tissue radiation biology

Tissue Radiation BiologyTissue Radiation Biology

Response to irradiation at the Response to irradiation at the tissue level; tissue level;

Tied to cellular division kinetics Tied to cellular division kinetics

In general cells have the same sensitivity In general cells have the same sensitivity to ionizing radiation as far as nuclear injury to ionizing radiation as far as nuclear injury is concerned. is concerned. – The DNA in all mammalian cells has about the The DNA in all mammalian cells has about the

same sensitivity to radiation injury.,same sensitivity to radiation injury.,

Response to irradiation at the Response to irradiation at the tissue level; tissue level;

Difference in response become apparent Difference in response become apparent at the tissue (organ) level.at the tissue (organ) level.

These differences in radiation sensitivity These differences in radiation sensitivity are due to the rate of replication inherent are due to the rate of replication inherent in the critical cells in that in the critical cells in that

"Law" of Bergonie' and Tribondeau "Law" of Bergonie' and Tribondeau

Radiation has a more rapid (is more Radiation has a more rapid (is more effective) effective against cell that are effective) effective against cell that are actively dividing, are undifferentiated and actively dividing, are undifferentiated and have a large dividing future. have a large dividing future.

Cell differentiation Cell differentiation

Undifferentiated cells are precursor or Undifferentiated cells are precursor or stem cells and have less specialized stem cells and have less specialized functions. Their major role is to reproduce functions. Their major role is to reproduce to replace themselves and to provide cells to replace themselves and to provide cells which mature into more differentiated which mature into more differentiated cells.cells.

Modified by Ancel and Vitemberger Modified by Ancel and Vitemberger

The appearance of radiation damage is The appearance of radiation damage is dependent on two factors: 1. The biologic dependent on two factors: 1. The biologic stress on the cell and 2. the conditions to stress on the cell and 2. the conditions to which the cell is exposed pre which the cell is exposed pre and and post post irradiation irradiation The most important biologic stress is The most important biologic stress is division therefore rapidly dividing cells division therefore rapidly dividing cells express damage earlier and slowly express damage earlier and slowly dividing cells later. dividing cells later.


The more specialized a cells function is, The more specialized a cells function is, the more differentiated it is. (examples are the more differentiated it is. (examples are the major organ cells, muscle and neuronsthe major organ cells, muscle and neurons

Highly differentiated cell Highly differentiated cell usuallyusually have less have less reproductive activity than undifferentiated reproductive activity than undifferentiated cells. (examples of undifferentiated cells cells. (examples of undifferentiated cells are bone marrow cells, intestinal crypt are bone marrow cells, intestinal crypt cells and basal cells of the skin. cells and basal cells of the skin.


Undifferentiated cells are precursor or Undifferentiated cells are precursor or stem cells and have less specialized stem cells and have less specialized functions. Their major role is to reproduce functions. Their major role is to reproduce to replace themselves and to provide cells to replace themselves and to provide cells which mature into more differentiated which mature into more differentiated cells.cells.Undifferentiated cells generally are Undifferentiated cells generally are actively dividing and have a long dividing actively dividing and have a long dividing future. future.

Rubin and CasarettRubin and Casarett

classification of cellular populationsclassification of cellular populations

based on reproductive kinetics: based on reproductive kinetics:

These classifications cells is an attempt to These classifications cells is an attempt to explain the difference in observed cellular explain the difference in observed cellular and tissue radiosensitivity based on the and tissue radiosensitivity based on the reproductive and functional characteristics reproductive and functional characteristics of various cell lines. of various cell lines.

Vegetative Intermitotic Cells.Vegetative Intermitotic Cells.(VIM)(VIM)

Undifferentiated rapidly dividing cells Undifferentiated rapidly dividing cells which generally have a quite short life which generally have a quite short life cycle. Examples are erythroblasts, cycle. Examples are erythroblasts, intestinal crypt cells and basal cells of the intestinal crypt cells and basal cells of the skin.skin.

Essentially continuously repopulated Essentially continuously repopulated throughout life.throughout life.

Differentiating Intermitotic Cells Differentiating Intermitotic Cells (DIM)(DIM)

Actively mitotic cells with some level of Actively mitotic cells with some level of differentiation. Spermatogonia are a prime differentiation. Spermatogonia are a prime example as well as midlevel cells in example as well as midlevel cells in differentiating cell lines. differentiating cell lines.

