Lecture 2 Cell Injury PDF

II. The Typical Pathological Processes Lecture 12 Cell Injury Classification of Cell Injuries. Pathogenetic Mechanisms of Cell Injuries. Disturbances of Structure and Functions of Cell Org...

II. The Typical Pathological Processes Lecture 12 Cell Injury Classification of Cell Injuries. Pathogenetic Mechanisms of Cell Injuries. Disturbances of Structure and Functions of Cell Organoids. Non - specific and Specific Signs of Cell Injuries. Defence - recovery Mechanisms of Cells Protecting them from Injury The onset of disease depends upon the alterations in the normal cells. With the advent of electron microscopy, we talk of disease at the level of injury to cell organelles (in the case of genetic diseases- even at the level of genes located on chromosomes, at the molecular level). Cell injury is disturbance of cellular structure and function under the influence of different pathogenic agents. As a result of damage to the cells, interrelations between cells and intercellular substance is disturbed, functions of organs and systems are changed and diseases develop. At the same time cellular injury of any degree and intricacy immediately is accompanied by the development of the defence-compensatory processes in the cell. For instance, suppression of oxidative processes in damaged cell is usually accompanied by the activation of another source of energy-glycolysis. In the damaged cells protease of lysosomas are activated; they digest injured cells and this way promote their removal from the organism. Cell injuries may be caused by exogenous and endogenous factors. According to their causes and main signs cell injuries are divided into the following groups (types of cell injury): 1. Hypoxic cell injury. Hypoxia is the most common cause of cell injury and cell death resulting in disease. The various causes of hypoxia resulting in inadequate perfusion of cells and tissues are atherosclerosis, thrombosis, anaemia, cardio-respiratory insufficiency and increased demand of the tissue for oxygen. Depending on the cell type, nutritional and hormonal status of the cell, hypoxia may produce reversible or irreversible cellular injury. For instance, neurons are highly susceptible to irreversible injury in 3-5 min; tissues like heart, liver and kidney take 30 min to 2 hours to produce irreversible injury, whereas fibroblasts, skeletal muscle, skin take very long to cause permanent damage of cells. In reversible cell injury the following sequential changes follow ischemia: decreased generation of ATP, reduction of the intracellular pH, clumping of nuclear chromatin, cellular swelling, formation of blebs at the cell surface, swelling of endoplasmic reticulum and mitochondria. If ischemia persists, the following changes lead to irreversible injury: mitochondrial dysfunction, cell membrane damage, liberation of hydrolytic enzymes from ruptured lysosome, nuclear changes (pyknosis, karyolysis, karyorrhexis), death of cell. 2. Physical cell injury-is caused by mechanical trauma, heat (burn), cold (frostbite), electricity, radiation, rapid changes in atmospheric pressure, etc. Injuries caused by mechanical force may lead to the state of shock. The radiant energy (ultraviolet, radioactive, X-rays) more easily affects the cells which rapidly proliferate. As a result of radiation often the genetic apparatus of cells is damaged and mutations come into being. Radiation injury to human by accidental or therapeutic exposure is of importance in treatment of persons with malignant tumors as well as may have carcinogenic influences. 3. Chemical cell injury-is caused by numerous chemical agents (drugs, strong acids, alcalies, poisons, environmental pollutants, insecticides, industrial processes, occupational hazards, etc.). Some chemicals (cyanide) cause death instantaneously by inactivation of cytochrome oxidase, and others cause prolonged effects or local injury. Bacterial toxins (tuberculosis, syphilis, leprosy, abdominal typhoid, dysentery, etc.) and some toxic substances of endogenic origin (for instance, surplus of phenylalanine, phenylpyruvic acid, phenyllactic acid, phenylacetic acid in phenylketonuria) also cause cell injury. Chemicals cause cell injury by one of the following two mechanisms:  a) direct cytotoxic effects(for instance, in mercuric chloride poisoning the greatest damage occurs to cells of the alimentary tract and kidney); b) conversion to reactive toxic metabolites(which then cause cell injury by direct cytotoxic effect or by formation of free radicals). Both physical (mechanical, thermal, electrical, etc.) and chemical (concentrated solutions of acids and alkalies) factors may cause traumatic cell injury. This may be caused also by physiologically active substances(especially enzymes) that are formed in the organism (damage of stomachal mucous membrane as a result of gastric juice hypersecretion, pancreatonecrosis as a result of activation of trypsin in internal pancreatic ducts, etc.). 4. Cell injury of vascular origin-occurs as a result of disturbance of blood flow in arteries. 5. Trophoneurotic cell injuries - are caused by disturbance of neural trophism in tissues (necrotic and necrobiotic processes in cells as a result of damage of the central and peripheral nervous systems, bedsore in chronic patients that are in bed rest for a long time). 