Tissue Radiation Biology
Response to irradiation at the
tissue level;
Tied to cellular division kinetics
In general cells have the same sensitivity
to ionizing radiation as far as nuclear injury
is concerned.
– The DNA in all mammalian cells has about the
same sensitivity to radiation injury.,
Response to irradiation at the
tissue level;
Difference in response become apparent
at the tissue (organ) level.
These differences in radiation sensitivity
are due to the rate of replication inherent
in the critical cells in that
"Law" of Bergonie' and Tribondeau
Radiation has a more rapid (is more
effective) effective against cell that are
actively dividing, are undifferentiated and
have a large dividing future.
Cell differentiation
Undifferentiated cells are precursor or
stem cells and have less specialized
functions. Their major role is to reproduce
to replace themselves and to provide cells
which mature into more differentiated
cells.
Modified by Ancel and Vitemberger
The appearance of radiation damage is
dependent on two factors: 1. The biologic
stress on the cell and 2. the conditions to
which the cell is exposed pre and post
irradiation
The most important biologic stress is
division therefore rapidly dividing cells
express damage earlier and slowly
dividing cells later.
Cell differentiation
The more specialized a cells function is,
the more differentiated it is. (examples are
the major organ cells, muscle and neurons
Highly differentiated cell usually have less
reproductive activity than undifferentiated
cells. (examples of undifferentiated cells
are bone marrow cells, intestinal crypt
cells and basal cells of the skin.
Cell differentiation
Undifferentiated cells are precursor or
stem cells and have less specialized
functions. Their major role is to reproduce
to replace themselves and to provide cells
which mature into more differentiated
cells.
Undifferentiated cells generally are
actively dividing and have a long dividing
future.
Rubin and Casarett
classification of cellular populations
based on reproductive kinetics:
These classifications cells is an attempt to
explain the difference in observed cellular
and tissue radiosensitivity based on the
reproductive and functional characteristics
of various cell lines.
Vegetative Intermitotic
Cells.(VIM)
Undifferentiated rapidly dividing cells
which generally have a quite short life
cycle. Examples are erythroblasts,
intestinal crypt cells and basal cells of the
skin.
Essentially continuously repopulated
throughout life.
Differentiating Intermitotic Cells
(DIM)
Actively mitotic cells with some level of
differentiation. Spermatogonia are a prime
example as well as midlevel cells in
differentiating cell lines.
Have substantial reproductive capability
but will eventually stop dividing or mature
into a differentiate cell line
Multipotential Connective
Tissue Cells
Cells which divide at irregular intervals
often in response to a need. Relatively
long cell life cycle.
Major examples are fibroblasts although
recently more examples of such cells have
been identified in a number of tissues
Reverting Postmitotic Cells
(RPM)
does not normally undergo division but
can do so if called upon by the body to
replace a lost cell population. These are
generally long lived cells.
Mature liver cells, pulmonary cells and
kidney cells make are examples of this
type of cell.
Fixed Postmitotic Cells. (FPM)
These cells do not and cannot divide.
They are highly differentiated and are
highly specialized in there morphology and
function.
May be very long lived or relatively short
lived but replaced by differentiating cells
below them in the cell maturation lines.
Examples are: Neurons, muscle cells and
RBCs
Perceived Radiation Sensitivity
VIM cells are the most sensitive cells to
radiation and FPM cells are most resistant.
The others are of intermediate sensitive in
the order presented.
However, this perception is a product of
the longer cell cycle time in more highly
differentiated cell lines
Michalowski Classification
A more modern type of classification which
essentially says the same thing in another
way.
Michalowski Classification
Stem cells –continuously divide and reproduce
to give rise to both new stem cells and cells that
eventually give rise to mature functional cells.
Maturing cells arising from stem cells and
through progressive division eventually
differentiate into an end-stage mature functional
cell.
Mature adult functional cells that do not divide
(H-type)
There are many cell types that progress
from the stem cell through the mature cell
with nonreversible steps along the way.
These cell lines are said to be
hierarchical (H-type) populations.
They include bone marrow, intestinal
epithelium, epidermis and many others.
F-type populations
There are other cell lines in which the
adult cells can under certain circumstance
be induced to undergo division and
reproduce another adult cell. These cell
are said to be flexible tissue (F-type
populations).
Examples include; liver parenchymal cells,
thyroid cells and pneumocytes as well as
others.
Michalowski Classification
These two types represent extremes and there
are many tissues which exhibit characteristics of
both types where mature cells are able to divide
a limited number of times.
The rapidity of response to and hence the
sensitivity to radiation at the tissue level is
dependent on the length of the life cycle and
the reproductive potential of the critical cell
line within that tissue.
“Critical Cells"
All tissues contain multiple cell types
contained in either the stromal
compartment or the parenchymal
compartment.
A cell in either compartment may be the
critical cell.