Have substantial reproductive capability Have substantial reproductive capability but will eventually stop dividing or mature but will eventually stop dividing or mature into a differentiate cell lineinto a differentiate cell line

Multipotential Connective Multipotential Connective Tissue CellsTissue Cells

Cells which divide at irregular intervals Cells which divide at irregular intervals often in response to a need. Relatively often in response to a need. Relatively long cell life cycle. long cell life cycle.

Major examples are fibroblasts although Major examples are fibroblasts although recently more examples of such cells have recently more examples of such cells have been identified in a number of tissuesbeen identified in a number of tissues

Reverting Postmitotic Cells Reverting Postmitotic Cells (RPM)(RPM)

does not normally undergo division but does not normally undergo division but can do so if called upon by the body to can do so if called upon by the body to replace a lost cell population. These are replace a lost cell population. These are generally long lived cells. generally long lived cells.

Mature liver cells, pulmonary cells and Mature liver cells, pulmonary cells and kidney cells make are examples of this kidney cells make are examples of this type of cell. type of cell.

Fixed Postmitotic Cells. (FPM)Fixed Postmitotic Cells. (FPM)

These cells do not and cannot divide. These cells do not and cannot divide. They are highly differentiated and are They are highly differentiated and are highly specialized in there morphology and highly specialized in there morphology and function. function. May be very long lived or relatively short May be very long lived or relatively short lived but replaced by differentiating cells lived but replaced by differentiating cells below them in the cell maturation lines. below them in the cell maturation lines. Examples are: Neurons, muscle cells and Examples are: Neurons, muscle cells and RBCs RBCs

Perceived Radiation SensitivityPerceived Radiation Sensitivity

VIM cells are the most sensitive cells to VIM cells are the most sensitive cells to radiation and FPM cells are most resistant. radiation and FPM cells are most resistant. The others are of intermediate sensitive in The others are of intermediate sensitive in the order presented. the order presented.

However, this perception is a product of However, this perception is a product of the longer cell cycle time in more highly the longer cell cycle time in more highly differentiated cell linesdifferentiated cell lines

Michalowski ClassificationMichalowski Classification

A more modern type of classification which A more modern type of classification which essentially says the same thing in another essentially says the same thing in another way. way.


Stem cells –continuously divide and reproduce Stem cells –continuously divide and reproduce to give rise to both new stem cells and cells that to give rise to both new stem cells and cells that eventually give rise to mature functional cells. eventually give rise to mature functional cells.

Maturing cells arising from stem cells and Maturing cells arising from stem cells and through progressive division eventually through progressive division eventually differentiate into an end-stage mature functional differentiate into an end-stage mature functional cell. cell.

Mature adult functional cells that do not divide Mature adult functional cells that do not divide

((H-typeH-type) )

There are many cell types that progress There are many cell types that progress from the stem cell through the mature cell from the stem cell through the mature cell with nonreversible steps along the way. with nonreversible steps along the way. These cell lines are said to be These cell lines are said to be hierarchicalhierarchical ( (H-typeH-type) populations. ) populations.

They include bone marrow, intestinal They include bone marrow, intestinal epithelium, epidermis and many others. epithelium, epidermis and many others.

F-typeF-type populations populations

There are other cell lines in which the There are other cell lines in which the adult cells can under certain circumstance adult cells can under certain circumstance be induced to undergo division and be induced to undergo division and reproduce another adult cell. These cell reproduce another adult cell. These cell are said to be are said to be flexibleflexible tissue ( tissue (F-typeF-type populations). populations). Examples include; liver parenchymal cells, Examples include; liver parenchymal cells, thyroid cells and pneumocytes as well as thyroid cells and pneumocytes as well as others. others.


These two types represent extremes and there These two types represent extremes and there are many tissues which exhibit characteristics of are many tissues which exhibit characteristics of both types where mature cells are able to divide both types where mature cells are able to divide a limited number of times. a limited number of times.

The rapidity of response to and hence the The rapidity of response to and hence the sensitivity to radiation at the tissue level is sensitivity to radiation at the tissue level is dependent on the length of the life cycle and dependent on the length of the life cycle and the reproductive potential of the critical cell the reproductive potential of the critical cell line within that tissue.line within that tissue.

““Critical Cells" Critical Cells"

All tissues contain multiple cell types All tissues contain multiple cell types contained in either the stromal contained in either the stromal compartment or the parenchymal compartment or the parenchymal compartment. compartment.

A cell in either compartment may be the A cell in either compartment may be the critical cell. critical cell.

