a labeled mitoses curve
with only one peak and may be rather inaccurate.
age
(hours)
Chart 6. Age distribution diagram draWn for the solid Ehrlich tumor
by assuming log normal distributions for the phase times listed in Table
2 and a growth fraction of 80%.
AUGUS T 1969
Downloaded from cancerres.aacrjournals.org on June 15, 2017. © 1969 American Association for Cancer
Research.
1531
L F. Tannock
able fit to the experimental points; however, because it is
based on a labeled mitoses curve with only one peak, this value
of growth fraction may be rather inaccurate.
For solid tumors a repeated labeling curve calculated for a
growth fraction of 80% was also a good fit to the experimental
points (Chart 4, lower section, solid line). Cell loss has no
effect on repeated labeling results if it occurs randomly from
the population. However, many abnormal mitotic figures were
observed in solid Ehrlich tumors, suggesting a high rate of cell
death in mitosis. This would lead to a lower labeling index
after repeated labeling, particularly in a population with a low
growth fraction (7. 277 . The age distribution diagram for the
solid Ehrlich tumor was therefore modified by assuming that
only 1.5 cells were produced in mitosis. Because this tumor
has a high growth fraction ("80%), the corresponding repeated
labeling curve (Chart 4, dashed line) differs only slightly from
that calculated by assuming random cell loss; either curve is an
adequate fit to the experimental points.
Most methods of analyzing cell proliferation results apply a
model to a set of experimental data, and the present model
classifies the cell population into two compartments, proliferating and nonproliferating cells. Proliferating cells are characterized by a cell cycle time distribution of a particular shape,
and the relative numbers of cells in the two compartments are
specified by the growth fraction. "Growth fraction" is therefore a theoretical parameter; it applies directly to the model
and only indirectly to the tumor population. By comparing
the experimental and calculated repeated labeling curves, the
present method of estimating growth fraction applies a more
rigorous test of the ability of the assumed model to simulate
experimental results than, for example, a comparison of expected and measured labeling indices. However, the method
depends on a labeled mitoses experiment, and labeled mitoses
curves are insensitive to cells with cycle times much longer
than the mean (277. The technic, therefore, suffers from the
same defect as other methods (15, 17) in that it gives rather
poor resolution between nonproliferating cells and those with
long cell cycle times. A value of growth fraction derived by
assuming the present model must therefore be regarded as an
approximation to the true biologic value. Since the model is
already sufficiently flexible to simulate results of labeled mitosis and repeated labeling experiments, more accurate estimates
of the growth fraction must await new experimental technics.
from the computed cell cycle time distributions (Chart 5). For
many tumors the turnover time is less than the population
doubling time (T 17, and this difference is due to cell loss. The
following equation allows the rate of cell loss to be expressed
as a fraction (r of the rate of entry of cells into mitosis (22,
23):
r
1 _T
T1
(B)
In the ascites tumors, r increases from zero in 2-day tumors
to about 40% in 10-day tumors (Table 2). In solid tumors the
cell population doubling time (TI) is equal to the tumor volume doubling time (TD) if the necrotic proportion of the
tumor (f/v) is not changing with time. This condition is approximately satisfied in solid Ehrlich tumors, and within the
weight range 0.5-2.0 gm the cell loss factor is about 90%.
Thus the rate of cell production is only a little greater than the
rate of cell loss.
Resorption of Necrotic Tissue
In grossly necrotic tumors the rate of growth depends on
three factors: the rates of cell production and cell death, and
the rate of resorption of necrotic tissue. At the present time
little is known about the rate at which necrotic material is
resorbed from tumors, and the following analysis has been
developed to measure resorption rate.
Cells are known to be lost from tumors into blood and lymphatic vessels, and by exfoliation from the surface if tumors are in
superficial sites. However, in grossly necrotic tumors, many
more cells probably die within the tumor than are lost by the
above routes, and it is assumed below that all modes of cell
loss except cell death within the tumor may be neglected. It is
also assumed that the total rate of necrotic resorption is proportional to the necrotic volume. In the absence of cell death,
necrotic material is then removed exponentially with time, and
one may define a halving time (TN) for the resorption of necrotic tissue.
In the following equations, N is the number of live cells, V is
the tumor volume, and far is the proportion of necrosis by
volume. For the cellular part of the tumor:
dN
dt = aN - bN
Cell Loss
The turnover time (7") of a cell population (i.e., its doubling
time in the absence of cell loss) may be calculated from the
cell cycle time (Tc) and the growth fraction (GF) by the formula (24):
log 2, = log (1 +
GF)
(A)
(C)
where a and b are the respective rate constants for cell production and cell death. The volume occupied by living tissue is
(1 - f / v ) V , and Equation C may be rewritten:
d
d-~ [(1 -- fN)V] = (a-b)
(i -- fN)V
(D)
r
The equation of growth for the necrotic region is:
This equation is accurate for an ideal population with a uniform cell cycle time, but approximate values of turnover time
may be calculated from median values Te; these are easily read
1532
d
d--~(f/v V) = b(1 --fN)V--cfNV
(E)
CANCER RESEARCH VOL. 29
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Solid and A scites Ehrlich Tumors
Here, c is the rate constant for resorption and the terms on the
right represent cell death into, and resorption from, the necrotic part of the tumor. Adding equations (D) and (E):
dV
d"-'f = a ( 1 - - f N ) V - - c f N V
(F)
and substituting the relations:
d V _ log2- V
at
TD
a =!og2
T
and
r
log2
a formula is obtained from which T/v may be calculated:
1
i.e.,
~D =
(1-fN)
T
fN
TN
(G)
The necrotic proportion of tumors OcN) may be calculated
by Chalkley's method (3), turnover time may be estimated
from labeling index, and tumor doubling time may be calculated from serial measurements of tumor volume. Values of
TN are very sensitive to variations in fN, and if there are large
intertumor variations in this necrotic proportion, estimates of
T/v are rather inaccurate. However, the proportion of necrosis
in solid Ehrlich tumors is fairly constant (meanfN = 0.6), and
the halving time for necrotic resorption was about 36 hours.
