Radiation biology is the study of effects of ionizing radiation on living systems. In this article we shall talk about the harmful effects of ionizing radiation on tissues, how does it occur, and how sensitive different types of cells are?
The effects of radiation on the tissues can be divided into 2 broad categories. The Deterministic and Stochastic effects.
Radiation acts on living organisms via direct and indirect effects.
Direct effects take place when energy of photons or secondary electrons ionizes biologic macromolecules.
Indirect effects take place when photons are absorbed by water molecules and ionize them. Thus, radiolysis of water takes place and free radicals are produced.
Radiolysis of water ultimately produces hydroperoxide free radicals and hydrogen peroxide. Both are oxidizing agents; can significantly alter biologic molecules, and cause cell death. These two chemicals are the major toxins produced by ionizing radiation.
Major changes in biologic molecules
- Breakage of single or both DNA strands
- Cross linking of DNA strands
- Change or loss of base molecules
- Disruption of hydrogen bond between DNA strands
- Activation of protein enzymes affecting at multiple levels
Radiation effects at cellular level
- Damaged nucleus due to DNA damage
- Chromosomal aberrations
- Mitochondria shows increased permeability, swelling and disorganization of cristae at 30-50 Gy dose
- Mitotic delay
- Cell death
Radiosensitivity of various cell types
Lymphocytes and oocytes are the most radiosensitive cells, inspite of being highly differentiated and non-dividing in nature.
The decreasing order of radiosensitivity of various tissues are as follows:
- Vegetative inter mitotic cells are most radiosensitive to radiation. Examples: spermatogenic and erythroblastic series cells, basal cell of oral mucous membrane.
- Differentiating inter mitotic cells are somewhat less radiosensitive. Examples are dividing inner enamel epithelium, cells of haemopoetic series.
- Multi potential connectivity tissue cells show intermediate radio sensitivity. Examples are vascular endothelial cells, fibroblasts and mesenchymal cells.
- Reverting post mitotic cells are generally radio resistant. Examples are acinal and ductal cells of salivary glands and pancreas, parenchymal cells of liver, kidney, and thyroid.
- Fixed post mitotic cells are most resistant. Examples are neurons, striated muscles, squamous epithelial cells
Radiation effects at the tissue level are divided into short term and long term. Short term effects are mainly due to mitosis linked cell death whereas the long term effects are due to irradiation of capillaries that cause swelling, degeneration and necrosis.
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Ref: Ref: White & Pharoah Oral Radiology Principles and Interpretation 5th Ed.