XIII Cell and Tissue
Kinetics
Lecture 13
Ahmed Group
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Cell cycle
Measurement of cell cycle parameters by 3H-thymidine
Measurement by flow cytometry, DNA staining and BrdU
Cell cycle synchronization techniques and uses
Effect of cell cycle phase on radiosensitivity
Cell cycle arrest and redistribution following irradiation
Cell cycle checkpoints, cyclins, cyclin dependent kinase
inhibitors
Tissue kinetics
Stem, progenitor, differentiated cells
Growth fraction
Cell loss factor
Volume doubling times
Tpot
Growth kinetics of clinical and experimental tumors
Lecture 13
Ahmed Group
The Cell Cycle
The average interval between successive mitoses or divisions is called the
cell cycle or mitotic cycle time (Tc)
Howard and Pelc were the
first to subdivide the mitotic
cycle by the use of a labeled
DNA prescursor.
Mitosis (M) is the only event that can be distinguished by light microscope. The DNA
synthetic phase (S) may be identified by autoradiography. The intervals of apparent
inactivity are G1 and G2.
Lecture 13
Ahmed Group
The proportion of cells in mitosis is called
the “mitotic index”.
MI = lTM/TC
Proportion of labeled cells is called the
“Labeling Index”.
LI = lTS/TC
Cells cannot be distributed uniformly in time around the cell cycle because they
double in number during mitosis. The simplest assumption is that they are
distributed as an exponential function of time.
Lecture 13
Ahmed Group
•Cell cycle
•Measurement of cell cycle parameters by 3H-thymidine
•Measurement by flow cytometry, DNA staining and BrdU
•Cell cycle synchronization techniques and uses
•Effect of cell cycle phase on radiosensitivity
•Cell cycle arrest and redistribution following irradiation
•Cell cycle checkpoints, cyclins, cyclin dependent kinase
inhibitors
•Tissue kinetics
•Stem, progenitor, differentiated cells
•Growth fraction
•Cell loss factor
•Volume doubling times
•Tpot
•Growth kinetics of clinical and experimental tumors
Lecture 13
Ahmed Group
Cell-labeling techniques
• Autoradiography
– [3H] Thymidine, Howard and Pelc,
1953
• Dye/staining
– 5-Bromodeoxyuridine
• Flow-cytometry
– Propidium Iodide
Lecture 13
Ahmed Group
Measurement of cell cycle
3
parameters by H-thymidine
Mouse Corneal Cells
Percentage of mitotic cells that carry a
radioactive
label
indicates
the
percentage of labeled mitoses.
The cell preparation was flash labeled some hours before with tritiated
thymidine, which was taken up by cells in S. By the time the autoradiograph was
made, the cell marked LM had moved around the cycle into mitosis; this is an
example of labeled mitotic figure. Other cells in mitosis are not labeled (UM).
Lecture 13
Ahmed Group
A plot of percentage of labeled cells as function of time
Percent-labeled mitoses curve for an
idealized cell population in which all of
the cells have identical mitotic cycle
times.
Typical percent-labeled mitoses curve
obtained in practice for the cells of a
tissue or tumor. It differs from the
idealized curve in that the only points
that can be identified with precision are
the peaks of the curve and the 50%
levels. The first peak is symmetric, and
the second peak is lower than the first
because the cells of a population have a
range of cell cycle times.
TG1 = Tc- (Ts + TG2 + TM)
Mitotic Index = TM / Tc
Lecture 13
Ahmed Group
In-vivo Labeling Index
Percent-labeled mitoses curve for two
transplantable rat sarcomas with
widely different growth rates. The
tumor in the upper panel has a gross
doubling time of 22 hours, compared
with 190 hours for the tumor in the
lower panel.
Lecture 13
Percent-labeled mitoses curve for an
EMT6 mouse tumor. Top: The
distribution of cell cycle times
consistent with the damped labeled
mitoses curve, obtained by computer
analysis of the data and the
mathematic model.