DISCUSSION
Deceleration of growth in this subline of the Ehrlich ascites
tumor was caused partly by large increases in phases of the cell
cycle, and partly by an increase in the rate of cell death; the
growth fraction remained close to one. These results may be
compared with those of Lala and Part (15, 16); they found no
detectable G 1 period in the Ehrlich ascites tumor and reported
that the growth fraction decreased to about 50% seven days
after injection of 106 cells. It may be that these differences are
due to the strain of animal and tumor subline used. The present observation of large increases in the duration of the S phase
does, however, confirm the observations of other workers (15,
29).
Results of labeled mitoses experiments on a variety of solid
tumors have failed to reveal cell populations with abnormally
long synthesis times (7, 12, 18, 19, 24, 26, 27). This was also
true of the present work on solid Ehrlich tumors. These tumors had a doubling time of about 10 days in the weight range
investigated; however, the median cell cycle time (17 hours)
was only a little greater than that of the fastest growing ascites
tumors (12 hours), and the growth fraction was 80%. The
difference between the rate of cell production and the rate of
tumor growth was due to a high rate of cell loss (~b = 90%).
There was a rapid deceleration of growth in smaller solid
tumors; because larger tumors retained a high rate of cell production, the main cause of this decelerating growth was probably an increase in the cell loss factor.
The present subline of the Ehrlich ascites tumor was also
grown in animals breathing 10% oxygen in nitrogen (air = 21%
oxygen). There was a delay in growth for about 1 day after
implantation, but thereafter tumors grew at the same rate as in
air (27). Tumor growth was thus dependent on blood oxygen
tension only for a short period after implantation. This result
is consistent with platinum electrode measurements of pO 2 in
an ascites hepatoma (5), and with cell survival curves obtained
after irradiation of the Ehrlich tumor (6), which have indicated that the environment of most ascites cells soon becomes
anoxic. Thus, cells in ascites tumors appear to be more resistant
to anoxia than in many solid tumors, where there is evidence
that cells may die soon after becoming anoxic (9, 26-28).
There are a number of possible explanations for this paradox.
Peristaltic action of the intestine probably causes movement of
ascites cells within the peritoneum, allowing repeated oxygenation from surrounding vascularized membranes for short
periods of their cycle; this may be sufficient to prevent cell
death. Also, conditions for cumulative cell death are more
favorable in solid than in ascites tumors. De Duve (4) has
reported that withdrawal of oxygen from a cell may lead to
rupture of lysosomal membranes; thus the probability of ceil
death may increase when cells become anoxic, regardless of
whether they are in solid or ascites tumors. Dead cells are
known to release lysosomal enzymes and other cytotoxic substances (25). Local high concentrations of such agents in solid
tumors could lead to cumulative cell death and necrosis, while
in ascites tumors similar agents might be rapidly diluted and
dispersed when released into the ascites fluid. The presence of
a dialyzable factor, highly toxic to normal cells, has been demonstrated in ascites fluid (10) and may attain a sufficiently
high concentration to cause cell death in older tumors.
The proportion of anoxic cells in ascites tumors increases
with tumor age (5). Such cells derive most of their energy
from glycolysis (2, 13), and lengthening of phases of the cell
cycle with age may be caused partly by the consequent decrease in energy output as compared with respiration. If hypoxic cells are only able to proliferate with long cell cycles,
they probably have little chance to divide in solid tumors
where their lifetime may be short. This hypothesis is consistent with results that indicate a decrease in growth fraction
in hypoxic areas of other tumors (26) and with the occurrence
of "islands" of apparently intact cells in the necrotic centres
of some tumors, including the solid Ehrlich tumor (27). Such
cells may generate tumors on transplantation (9) but have not
been observed to proliferate in situ.
ACKNOWLEDGMENTS
I would like to thank Mrs. J. Lucas for expert technical assistance,
and Professor L. F. Lamerton and Dr. G. G. Steel for their encouragement and useful discussion.
AUGUST 1969
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1. F. T a n n o c k
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CANCER RESEARCH VOL. 29
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Research.
A Comparison of Cell Proliferation Parameters in Solid and
Ascites Ehrlich Tumors
I. F. Tannock
Cancer Res 1969;29:1527-1534.
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