PATHOPHYSIOLOGY OF CELL. MECHANISMS OF CELL DAMAGE.
IONIZING RADIATION ACTION UPON ORGANISM
PATHOPHYSIOLOGY OF CELL. MECHANISMS OF CELL DAMAGE. IONIZING RADIATION ACTION UPON ORGANISM
Ionizing rays characteristics. Ionizing radiation – is a high-energetic electromagnetic or corpuscular radiation. X-rays belong to electromagnetic radiations. X-rays arise of substance or the X-ray tube anode electrons internal atoms. Gamma rays are a neutral particles stream, which arise with radioactive atoms disintegration. Inhibitory radiation occurs while high-energy charged particles inhibit an electric space.
Other ionizing radiation kinds have got a corpuscular nature and represent elementary particles streams. They are:
· electrons
· protons
· neutrons
· helium nucleas
Negatively charged electrons which are presented at all stable atoms. They are easily radiated during radioactive disintegration and then they get called β-rays. Protons are positively charged particles; they are nucleus parts. Their weight approximately equals to neutron weight and is 2000 times greater of electron weight. Neutrons are the particles with proton weight, but without any charge (electroneutral), so they penetrate organism tissues deeply. Neutrons are nucleas parts. In fact of heavy radioactive elements nuclear division, they are radiated as a side-product. α-Particles are the helium atoms nucleas, orbital electrons deprived. They consist of two protons and two neutrons, which are positive charged. Injury arises mostly with radioactive of uranium and radium disintegration.
Different ionizing radiation kinds are characterized by different penetrating ability. Charged alpha and beta particles have small pathetic ability, they mainly act upon those superficial body areas which were direct irradiated. Gamma radiation, X-rays, neutrons have high penetrating ability and cause the whole organism damage though.
Penetrating ability of ionizing rays
Initial ionizing radiation effects. The common all ionizing radiation kinds property is their energy exceeding the intramolecular and intraatomic connections energy; this occurs in alive object water space, which makes 70 % from weight of a body. Therefore the radiation biological effect gets achieved in two ways – direct and indirect ones.
Direct way is the straight ionizing radiation influence upon high-molecular connections of an organism: proteins, lipids, enzymes, nucleic acids, nucleoproteids, lipoproteids. The energy absorbed with macromolecule, migrates in there and breaks the most labile connections. Proteins lose their enzyme and immune properties after such irradiation.
Nucleic acids and their albuminous complexes – nucleoproteids are very sensitive to radiation. Indirect radiation action is connected to water radiolysis. In result positively and negatively charged ions with oxydizing and regenerative ability get produced first. They react with the activated water molecule and in combinative way, form hydrogen peroxyde H2O2, hydroperoxyde HO2, atomic oxygen O, superoxyde radical O2, etc. Water radiolysis roducts are very chemically active. They reach biologically important molecules and get them oxidized. Free radicals attack the same molecules, which are injured by the direct ionizing rays action: proteins, enzymes, nucleic acids, lipids. So, their damage is the result of direct and indirect ionizing rays action.
Except of free radicals, one more bio-active substances group-radiotoxins in radiated organism appears. There are two radiotoxins kinds: lipidic and quinonic. The first appear in fact of nonsaturated fatty acids oxidation, for example аrachidonic; the second – oxidation amino acids and their derivates oxidation (tyrosine, tryptophan, serotonin, adrenalin).
Thus, superfluous radiation energy is transformed into changed macromolecules which are the products of direct action, water radiolysis. Their occurrence and accumulation conducts the infringement of the major cell vital ability processes. The main cellular targets are:
а) enzymes, which cause metabolism depression, while being inhibited;
b) cell membranes and subcellular structures (mitochondrias, lysosomas); the cell energy suffersat in case of mitochondrial membranes infringement; in fact of lysosomal membranes permeability increase cell lysis enzymes get poured out into the cytoplasm;
c) a nucleus, where the incorporated genetic information locates; in fact of its damage mutations appear.
Not all cells are equally sensitive to ionizing irradiation. They can be divided into three groups:
а) very sensitive bone marrow and lymphoid cells
b) low sensitive digestive and sexual glands epithelial cells, skin epithelium, vessels endothelium
c) resistant cells of cartilages, bones, muscles, nerves
Direct organism cells damage consequence is radiation disease – acute or chronic
ACUTE RADIATION DISEASE
This term designates the general injury of an organism with the big ionizing radiation doze.
There are three forms of acute radiation disease:
· bone marrow form (a doze of 1-10 gr)
· intestinal (a doze of 10-50 gr)
· cerebral (a doze of 50 gr)
Bone marrow form is characterized by the foreground red bone marrow defeat which determines specificity of illness.