6. Microbial cell injury. Bacteria usually produce harmful effect by toxins. Viruses may cause direct cytopathic effect or may be oncogenic. 7. Allergic cell injury-originates as a result of hypersensitivity of the organism to heterologous substances. For example, the changes connected with skin tuberculin test, necrotic processes in tissues as a result of Arthus phenomenon, etc. 8. Psychological cell injury-is caused by mental stress and strains, anxiety, overwork, and frustration which account for some acquired mental diseases (depression, etc.). 9. Cell injuries connected with hereditary enzymopathies-are observed in different types of hereditary glycogenoses (Gierke’s disease, Pompe’s disease, etc.) and systemic lipidoses (Tay-Sachs disease, Gaucher’s disease, Niemann-Pick disease, etc.). Pathogenetic mechanisms of the cell injury are the following: 1. Damage to membrane apparatus of the cell. 2. Disturbance of energy supply of processes taking place in the cell. 3. Disturbance in the ferment systems of the cell. 4. Disbalance of ions and fluid in the cell. 5. Disorder in the genetic program of the cell or in the mechanisms of its realization. 6. Disturbance of intracellular mechanisms of regulation of cell functions. According to the rate of development, cell injuries may be divided into two groups: acute and chronical. Mechanical flow, high and low temperature, intoxications (with the compounds of phosphorus, arsenic, mercury, etc.), acute infectious diseases (diphtheria, dysentery, pneumonia, etc.) cause acute cell injury. Chronical cell injuries are caused, in the first place, by some diseases resulting in hypoxia (chronical diseases of cardiovascular system and lungs, etc.), chronic alcoholism, avitominoses, etc. In multicellular organisms cell injuries are manifested in the form of dystrophy and necrosis. Dystrophy is the structural changes occurring as a result of metabolic disorders in cells and tissues. It may be caused by intracellular and extracellular factors. In many cases dystrophy is the initial stage of development of necrosis. Necrosis is the pathological process resulting in death of cells and tissues. The following successive stages are distinguished in the process of necrosis: a) paranecrosis-in favourable conditions the life of cell in this stage may be restored; b) necrobiosis-consists of irreversible dystrophic cellular changes; catabolism prevails over anabolism, but the cells can fulfill some of their functions; c) necrosis-is characterized by complete cessation of cell’s vital activity; d) autolysis-disintegration of dead cells(by hydrolytic enzymes that are released from lysosomas; their remains are phagocytized by macrophages. One of the significant indices of cell injury is disturbance of structure and functions of cell organoids-nuclei, endoplasmic reticulum, mitochondria, lysosomes, ribosomes. In damaged cells dissolving (karyolysis), induration (karyopyknosis) and destruction (karyorrhexis) of nuclei are observed. The changes of cytoplasm include coagulation plasmorrhexis (destruction of cytoplasm), plasmolysis (dissolving of cytoplasm).  The main and initial sign of damage of mitochondria is swelling. In severe forms of cell injury their crests and membranes swell. Damage to lysosomas causes destructions of their membrane; about 40 enzymes that are in lysosomas, are released. They damage the cytoplasm. So, enzymes of lysosoma closely participate in autolysis of necrotic cells. The main signs of damage of endoplasmic reticulum are changes of its configuration, swelling, degranulation. Number of ribosomes in membrane of the damaged endoplasmic reticulum is decreased, loops are deformed. The substances inhibiting synthesis of proteins increase number of ribosomes in cells, whereas the substances accelerating growth and reproduction of cells decrease their number. Number and configuration of ribosomes may be changed. The pathophysiological signs of cell injuries divided into two groups:non-specific and specific. Non-specific signs of cell injury are changes of the same character caused by different damaging factors. The morphological and functional changes of cells which depend on the character of some damaging factors are specific signs of cell injury. Frequently specific signs of cell injury appear earlier than non-specific signs, but sometimes non- specific signs may appear first. Non-specific disturbances are found in all types of cell injury regardless of the type of pathegenic agent. Non-specific signs of cell injury are the following: 1. Disturbance of the cell membrane permeability. All of the damaging factors (especially the free radicals, salts of heavy metals) cause changes in the structure and functions of the cell membrane. Usually permeability of injured cell membrane is increased both for macromolecules (proteins, colloidal dyestuffs) and substances with the low molecular mass (amino acids, membranes of cell organoids). Amount of free radicals which affect cell membrane powerfully, s sharply increased under the influence of ionizing and ultraviolet rays. Carcinogenic substances and some tissue poisons (for instance, alcoholic intoxication) also increase their quantity. The main sources of free radicals are water (H +, OH-, O2H-) and oxygen ( O O ). Cell membrane injury may be conditioned by the damage of its lipide components as well as protein (enzyme) components. Increase of membrane permeabilty is connected by disturbance of the activity of cell pumps. 