Ahmed Group
Lecture 13
Ahmed Group
•Cell cycle
•Measurement of cell cycle parameters by 3H-thymidine
•Measurement by flow cytometry, DNA staining and BrdU
•Cell cycle synchronization techniques and uses
•Effect of cell cycle phase on radiosensitivity
•Cell cycle arrest and redistribution following irradiation
•Cell cycle checkpoints, cyclins, cyclin dependent kinase
inhibitors
•Tissue kinetics
•Stem, progenitor, differentiated cells
•Growth fraction
•Cell loss factor
•Volume doubling times
•Tpot
•Growth kinetics of clinical and experimental tumors
Lecture 13
Ahmed Group
Measurement by flow cytometry,
DNA staining and BrdU
Pulsed Photo Cytometry
• Excited the stained DNA
with a laser beam and
subsequently detected
using a fluorescence
detector.
• As per the DNA content,
the cells are distributed
in different phases of
cell cycle.
Lecture 13
Ahmed Group
The flow cytometric analysis of cellular bromodexyuridine (BrdUrd) and DNA
content for cells stained with fluorescein (linked to BrdUrd) and propidium iodide
(linked to DNA) using a single biopsy specimen taken 4 to 8 hours after the
injection of a tracer amount of a thymidine analogue (BrdUrd or Iododeoxyurine).
The LI is the green fluorescence.
Ts is mean red fluorescence of S cells
relative to G1 and G2 cells. The DNA
content in G2 is double that in G1.
Lecture 13
Ahmed Group
Autoradiography / Dye staining
• Pulse labeling
Lecture 13
Ahmed Group
Autoradiograph / Bromodeoxyuridine staining
Lecture 13
Ahmed Group
•Cell cycle
•Measurement of cell cycle parameters by 3H-thymidine
•Measurement by flow cytometry, DNA staining and BrdU
•Cell cycle synchronization techniques and uses
•Effect of cell cycle phase on radiosensitivity
•Cell cycle arrest and redistribution following irradiation
•Cell cycle checkpoints, cyclins, cyclin dependent kinase
inhibitors
•Tissue kinetics
•Stem, progenitor, differentiated cells
•Growth fraction
•Cell loss factor
•Volume doubling times
•Tpot
•Growth kinetics of clinical and experimental tumors
Lecture 13
Ahmed Group
Cell cycle synchronization
techniques and uses
A synchronized culture is one where cells pass through the division
cycle as a relatively uniform cohort and represent, at different time
points, cells of different cell cycle ages. In passing through the cell
cycle a newborn eukaryotic cell first passes through the G1 phase
(absence of DNA synthesis), then S phase (period of DNA synthesis),
then G2 phase (absence of DNA synthesis), with division occurring at
M phase (mitosis). If many cells in a culture approximate this pattern
as a group, these cells would be called a synchronized culture.
Cooper, 2003
Lecture 13
Ahmed Group
Cell cycle synchronization techniques
• Mitotic shake-off
Terasima and Tolmach
• Use of drug/pharmacological agents
Hydroxyurea (Adverse cellular perturbations)
• Serum deprivation
• Contact inhibition
• Centrifugal elutriation
• Fluorescence activated cell sorting
Batch treatments (i.e., treatments that affect all cells equally)
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Lecture 13
Many methods related to serum starvation may be grouped as methods that
arrest cells at a specific point in the G1 phase, sometimes referred to as the
‘restriction point.’
Other synchronization methods such as the double-thymidine block method or
hydroxyurea inhibition affect DNA synthesis and are proposed to arrest cells in
S phase.
A third class of batch synchronization methods arrests cells at mitosis. The
classical inhibitor for such a process is nocodazole.
Ahmed Group
Mode of action of HU
Lecture 13
Ahmed Group
HU effect on Vicia root tip
Lecture 13
Ahmed Group
Serum starvation
Serum starvation or low serum concentration is believed to arrest
cells at a particular point in the cell cycle. The arrested cells with a G
phase amount of DNA are proposed to be arrested at this point in the
G1 phase or in what is generally called the G0 phase.
