A study on the histological and electrical impedance changes in biological tissue and rats exposed to ionizing radiation

Abstract

Awareness of radiation exposure is increasing due to the recent nuclear accident caused by the Chernobyl and Fukushima tsunamis, the use of radiation for medical diagnosis and treatment, and the use of radioactive isotopes for industrial and medical purposes. Even in everyday life, concerns about radiation risks to the general public are growing due to radiation hazards that can occur around nuclear power plants, Japanese seafood consumption, asphalt or bed mattresses. Here, radiation refers to ionizing radiation, and when interacting with a substance, it is energy that can release orbital electrons of atoms and neutrons and protons in the nucleus. The biological effects of ionizing radiation lead to oxidative metabolism by altering molecular structure through the interaction of DNA with organelles. When a living body is exposed to radiation, it acts directly on the DNA and cuts the chain into one or two strands depending on the energy, and also gives energy to areas with a lot of water such as the cytoplasm or body fluid. Thereafter, reactive oxygen species(ROS) such as hydrogen peroxide(H2O2), superoxide(O2-), and hydroxyl radicals(OH-) are generated. These ROS not only structurally change cells, but also induce various biological damages, resulting in different pathological characteristics depending on the period after exposure. As a result, tissue damage and disruption of cellular function occur at the molecular level, and when cellular homeostasis is disrupted, the induced biological changes can persist and propagate to the next generation of cells. Depending on the radiation dose, dose rate, quality, extent of exposure, and radiation susceptibility, these protective mechanisms may or may not be sufficient to cope with stress, causing cells or tissues to be damaged or repaired. As for the effects of radiation dose on living things, necrosis occurs when a dose above a certain level is irradiated, and mutation and apoptosis can occur even through the recovery process. Changes at the cellular level caused by ionizing radiation are generally known as an increase in cell size, in particular, an increase in cell nucleus, multinucleate, and fibrosis. Exposure to radiation above the threshold dose causes inflammation and fibrosis formation mainly due to the leakage of factors constituting cells or an initial tissue reaction due to cell loss. Inflammation plays a pivotal role in the detrimental effects of ionizing radiation on radiation exposure as a result of radiation therapy or in radiation accidents. An inflammatory response occurs and the extracellular matrix directs cell differentiation, migration, proliferation, and fibrotic activation or inactivation. Radiation-induced fibrosis(RIF) occurs as a result of erroneous healing activated by the extracellular matrix. RIF leads to abnormal accumulation of fibrin in the intravascular, perivascular, and extravascular compartments. The intrinsic electrical properties in normal tissues depend on the structure of the constituent materials and the amount of water. Cells are largely composed of a cell nucleus, cytoplasm, and cell membrane, and their electrical properties are also different because their components and structures are different. By using these characteristics, it is possible to analyze the electrical characteristics inside the living body. Methods for analyzing the electrical properties inside a living body are widely used due to low system interference, comprehensive process information, and reliable results. As a method of analyzing in vivo electrical properties, electrical impedance measurement can measure changes in living tissue non-invasively and rapidly. In a living body exposed to ionizing radiation, histopathological changes in the cell nucleus, cytoplasm, and cell membrane increase as the dose increases. These changes destroy the intrinsic electrical properties of the cell and increase the dielectric constant. And the tissue exposed to the ionizing radiation generates active oxygen ions and the movement of ions becomes active. The generated ions can move more smoothly in and out of the cell due to the loosened cell bonds and the destruction of the cell membrane. As a result, the living body exposed to ionizing radiation over a certain dose increases electrical conductivity and permittivity. Since electrical impedance measurement uses these characteristics, it is proposed that it is a suitable tool as a method to know changes in the living body caused by ionizing radiation. For a stable and reproducible experiment, pork tenderloin tissue was selected as the biological medium. Accurate doses were investigated using a linear accelerator and divided into groups for doses of 1Gy, 2Gy, 4Gy and 10Gy. The difference in electrical impedance before and after irradiation for each group was checked and compared. In order to observe changes in the microstructure at the cell level, changes in the pig tenderloin tissue according to the dose difference were confirmed using a transmission electron microscope. As the radiation dose increased, the electrical impedance decreased and the difference increased. An increase and enlargement of the number of mitochondria were observed, and the sarcomere spacing was loosened. These results confirmed that changes in vivo exposed to ionizing radiation could be known through electrical impedance measurement. This study attempted to confirm changes in the whole-body of living organisms based on the results of experiments with biological medium. To date, no model has demonstrated the relationship between tissue and electrical resistance in the whole-body due to radiation exposure in living animals. Sprague-Dawley rats were selected as experimental animals, and the electrical impedance of the whole-body and histological changes in skeletal muscle, liver and spleen were investigated according to the increase in radiation dose. Each group received 1Gy, 5Gy, 10Gy and 20Gy whole-body exposure, and the unirradiated group was used as a control for histological comparison. Histopathological changes in living animals due to exposure to ionizing radiation required a time course due to recovery and damage. As a result of daily monitoring, the appropriate period for the doses investigated was 4 days. Therefore, histopathological changes were observed before and 4 days after irradiation, and electrical impedance was measured. As the radiation dose increased, the impedance after 4 days was decreased compared to before irradiation. In gross observation, the pattern of acute radiation syndrome was observed, and as the dose increased, the weight decreased and the reduced physique was confirmed. In H&E staining, changes in the nucleus and cytoplasm were found, and the analysis result of MT staining showed changes in the extracellular matrix due to collagen deposition. As a result of quantitative analysis, it was found that atrophy in muscle fibers and collagen deposition in liver, spleen, and skeletal muscle increased as the radiation dose increased. In this study, electrical impedance measurement was presented as an effective approach to characterize changes in pork tenderloin tissue induced by ionizing radiation. It was found that the impedance change caused by ionizing radiation is due to changes in cell components such as an increase in the amount of electrolyte movement due to an increase in permeability due to cell membrane destruction rather than an electrical response of the cell. In experiments on living rats, it was confirmed that as the radiation dose increased, changes in tissues increased and the electrical resistance of the whole-body decreased. With these results, the hypothesis that measuring the whole-body impedance before and after irradiation can know the degree of decrease in electrical impedance induced by histomorphological changes was confirmed. In summary, changes in the structure or moisture content of tissues and cells due to ionizing radiation in the living body are measured as changes in electrical impedance, so that the exposure amount can be known. The relationship between the electrical properties and biological changes caused by ionizing radiation in the living body has not yet been studied. Current commercial radiation measurements used values using ionization chambers, scintillators, film badge, and substitutes such as air or film, such as TLD(thermoluminesence dosimeter). Although these measurement methods can provide stable and reproducible results by using a substitute material in the living body, they do not reflect the change in the living body. Films and thermofluorescent materials have time constraints because they measure the cumulative dose within a specific period. In addition, since ionizing radiation cannot be directly irradiated to humans for experimentation, body changes in response to high doses would have been mathematically reported as experimental data or epidemiological investigations. Therefore, the electrical impedance measurement can evaluate the effect of ionizing radiation on individual living bodies, and the biological response to a specific dose can be known as an immediate quantitative value. In addition, if an electrical measurement method is used for exposure to ionizing radiation, customized management is possible by evaluating each individual's various biological responses after radiation treatment, and it can also help to individually measure the response after a radiation accident.