2. Denaturation of proteins-may be caused by the high temperature, sharp changes in pH of medium, action of heavy metal salts, etc. As a result of denaturation physicochemical properties of proteins are changed (for instance, their solubility is increased), enzymatic activity is disturbed, some functional groups (SH-group of cystein, OH-group of tyrosine, etc.) that are in protein molecule in the form of complex compounds, are released. 3. Disturbance of water metabolism. In damaged cell water is released from its protoplasm and goes out into the medium surrounding the cell. Accourdingly the quantity of extracellular water is increased in damaged tissue. This causes traumatic edema (for instance, brain edema in concussion of the brain). 4. Disturbance of intracellular metabolism. One of the main indices of this process is release of potassium ions from the cells into the blood. This is connected with the damage of potassium and sodium pumps. 5. Change of intracellular enzymes activity-occurs as a result of damage of mitochondria, destruction of lysosomas, endoplasmic reticulum, ribosomes and other intracellular organelles. Enzymes of the organoids go out into protoplasm and surrounding medium. At the same time in the damaged cells activity of natural inhibitors of proteases is decreased. This leads to activation of intracellular proteases which rapidly decompose protein-lipoid components of protoplasm. The products of proteolysis, as low- molecular substances, change colloido-osmotic pressure and pH of plasma. As a result of accumulation of acid products of proteolysis (polypeptides, amino acids) pH of protoplasm in injured cells becomes lower. These processes underlie autolysis of damaged cells. In the mitochondria of the damaged cells also activity of intracellular oxidative enzymes is disturbed, and absorption of oxygen is reduced. Simultaneously anaerobic glycolysis is increased. Because activity of enzymes which inhibit this process in the presence of oxygen (Pasteur’s effect) is suppressed. 6. Reduction of oxidative phosphorylation. In the conditions of hypoxia during 3 hours the coefficient P/O2 for the isolated kidney tissues is decreased twice. This coefficient is important not only for appreciation of the degree of damage of the organ, but also when the problem of viability and usefulness of the organ for transplantation is solved. In hypoxia as a result of decrease of macroergic phosphoric compounds (ATP) the activity of Ca pump, Na-K pump and other pumps is disturbed. Prolonged hypoxia causes irreversible changes and damage of cell membranes. 7. Acidosis of injury. All types of the damage cause acidosis in protoplasm. The primary acidosis of cell injury must be differed from the secondary acidosis in inflamed tissue which occurs considerably later after the damage is caused. The primary acidosis of cell injury is resulted from activation of proteolysis, lipolysis, glycogenolysis and glycolysis in the damaged cell. Great is significance of damage of lysosomas which release about 40 hydrolytic enzymes. 8. Mediators of injury – are physiologically active substances which come into being under the influence of pathogenic factors which injure the cell: histamine, adenosine, different polypeptides, kinines, serotonin, norepinephrine, acetylcholine, some metabolic products, etc. These substances are absorbed from the injured areas into the blood, spread in the organism and cause local reactions in different tissues and organs. Their general influence on the organism causes a number of complications. 9. Decrease of membrane potential (damage potential) – may be conditioned by breach of structure of cytoplasmic membrane or disturbance of activity of ionic pumps which maintain transmembrane difference of potentials. 10. Decrease of resistance of tissues to the electric current. The total value of resistance of living cells to the alternating current (impedance) consists of ohmic and capacity resistance. Different damages cause mainly decrease of ohmic resistance. 11. Increase of sorptional properties of cells - is determined according to absorption of dyes and other substances by damaged cells. 12. Intensification of ultraweak luminescence. Ultraweak luminescence is discharge of comparatively small amounts of light energy by tissues. It is used for appreciation of state of vitality (or degree of damage) of preserved tissues. 