Upon restoration of serum, these arrested cells are assumed to pass
synchronously through the cell cycle.
Cooper, 2003
Lecture 13
Ahmed Group
Membrane-elution
Exponentially growing cells bound to a membrane (Cells grow normally on
the membrane)
At division, one of the daughter cells remains attached to the membrane
while the other daughter cell is released into the eluate
In a typical membrane-elution synchronized culture, one can produce three
clear synchronized divisions with clear plateaus between the doublings in
cell number.
Temperature and medium are maintained constant throughout the procedure. DNA content
distributions as well as size distributions indicate that the cells eluted from the membrane-elution
apparatus reflect the normal pattern of growth during the division cycle. The newborn cells are
appropriately small (equivalent in size to the cells at the low-size end of the size distribution of
the exponential culture) and these newborn cells have a G1 phase DNA content.
The membrane- elution method therefore shows that without any starvation, one can have a
narrowing in the size distribution and a narrow DNA content that reflects the normal cell cycle.
Cooper, 2003
Lecture 13
Ahmed Group
Cell cycle synchronization uses
Understanding the molecular and biochemical basis of cellular growth and
division involves the investigation of regulatory events that most often occur in
a cell-cycle phase-dependent fashion. Studies examining cell-cycle regulatory
mechanisms and progression invariably require cell-cycle synchronization of
cell populations.
Davis et al ., 2001
Lecture 13
Ahmed Group
•Cell cycle
•Measurement of cell cycle parameters by 3H-thymidine
•Measurement by flow cytometry, DNA staining and
BrdU
•Cell cycle synchronization techniques and uses
•Effect of cell cycle phase on radiosensitivity
•Cell cycle arrest and redistribution following irradiation
•Cell cycle checkpoints, cyclins, cyclin dependent kinase
inhibitors
•Tissue kinetics
•Stem, progenitor, differentiated cells
•Growth fraction
•Cell loss factor
•Volume doubling times
•Tpot
•Growth kinetics of clinical and experimental tumors
Lecture 13
Ahmed Group
Effect of cell cycle phase on
radiosensitivity
Chinese hamster cells
Lecture 13
Ahmed Group
Radiation response in different phases of
cell cycle
Chinese hamster cells
Lecture 13
Most sensitive:
Most resistant:
Intermediate:
G2 and M
Late S
G1 and early S
Ahmed Group
Synchronized human kidney cells
show
a
differential
survival
depending on cell cycle phase in
which they are irradiated. Cells are
most sensitive to irradiation during
mitosis and in G2, less sensitive in
G1, and least sensitive during the
latter part of S phase.
Pawlik and Keyomarsi, 2004
Lecture 13
Ahmed Group
Radiation response and the length of G1
• Longer G1 leads to resistance in early G1
followed by enhanced sensitivity in the end of G1.
Lecture 13
Ahmed Group
Radiation response in-vivo Age-response
Lecture 13
Ahmed Group
Summary
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Cell cycle phases: M, G1, S and G2
CDK family regulates the phases of cell cycle
Fastest cycling cells: crypt cells (9-10 hrs)
Slowest cycling cells: mouse skin stem cells (200hrs)
M and G2 are radiosensitive and late S is radioresistant
Long G1 results in a second resistant peak in early G1
Radiation induces G2 arrest through molecular checkpoint
genes
• The age-response for crypt cells in tissue is similar to that
of cells in culture
Lecture 13
Ahmed Group
•Cell cycle
•Measurement of cell cycle parameters by 3H-thymidine
•Measurement by flow cytometry, DNA staining and BrdU
•Cell cycle synchronization techniques and uses
•Effect of cell cycle phase on radiosensitivity
•Cell cycle arrest and redistribution following irradiation
•Cell cycle checkpoints, cyclins, cyclin dependent kinase
inhibitors
•Tissue kinetics
•Stem, progenitor, differentiated cells
•Growth fraction
•Cell loss factor
•Volume doubling times
•Tpot
•Growth kinetics of clinical and experimental tumors
Lecture 13
Ahmed Group
Cell cycle arrest and redistribution
following irradiation
Several chemotherapeutic drugs and ionizing radiation (IR) can arrest the
cells in different phases of the cell cycle leading to cell cycle phase
redistribution and may lead to partial synchronization with time.