This illness is devided into four periods (stages):
· initial
· latent
· the period of expression
· the period of mentioned functions recovery
The initial stage lasts several hours up to two days. Characteristic attributes are: acute nervous excitation, headache, dizziness, pulse frequency increase, sometimes faintness and vomitting, high temperature. The behaviour reminds of alchohol intoxication. Bone marrow cellular elements initial reduction and regeneretive abilities inhibition takes place. It displays of lymphocytes amount reduction in peripheral blood (they are not sensitive to ionizing rays action and their life term is short.
The latent period lasts 1-2 weeks. Symptoms of defeat disappear. Mitosis activity of bone marrow cells raises. The young forms amount in a bone marrow grows. This phenomenon constrains formal elements in blood fall, but it is temporary. During the second period leukopenia, lymphopenia, thrombocytopenia, reticulocytes amount decrease is observed anyway, that testifies of erythropoietic bone marrow sprout exhaustion. Vessels walls penetration grows.
The period of expression is the all illness attributes complete display. It includes such syndromes:
1. Bone marrow injury syndrome – a secondary devastation of a bone marrow, its aplasia and replacement with fatty tissue with lymphopenia, granulocytopenia, thrombocytopenia, anemia.
2. Hemorrhagic syndrome which has three phenomena groups:
a) hemopoesis infringement (thrombocytopenia and thrombocytopathy);
b) blood curtailing oppression owing to appropriate factors synthesis insufficient, activation fibrinolytic and anticoagulative systems;
c) vascular walls permeability increase (epithelial separation, loss of basal membranes durability, desmosomal destruction, litic collagenase enzymes increase, elastase, gialuronidase, protease).
Hemorrhagic syndrome
3. Enteral syndrome, which is shown by inflammation and dystrophy mucous digestive canal, intestinal intoxication, digestive glands function oppression, intestinal microflora development.
4. Immune reactivity, connecting to infectious complications (pneumonia, necrotic quinsy) is reducted as a result of antibody production, phagocytosis, T-cellular immunity oppression.
The fourth period is gradual restoration of mentioned functions. Disturbance of hemopoiesis may be remain long.
Sometimes illness passes into the chronic form. Chronic radiation disease arises also in cases, when the organism repeatedly gets under the low radiation dozes actions.
Mutagen ionizing rays action is shown at absorbed radiation dozes up to 1 gr. Straight ionizing line and of free radicals association injury actions upon DNA is necessary for mutations occurrence.
The risk of a mutation exists for all kinds of radiated cells is somatic and sexual, but their appearence consequences of are different.
Mutations in sexual cells in certain cases have pathological posterity. Genic and chromosomal aberrations eliminate with intra-uterine zygote destruction in 95 % of cases. Another 5 % survive and result with translocations, mosaicism or aneuploidy. Genic mutations increase pathological phenotypes number in posterity.
Following mechanism of genome mutations – is anaphase deficiency. During the of anaphase moving from equator down to the pole one of chromosomes gets retarded and lost. One from daughter gametes gets the normal amount of chromosomes, and the other one – less. After the impregnation, zygote can be normal (46), or monosomic (45).
Undivergence of chromosomes Anaphase deficiency
The chromosomal mutations arise in those cases, when amount of chromosomes does not change, but their structure ruines. Each structural chromosome alteration begins with its break. Herewith, DNA breaks. Sometimes chromosome fragments endings successfully connect to each other with reparative enzymes.
Chromosome becomes intact again. But happens so, that the chromosomes fragments do not connect at all or unite in break points of other chromosomes. So arise the diverse chromosome violations types (aberrations, anomalies). It is known more then 30 of them.
More frequent the one can meet following chromosome variety:
· deletion – chromosome area loss
Deletion
· inversion – two breaks appear in chromosome, free fragment turns over in 180o and gets combined with chromosome again;
Inversion
· translocation – is fragment transfer from one chromosome onto the other, or mutual exchange by fragments.
Translocation
Mutations make somatic cells get divided intensively, and may cause their ability to uncontrollable duplications. From them malignant tumours or leukosis develop (ionizing rays cancerogenic action).
CELL INJURY AND DEATH
Cells can be injured in many ways. The extent to which any injurious agent can cause cell injury and death depends in large measure on the intensity and duration of the injury and the type of cell that is involved. Cell injury is usually reversible to a certain point, after which irreversible cell injury and death occur. Whether a specific stress causes irreversible or reversible cell injury depends on the severity of the insult and on vari ables such as blood supply, nutritional status, and regenerative capacity. Cell injury and death are ongoing processes, and in the healthy state, they are balanced by cell renewal.
