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.
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.
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.
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
A more modern type of classification which essentially says the same thing in another way.
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
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.
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.
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.
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.
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
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
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
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
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 tissuse target cell population
Specific tissue functional capability
Lethality - death of the organism from radiation of that tissue
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
– 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
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
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.
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
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.
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