For example: IR can retard the rate of progression of proliferating cell
populations through various phases of the cell cycle causing cells to
accumulate in G2 phase and keeping cells from undergoing mitotic division.
Split or fractionated radiation may therefore be more efficacious by inducing
a transient cell cycle arrest after which a secondary RT fraction is
administered exactly at the height of cell accumulation in the most
radiosensitive cell cycle phase (G2/M).
This suggests that the redistribution of cells in a particular phase would
determine the response of an initially asynchronous population to
fractionated RT.
Lecture 13
Ahmed Group
•Cell cycle
•Measurement of cell cycle parameters by 3H-thymidine
•Measurement by flow cytometry, DNA staining and BrdU
•Cell cycle synchronization techniques and uses
•Effect of cell cycle phase on radiosensitivity
•Cell cycle arrest and redistribution following irradiation
•Cell cycle checkpoints, cyclins, cyclin dependent kinase
inhibitors
•Tissue kinetics
•Stem, progenitor, differentiated cells
•Growth fraction
•Cell loss factor
•Volume doubling times
•Tpot
•Growth kinetics of clinical and experimental tumors
Lecture 13
Ahmed Group
Cell cycle checkpoints, cyclins, cyclin
dependent kinase inhibitors
Molecular checkpoint genes
Lecture 13
Ahmed Group
Genes regulate cell cycle phases
• Cyclin-dependent kinases
• Cyclins A, B, D1, K1, K4 and E
• Rb
Blockers
• p53 mediated p21 inhibits
CDKs
Lecture 13
Ahmed Group
Cell Cycle Regulation
The current concept of the cycle and its regulation by
protein kinases, activated by cyclins.
Lecture 13
Ahmed Group
Progression through the cell cycle from one phase
to the next is governed by protein kinases,
activated by cyclins. In mammals, cyclins A
through H have been described: each cyclin
protein is synthesized at a discrete phase of the
cell cycle. Cyclin levels oscillate with phase of
cycle, as shown schematically in this figure
Lecture 13
Ahmed Group
DNA damage response reactions in mammalian cells
The four responses (DNA repair, transcriptional response, DNA damage checkpoints, and
apoptosis) may function independently, but frequently a protein primarily involved in one
response may participate in other responses.
Sancar et al., 2004
Lecture 13
Ahmed Group
Components of the DNA damage checkpoints in human cells
Current model of the mammalian DNA damage and
replication checkpoints. A line ending with an arrowhead
indicates activation. A line ending with a bar indicates
inhibition.
Li and Zou, 2005
The damage is detected by sensors that, with
the aid of mediators, transduce the signal to
transducers. The transducers, in turn, activate
or inactivate other proteins (effectors) that
directly participate in inhibiting the G1/S
transition, S-phase progression, or the G2/M
transition.
Lecture 13
Sancar et al., 2004
Ahmed Group
Sancar et al., 2004
Lecture 13
Ahmed Group
A simplified scheme of cell-cycle checkpoint
pathways induced in response to DNA damage
(here DSBs), with highlighted tumor suppressors
shown in red and proto-oncogenes shown in green.
The proximal checkpoint kinases ATM and ATR
phosphorylate diverse components of the network,
either directly (red ‘P’) or through the transducing
kinases CHK2 and CHK1 (black ‘P’). (For simplicity,
some candidate damage sensors and several
ATM/ATR and CHK1/CHK2 substrates have been
omitted.) The BRCA1 protein also contributes to
cell-cycle arrest and DNA repair by homologous
recombination, whereas p53 controls genes
involved in cell death and DNA-repair mechanisms.