13. Calcification, that is, accumulation of phosphoric compounds of calcium and then magnesium and sodium carbonate in damaged cells. It is observed in tuberculosis, arteriosclerosis, infarction, etc., and may be caused by chronical hypoxia. Although each of these signs is common for all or most of pathogenic factors causing cell injury, some of them are specific for one of these factors. That is, each damaging agent is able to cause in cells the changes that are characteristic of only this agent. The specific signs of cell injury are the following: 1) in the mechanical injury – breach of integrity of tissue, cells, subcellular and intercellular structures; 2) in the thermal injury (as well as in injuries caused by strong acids and alcalies) – coagulation and denaturation of protein – lipoid structures of cells; 3) in cell injuries caused by ionizing radiation – formation of free radicals; 4) in specific chemiccal (toxic) injury – inhibition of activity of different enzymes (for instance, activity of cytochrome oxidase is inhibited by cyanides, of succinate dehydrogenase – by the salts of malonic acid, etc.) In a number of specific injuries caused by inhibition of different enzymes metabolism in cells is considerably changed. For instance, inhibition of processes of phosrylation of glucose in epithelium of renal tubules by phloridzin (which inhibits activity of hexokinase). Damaging action of many poisons (snake venom, bacterial toxin of gas gangrene) is connected with their ability to dissolve in membrane lipides and selectively change the activity of a number of hydrolytic enzymes. In the process of evolution cells of organism have acquired a number of defence-recovery mechanisms which protect them from the injury:  1) antimutation system; 2) antioxidant system; 3) detoxication system; 4) buffer system. In human cells special enzymatic system promotes recovery of normal structure of DNA (deoxyribonucleic acid). This antimutation system serves chiefly protection of skin’s cells from ultraviolet rays. Destruction of DNA molecules caused by ionizing radiation, ultrasound and other mutagenous agents, and disturbance of DNA’s matrix activity may be eliminated by the help of this system. The following stages are distinguished in the process of elimination of injuries in DNA molecules, that is, in the process of reparation: 1. The special enzyme of ring – shaped molecular structure “inspects” DNA molecules in longitudinal direction and reveals the local foci of damage (defect). 2. By the participation of enzyme endonuclease the nucleotides situated between the normal part of DNA molecule and its damaged part are separated by the hydrolytic way. 3. By the participation of enzyme exonuclease the damaged area of DNA spiral (with some of its normal part) is separated from the molecule. 4. By the action of exonuclease the polynucleotide chain which has been separated from the DNA, is decomposed into its constituent parts (nucleotides). 5. By the participation of enzyne DNA - polymerase the new healthy spiral is synthesized instead of polynucleotide which was separated from the DNA molecule. 6. By the participation of enzyme ligase the new synthesized polynucleotide chain is joined up with the DNA molecule. When the antimutation system is weakened, sensitivity of the organism to mutagenous factors is increased. One of the diseases which are connected with the pathology of this system is xeroderma pigmentosum (Kaposi’s disease). This is the hereditary disease characterized by hypersensitivity of the skin to ultraviolet rays. In this disease deficiency of enzyme DNA – endonuclease in skin was revealed. Antioxidant system of cells protects lipides (which are the main components of cell membrane) from oxidative reactions, especially from the peroxide oxidation. This system includes vitamin E, ubichinon, some enzymes (superoxiddysmutase, catalase, glutathione peroxidase, etc.) The enzymatic detoxication system of cells takes part in decontamination and removal from the organism of exogenous and endogenous toxic substances. The main principle of the intracellular detoxication is based on the conversion of hydrophobic (insoluble) substances into polar (dissoluble) compounds whose toxic action on the biological membranes is weaker. This system includes cytochrome P450, glutathione – S – transferase and other enzymes. The buffer systems take part in protection of cells from damaging substances of acid and alkaline character. The injury caused by different pathogenic factors, besides local changes, leads also to development of general reactions of the organism. They include stress, shock, coma, fever, “acute phase” reaction, etc. The basis of “acute phase” reaction is formation and releasing of peptide – interleukine – 1 in the acute period of injury especially in the cases with development of infectious process, activation of mononuclear, phagocytic and immune systems and inflammation. This leads to intensification of synthesis of proteins of acute period (c – reactive protein, haptoglobin, components of complement, ceruplasmin, fibrinogen) in liver, stimulation of development of neutrophils in bone marrow, activation of thermoregulation center in hypothalamus, stimulation of protein catabolism in muscles, activation of T and B lymphocytes. Laboratory Studies Disturbance of the Specific Motor Function of Frog’s Ciliary Epithelial Cells During Alteration of the Oral Cavity Mucous Membrane The equipment: frog, small cork plank. scissors, pins, thread, 1% solution of hydrochloric acid.  The frog is made motionless by the way of destroying the brain and spinal cord and is fixed on the cork plank on its back. The lower jaw is cut off. Several pieces (1mm) of thread are put on the mucous membrane of the palate and during 3-5 min. their movement to the esophagus is observed. Then the mucous membrane is smeared with 1 % hydrochloric acid solution and again pieces of thread are put on it. As a result of mucous membrane’s alteration, the motor function of epithelial cells is disturbed, and the pieces of thread do not move.