“Critical Cells"
the endothelial cells lining the blood
vessels were thought for many years to be
the critical cells in tissues however
"critical cells" have been identified in many
tissues.
The time required for the tissue to
respond to radiation injury can be
predicted on the basis of the cell
cycle kinetics of these critical cells.
Biologic Factors moderating Cell
injury by irradiation.
Cell Cycle.
Intracellular repair
Hypoxia
Cell Cycle.
The point that a cell is in the cell cycle has
a marked influence on its response and
survival of irradiation.
G1 & G0 are relatively insensitive to
radiation injury.
S phase is generally considered to be the
most resistant to radiation injury.
Cycle Phase Influence on
Sensitivity
Intracellular repair
The shoulder on the cell survival curve
indicates that there is some degree of
repair by cells of radiation injury.
Amount of repair differs between cell lines
However the rate of repair is the same
Intracellular Repair
Intracellular repair
Intracellular repair
Studies have shown that although repair
can be an ongoing process, the vast
majority of the repair is finished by 6 hours
post irradiation.
Once repair is complete the remaining cell
population will respond to subsequent
dose of radiation as though the original
irradiation had not occurred
Hypoxia
Oxygen is a potent preventer of repair
Hypoxia markedly improves the ability of
the cells to repair radiation injury
However it is quite rare for a normal
somatic cell to be hypoxic.
Measurement or radiation injury at
the tissue level
Assay systems are needed to construct
survival and injury curves for irradiation at
the tissue level.
Such assays must be quantifiable
The effect measured must increase with
dose
Types of Assays
Clonogenic (related to reproductive
potential of stem cells in the tissue target
cell population
Specific tissue functional capability
Lethality - death of the organism from
radiation of that tissue
Clonogenic assays
May be performed in vivo or in vitro
In an in vitro assay cells are harvested
from tissue irradiated in living tissue and
the cells are grown out in cell culture and
the number of colonies growing out is
compared to that for a control
In vivo assays are performed by
evaluation of cellular reproductive activity
in the living animal
In Vitro Assays
Cells harvested from culture and plated
out – many, many flasks or dishes
Dishes are irradiated at different levels
The number of colonies are counted after
a specific time.
# of colonies compared to control sample
Survival curves generated
In vivo Assays
Two types
– In Situ Assays
– Transplantation Assays
In Situ Assays
The tissue or organ is irradiated in the
whole animal. At a given time after
irradiation the organism (animal or plant) is
sacrificed and the organ of interest is
evaluated for cell survival of the cell of
interest.
Classic example is the intestinal crypt cell
studies
In Situ Assays
Another example is irradiation of testes
and then assaying the testicle for surviving
spermatogonia in the tubules of the
testicle.
In Situ Assays
Classically these assays have been used
to evaluate the radiation effects in acutely
responding (rapidly dividing) cell lines
such as the intestinal villi, the testes and
the skin.
Recently these types of assays have been
extended to evaluation (slowly dividing)
cell lines.
In situ Assays
These assays have shown that the Do for
slowly dividing cells in this assay is about
1.5 Gy or about the same as for the rapidly
responding tissues. The difference in time
required for the cell killing to occur is a
manifestation of the slow turnover rate of
the cells.
In Situ Assays
Tissue is irradiated in vivo and returned to
subject.
After a period of time the number of viable
cell groups in irradiated area is measured
Generally done in mice as large numbers
are required.
Intestinal and gonadal epithelium are the
classic tissues studied.
In Situ Assays
Studies of RPM and FPM tissues and cell
lines requires much longer experiment
May require use of larger more expensive
and long lived animals
These studies are very expensive to do
Transplantation Assays
Often used to study tumor sensitivity
Usually done in immune compromised
animals.
Done to mimic metastatic disease or to
remove immune system effects in live
animal
Transplantation Assays
Tumor or test tissue is irradiated while still
in donor animal.
Irradiated tissue is then removed and cells
suspended in solution.
Cells then injected into recipient animal
After a period of growth, the animals are
sacrificed and the number of tissue
colonies or size of colony is measured.
Functional Assays
Measure organ functional capacity
– Most organs have clinical functional reserve
– Tests measure complete functional capacity
Done in a live animal with “in situ” organs
Do not require sacrifice of the animal
– Multiple dose levels can be studied
Measure effect at sub clinical levels.
Heart, lungs, liver, kidneys applicable
Functional Assays
Measure effects of regional irradiation
Very important in radiation therapy
– Helps predict effects of irradiation plan
Also used to study effects of ingested
radionuclides either medical or accidental
Useful for studies on effect modifiers such
as chemotherapy.
Lethality Assays
Measures clinical effects
Measures doses required to cause death.
– Whole body irradiation
– Regional body irradiation (brain, heart, liver,
etc.
Generally expressed in terms of % death
in a given time. i.e. LD30/90
– 30% of subjects die by 90 days post
irradiation