The cell-cycle phase and the duration of the
blockade affected by the effector pathways are
indicated, including the potential permanent arrest
(senescence), as mediated by p53. The global
checkpoint network regulated by ATM/ATR and
CHK2/CHK1 also affects cellular responses other
than cell cycle progression, including DNA repair,
transcription, chromatin assembly and cell death.
Kastan and Bartek, 2004
Lecture 13
Ahmed Group
The G1/S checkpoint
DNA damage is sensed by ATM after doublestrand breaks or by ATR, Rad17-RFC, and the 9-11 complex after UV-damage. ATM/ATR
phosphorylates Rad17, Rad9, p53, and Chk1/Chk2
that in turn phosphorylates Cdc25A, causing its
inactivation by nuclear exclusion and ubiquitinmediated degradation. Phosphorylated and
inactivated Cdk2 accumulates and cannot
phosphorylate Cdc45 to initiate replication.
Maintenance of the G1/S arrest is achieved by p53,
which is phosphorylated on Ser15 by ATM/ATR
and on Ser20 by Chk1/Chk2. Phosphorylated p53
induces p21WAF-1/Cip1 transcription, and
p21WAF-1/Cip1 binds to the Cdk4/CycD
complex, thus preventing it from phosphorylating
Rb, which is necessary for the release of the E2F
transcription factor and subsequent transcription
of S-phase genes. p21WAF-1/Cip1 also binds to
and inactivates the Cdk2/CycE complex, thus
securing the maintenance of the G1/S checkpoint.
Sancar et al., 2004
Lecture 13
Ahmed Group
The ATM-regulated intra-S-phase checkpoint
In response to double-strand breaks induced
by ionizing radiation, ATM triggers two
cooperating parallel cascades to inhibit
replicative DNA synthesis. ATM, through
the intermediacy of MDC1, H2AX, and
53BP1, phosphorylates Chk2 on Thr68 to
induce ubiquitin-mediated degradation of
Cdc25A phosphatase. The degradation locks
the S phase–promoting Cyclin E/Cdk2 in its
inactive, phosphorylated form and prevents
the loading of Cdc45 on the replication
origin. ATM also initiates a second pathway
by phosphorylating NBS1 of the M/R/N
complex, as well as SMC1, BRCA1, and
FANCD2.
Sancar et al., 2004
Lecture 13
Ahmed Group
The ATR-mediated intra-S-phase checkpoint
The ATR-ATRIP complex,
Rad17-RFC, the 9-1-1 complex,
and Claspin are independently
recruited to RPA(A)- coated
single-stranded regions of the
stalled replication fork. ATR
then phosphorylates Chk1 and
other substrates, and activated
Chk1 phosphorylates Cdc25A,
which leads to inactivation of
Cdk2/Cyclin E complex. Singlestrand DNA gaps can also be
sensed by ATR and following
ATR activation, origin firing and
consequently DNA replication is
inhibited
through
downregulation of Cdc7/Dbf4
protein kinase activity.
Sancar et al., 2004
Lecture 13
Ahmed Group
The G2/M checkpoint
The ionizing radiation- (ATM) and UV
damage responsive sensor proteins (ATRATRIP, Rad17-RFC, and 9-1-1) are
recruited to the damage site. The mediator
proteins such as MDC1, BRCA1 and/or
53BP1 communicate the DNA damage
signal to Chk1 and/or Chk2, thereby
regulating the Cdc2/CyclinB, Wee1, and
Cdc25A proteins that are crucial for the
G2/M transition by changing their
expression, phosphorylation, and cellular
localization.
Sancar et al., 2004
Lecture 13
Ahmed Group
The Replication Checkpoint (S/M Checkpoint)
The replication checkpoint is the process by which mitosis is inhibited while DNA
replication is ongoing or blocked. In both the G2/M and replication checkpoints, the ATRChk1-Cdc25 signal transduction pathway is utilized to inhibit mitosis, although the
initiating signals for the two checkpoints are different. Ongoing replication or replication
forks blocked by DNA damage or nucleotide starvation initiate the replication checkpoint.