““Critical Cells" Critical Cells"

the endothelial cells lining the blood the endothelial cells lining the blood vessels were thought for many years to be vessels were thought for many years to be the critical cells in tissues howeverthe critical cells in tissues however

"critical cells" have been identified in many "critical cells" have been identified in many tissues. tissues.

The time required for the tissue to The time required for the tissue to respond to radiation injury can be respond to radiation injury can be predicted on the basis of the cell predicted on the basis of the cell cycle kinetics of these critical cells. cycle kinetics of these critical cells.

Biologic Factors moderating Cell Biologic Factors moderating Cell injury by irradiation. injury by irradiation.

Cell Cycle. Cell Cycle.

Intracellular repair Intracellular repair

HypoxiaHypoxia

Cell Cycle. Cell Cycle.

The point that a cell is in the cell cycle has The point that a cell is in the cell cycle has a marked influence on its response and a marked influence on its response and survival of irradiation. survival of irradiation.

G1 & G0 are relatively insensitive to G1 & G0 are relatively insensitive to radiation injury. radiation injury.

S phase is generally considered to be the S phase is generally considered to be the most resistant to radiation injury. most resistant to radiation injury.

Cycle Phase Influence on Cycle Phase Influence on SensitivitySensitivity


The shoulder on the cell survival curve The shoulder on the cell survival curve indicates that there is some degree of indicates that there is some degree of repair by cells of radiation injury. repair by cells of radiation injury.

Amount of repair differs between cell linesAmount of repair differs between cell lines

However the rate of repair is the sameHowever the rate of repair is the same

Intracellular RepairIntracellular Repair

Intracellular repairIntracellular repair


Studies have shown that although repair Studies have shown that although repair can be an ongoing process, the vast can be an ongoing process, the vast majority of the repair is finished by 6 hours majority of the repair is finished by 6 hours post irradiation.post irradiation.

Once repair is complete the remaining cell Once repair is complete the remaining cell population will respond to subsequent population will respond to subsequent dose of radiation as though the original dose of radiation as though the original irradiation had not occurredirradiation had not occurred

HypoxiaHypoxia

Oxygen is a potent preventer of repair Oxygen is a potent preventer of repair

Hypoxia markedly improves the ability of Hypoxia markedly improves the ability of the cells to repair radiation injurythe cells to repair radiation injury

However it is quite rare for a normal However it is quite rare for a normal somatic cell to be hypoxic.somatic cell to be hypoxic.

Measurement or radiation injury at Measurement or radiation injury at the tissue level the tissue level

Assay systems are needed to construct Assay systems are needed to construct survival and injury curves for irradiation at survival and injury curves for irradiation at the tissue level. the tissue level.

Such assays must be quantifiable Such assays must be quantifiable

The effect measured must increase with The effect measured must increase with dose dose

Types of AssaysTypes of Assays

Clonogenic (related to reproductive Clonogenic (related to reproductive potential of stem cells in the tissue target potential of stem cells in the tissue target cell population cell population

Specific tissue functional capabilitySpecific tissue functional capability

Lethality - death of the organism from Lethality - death of the organism from radiation of that tissue radiation of that tissue

Clonogenic assays Clonogenic assays

May be performedMay be performed in vivo in vivo or or in vitroin vitro In an In an in vitroin vitro assay cells are harvested assay cells are harvested from tissue irradiated in living tissue and from tissue irradiated in living tissue and the cells are grown out in cell culture and the cells are grown out in cell culture and the number of colonies growing out is the number of colonies growing out is compared to that for a control compared to that for a control In vivoIn vivo assays are performed by assays are performed by evaluation of cellular reproductive activity evaluation of cellular reproductive activity in the living animal in the living animal

In VitroIn Vitro Assays Assays

Cells harvested from culture and plated Cells harvested from culture and plated out – many, many flasks or dishesout – many, many flasks or dishes

Dishes are irradiated at different levelsDishes are irradiated at different levels

The number of colonies are counted after The number of colonies are counted after a specific time.a specific time.

# of colonies compared to control sample# of colonies compared to control sample

Survival curves generatedSurvival curves generated

In vivo In vivo AssaysAssays

Two typesTwo types– In SituIn Situ Assays Assays– Transplantation Assays Transplantation Assays

In SituIn Situ Assays Assays

The tissue or organ is irradiated in the The tissue or organ is irradiated in the whole animal. At a given time after whole animal. At a given time after irradiation the organism (animal or plant) is irradiation the organism (animal or plant) is sacrificed and the organ of interest is sacrificed and the organ of interest is evaluated for cell survival of the cell of evaluated for cell survival of the cell of interest. interest.