Evidence from in vitro studies with Xenopus egg extracts indicates that the initiating signal
is a component of the replication fork, and experiments with various replication inhibitors
suggested that the signal might actually be the RNA primer of Okazaki fragments.
However, more recent experiments have raised questions about the true identity of the
replication checkpoint signal, as the inhibitors used to assign it to RNA primers inhibited
the entire replisome assembly.
The replication checkpoint has been observed in all model systems, including yeast and
mammalian cells. Surprisingly, however, recent evidence indicates that in mice the
replication checkpoint activated by hydroxyurea and aphidicolin is independent of ATM
and ATR, but the UV- and ionizing radiation induced replication checkpoint is dependent
on these damage sensor/signal transducer kinases, raising the possibility that replication
forks and DNA damage during S phase may inhibit mitosis by different signaling
mechanisms.
Sancar et al., 2004
Lecture 13
Ahmed Group
Schematic Representation of the DNA damage and replication checkpoint in yeast
and mammals. Arrows represent either biochemical or genetic evidence for a
connection. Regulators of the DNA damage response are indicated in red.
Regulators that act only in the replication/intra-S-phase checkpoint are indicated in
blue. Kinases that are used by both branches of the pathway are in green.
McGowan, 2002
Lecture 13
Ahmed Group
Cyclin dependent kinase inhibitor; p21
Schematic model of the p21CIP1 protein and its key functional activities mediated by
binding to different partner proteins. Apparently, p21CIP1 can localize to different
cellular compartments including the nucleus, the cytoplasm, and the centrosome.
Samuel et al., 2002
Lecture 13
Ahmed Group
Links to cancer and genetic instability
Loss of cell cycle checkpoint control has emerged as a central cause of genetic instability.
Consequently, chances that these unstable cells progress to cancer are increased. This notion
has several important implications:
1. Since checkpoints may determine the ultimate response (arrest vs. apoptosis), the integrity
of these checkpoints influences the susceptibility of cells to DNA damage. This is relevant
either to the cells’ fate after accumulation of undesired DNA damage or to the cells’ sensitivity
to desired damage during chemo-/radiotherapy.
2. Exploring the early checkpoint defects in cancerous or pre-cancerous lesions may serve as a
prognostic or, in certain tissues, as an additional diagnostic marker.
3. Known defects of pivotal checkpoint genes may help to predict treatment outcome or to
design more specific therapeutic strategies. In addition, checkpoint components which are
defective in certain cancer cells may be targeted during therapy to enhance the anti-tumor
effect, e.g., by preventing arrest and/or by forcing cells into apoptosis. Work is in progress to
develop novel therapeutic strategies with an increased therapeutic index.
4. Strategies could be considered to restore missing or dysfunctional checkpoints in order to
provide additional time for DNA repair and delay the onset of cancer.
5. Since some of the components that are involved in the DNA damage checkpoint are also
involved in other cellular regulatory activities, e.g., during senescence, differentiation, or certain
immunological responses, this could lead to cross-signaling into other pathways and might
permit new strategies to influence related cellular functions.
Samuel et al., 2002
Lecture 13
Ahmed Group
Kastan and Bartek, 2004
Lecture 13
Ahmed Group
Schematic representation of two main steps that contribute to a spectrum of mutations leading to
cancer development. If DNA damage is repaired efficiently, the likelihood of tumor development
is low. If cells have mutations in DNA-damage response signaling pathways — either sporadic or
inherited — this will lead to enhanced genomic abnormalities. Cells with damaged DNA
frequently arrest or do not survive, thus reducing the probability that they will progress to
malignancy. Mutations in apoptosis pathways, DNA-damage, DNA-repair or mitotic-checkpoint
pathways can permit the survival or continued growth of cells with genomic abnormalities, thus
enhancing the likelihood of malignant transformation.