Classic example is the intestinal crypt cell Classic example is the intestinal crypt cell studies studies


Another example is irradiation of testes Another example is irradiation of testes and then assaying the testicle for surviving and then assaying the testicle for surviving spermatogonia in the tubules of the spermatogonia in the tubules of the testicle. testicle.


Classically these assays have been used Classically these assays have been used to evaluate the radiation effects in acutely to evaluate the radiation effects in acutely responding (rapidly dividing) cell lines responding (rapidly dividing) cell lines such as the intestinal villi, the testes and such as the intestinal villi, the testes and the skin.the skin.

Recently these types of assays have been Recently these types of assays have been extended to evaluation (slowly dividing) extended to evaluation (slowly dividing) cell lines.cell lines.

In situIn situ Assays Assays

These assays have shown that the Do for These assays have shown that the Do for slowly dividing cells in this assay is about slowly dividing cells in this assay is about 1.5 Gy or about the same as for the rapidly 1.5 Gy or about the same as for the rapidly responding tissues. The difference in time responding tissues. The difference in time required for the cell killing to occur is a required for the cell killing to occur is a manifestation of the slow turnover rate of manifestation of the slow turnover rate of the cells. the cells.


Tissue is irradiated in vivo and returned to Tissue is irradiated in vivo and returned to subject.subject.

After a period of time the number of viable After a period of time the number of viable cell groups in irradiated area is measuredcell groups in irradiated area is measured

Generally done in mice as large numbers Generally done in mice as large numbers are required.are required.

Intestinal and gonadal epithelium are the Intestinal and gonadal epithelium are the classic tissues studied. classic tissues studied.


Studies of RPM and FPM tissues and cell Studies of RPM and FPM tissues and cell lines requires much longer experimentlines requires much longer experiment

May require use of larger more expensive May require use of larger more expensive and long lived animalsand long lived animals

These studies are very expensive to doThese studies are very expensive to do

Transplantation AssaysTransplantation Assays

Often used to study tumor sensitivityOften used to study tumor sensitivity

Usually done in immune compromised Usually done in immune compromised animals.animals.

Done to mimic metastatic disease or to Done to mimic metastatic disease or to remove immune system effects in live remove immune system effects in live animalanimal

Transplantation AssaysTransplantation Assays

Tumor or test tissue is irradiated while still Tumor or test tissue is irradiated while still in donor animal. in donor animal.

Irradiated tissue is then removed and cells Irradiated tissue is then removed and cells suspended in solution. suspended in solution.

Cells then injected into recipient animalCells then injected into recipient animal

After a period of growth, the animals are After a period of growth, the animals are sacrificed and the number of tissue sacrificed and the number of tissue colonies or size of colony is measured.colonies or size of colony is measured.

Functional AssaysFunctional Assays

Measure organ functional capacityMeasure organ functional capacity– Most organs have clinical functional reserveMost organs have clinical functional reserve– Tests measure complete functional capacityTests measure complete functional capacity

Done in a live animal with “in situ” organsDone in a live animal with “in situ” organs

Do not require sacrifice of the animal Do not require sacrifice of the animal – Multiple dose levels can be studiedMultiple dose levels can be studied

Measure effect at sub clinical levels.Measure effect at sub clinical levels.

Heart, lungs, liver, kidneys applicableHeart, lungs, liver, kidneys applicable

Functional AssaysFunctional Assays

Measure effects of regional irradiationMeasure effects of regional irradiation

Very important in radiation therapyVery important in radiation therapy– Helps predict effects of irradiation planHelps predict effects of irradiation plan

Also used to study effects of ingested Also used to study effects of ingested radionuclides either medical or accidentalradionuclides either medical or accidental

Useful for studies on effect modifiers such Useful for studies on effect modifiers such as chemotherapy.as chemotherapy.

Lethality AssaysLethality Assays

Measures clinical effectsMeasures clinical effectsMeasures doses required to cause death.Measures doses required to cause death.– Whole body irradiationWhole body irradiation– Regional body irradiation (brain, heart, liver, Regional body irradiation (brain, heart, liver,

etc.etc.

Generally expressed in terms of % death Generally expressed in terms of % death in a given time. i.e. LDin a given time. i.e. LD30/9030/90

– 30% of subjects die by 90 days post 30% of subjects die by 90 days post irradiationirradiation

tissue radiation biology

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stem cells

basal cells

intermitotic cells

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classifications cells

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