Kastan and Bartek, 2004
Lecture 13
Ahmed Group
•Cell cycle
•Measurement of cell cycle parameters by 3H-thymidine
•Measurement by flow cytometry, DNA staining and BrdU
•Cell cycle synchronization techniques and uses
•Effect of cell cycle phase on radiosensitivity
•Cell cycle arrest and redistribution following irradiation
•Cell cycle checkpoints, cyclins, cyclin dependent kinase
inhibitors
•Tissue kinetics
•Stem, progenitor, differentiated cells
•Growth fraction
•Cell loss factor
•Volume doubling times
•Tpot
•Growth kinetics of clinical and experimental tumors
Lecture 13
Ahmed Group
Stem, progenitor, differentiated cells
Stem cell population
• Sole purpose is to divide to (1) first maintain its own population (I.e. self
renewal) And (2) produce cells for another population.
• Undifferentiated
• Basal cells in the epidermis of the skin, cells in the bone marrow, the cells
in the crypts of lieberkuhn in the intestine, and spermatogonia in the testis.
• Tissues and organs containing stem cells are referred to as self renewing.
• Radiosensitive
Progenitor cells
• Transit cell populations.
• cells are on their way from one place (stem cell compartment) to another
place (end cell compartment).
• While in transit these cells may or may not divide. An example of cell that
divides is the nucleated red cell. A cell that does not divide is the reticulocyte
in the bone marrow.
Lecture 13
Ahmed Group
Differentiated cells
• A cell that is specialized functionally and/or morphologically (structurally).
• A mature cell or end cell in a population.
• An example of tissue that contains a series of cells in various stages of
differentiation is the testis.
• Another example is red blood cell.
• According to the law of Bergonie and Tribondeau, the degree of cellular
differentiation is inversely related to the radiosensitivity of cells.
Lymphocytes are exception to this rule.
Diagrammatic representation of testes illustrating differentiation. The cell becomes more
differentiated as it progresses from spermatogonium (stem cell) to sperm (end cell).
Lecture 13
Ahmed Group
Growth Fraction
The Growth Fraction (GF) is defined as a ratio of the number of proliferating cells to the total number of cells
(P, proliferating +Q, quiescent). The GF is also given by the expression of fraction of cells labeled divided by
the fraction of mitoses labeled.
GF = P / P + Q
GF=
Lecture 13
Fraction of Cells labeled
Fraction of mitoses labeled
Ahmed Group
Cell Loss
The cell-loss factor represents the ratio of the rate of cell loss to the rate of new cell production.
F = 1-Tpot/Td
Lecture 13
Ahmed Group
Overall pattern of tumor growth
Lecture 13
Ahmed Group
Volume doubling times
Time taken for a tumor to double in volume.
Depends on three factors:
• The cell cycle time of the proliferating cells
• The growth fraction
• The degree of cell loss from the tumor
Lecture 13
Ahmed Group
Tumor potential doubling time
• Tpot is a measure of the rate of increase of cells capable of
continued proliferation and therefore may determine the outcome of
fractionated radiation therapy over an extended period.
Tpot = lTS/TLI
• Tumors with short Tpot will repopulate if fractionation is extended
too long a period.
Lecture 13
Ahmed Group
Growth kinetics of tumors
1. The cell cycle of the proliferative cells in a tumor
2. The growth factor, that fraction of cells in the population that is P as opposed to Q.
3. The rate of cell loss, either by cell death or loss from the tumor.
Lecture 13
Ahmed Group
Lecture 13
Ahmed Group
Definitions
• The proportion of cells in mitosis is called the “mitotic index”. MI =
lTM/TC
• Proportion of labeled cells is called the “Labeling Index”. LI = lTS/TC
• Tpot is a measure of the rate of increase of cells capable of
continued proliferation and therefore may determine the outcome of
fractionated radiation therapy over an extended period. Tpot=
lTS/TLI
• The Growth Fraction (GF) is defined as a ratio of the number of
proliferating cells to the total number of cells (P, proliferating +Q,
quiescent)
• The GF is also given by the expression of fraction of cells labeled
divided by the fraction of mitoses labeled.
• The cell-loss factor represents the ratio of the rate of cell loss to the
rate of new cell production. F = 1-Tpot/Td
Lecture 13
Ahmed Group