Causes of Cell Injury
Cell damage can occur in many ways. For purposes of discussion, the ways by which cells are injured have been grouped into five categories: (1) injury from physical agents, (2) radiation injury, (3) chemical injury, (4) injury from biologic agents, and (5) injury from nutritional imbalances.
Injury From Physical Agents
Physical agents responsible for cell and tissue injury include mechanical forces, extremes of temperature, and electrical forces. They are common causes of injuries due to environmental exposure, occupational and transportation accidents, and physical violence and assault.
Mechanical Forces.
Injury or trauma caused by mechanical forces occurs as the result of body impact with another object. The body or the object can be in motion or, as sometimes happens, both can be in motion at the time of impact. These types of injuries split and tear tissue, fracture bones, injure blood vessels, and disrupt blood flow.
Extremes of Temperature.
Extremes of heat and cold cause damage to the cell, its organelles, and its enzyme systems. Exposure to low-intensity heat (43° to 46°C), such as occurs with partial-thickness burns and severe heat stroke, causes cell injury by inducing vascular injury, accelerating cell metabolism, inactivating temperature-sensitive enzymes, and disrupting the cell membrane. With more intense heat, coagulation of blood vessels and tissue proteins occurs. Exposure to cold increases blood viscosity and induces vasoconstriction by direct action on blood vessels and through reflex activity of the sympathetic nervous system. The resultant decrease in blood flow may lead to hypoxic tissue injury, depending on the degree and duration of cold exposure. Injury from freezing probably results from a combination of ice crystal formation and vasoconstriction. The decreased blood flow leads to capillary stasis and arteriolar and capillary thrombosis.
Electrical Injuries.
Electrical injuries can affect the body through extensive tissue injury and disruption of neural and cardiac impulses. The effect of electricity on the body is mainly determined by its voltage, the type of current (i.e., direct or alternating), its amperage, the resistance of the intervening tissue, the pathway of the current, and the duration of exposure.2,4 Lightning and high-voltage wires that carry several thousand volts produce the most severe damage.2 Alternating current (AC) is usually more dangerous than direct current (DC) because it causes violent muscle contractions, preventing the person from releasing the electrical source and sometimes resulting in fractures and dislocations. In electrical injuries, the body acts as a conductor of the electrical current. The current enters the body from an electrical source, such as an exposed wire, and passes through the body and exits to another conductor, such as the moisture on the ground or a piece of metal the person is holding. The pathway that a current takes is critical because the electrical energy disrupts impulses in excitable tissues. Current flow through the brain may interrupt impulses from respiratory centers in the brain stem, and current flow through the chest may cause fatal cardiac arrhythmias. The resistance to the flow of current in electrical circuits transforms electrical energy into heat. This is why the elements in electrical heating devices are made of highly resistive metals. Much of the tissue damage produced by electrical injuries is caused by heat production in tissues that have the highest electrical resistance. Resistance to electrical current varies from the greatest to the least in bone, fat, tendons, skin, muscles, blood, and nerves. The most severe tissue injury usually occurs at the skin sites where the current enters and leaves the body. After electricity has penetrated the skin, it passes rapidly through the body along the lines of least resistance— through body fluids and nerves. Degeneration of vessel walls may occur, and thrombi may form as current flows along the blood vessels. This can cause extensive muscle and deep tissue injury. Thick, dry skin is more resistant to the flow of electricity than thin, wet skin. It is generally believed that the greater the skin resistance, the greater is the amount of local skin burn, and the less the resistance, the greater are the deep and systemic effects.
Radiation Injury
Electromagnetic radiation comprises a wide spectrum of wavepropagated energy, ranging from ionizing gamma rays to radiofrequency waves. A photon is a particle of radiation energy. Radiation energy above the ultraviolet (UV) range is called ionizing radiation because the photons have enough energy to knock electrons off atoms and molecules. Nonionizing radiation refers to radiation energy at frequencies below that of visible light. UV radiation represents the portion of the spectrum of electromagnetic radiation just above the visible range. It contains increasingly energetic rays that are powerful enough to disrupt intracellular bonds and cause sunburn.
Ionizing Radiation. Ionizing radiation affects cells by causing ionization of molecules and atoms in the cell, by directly hitting the target molecules in the cell, or by producing free radicals that interact with critical cell components.1,2,5 It can immediately kill cells, interrupt cell replication, or cause a variety of genetic mutations, which may or may not be lethal. Most radiation injury is caused by localized irradiation that is used in treatment of cancer . Except for unusual circumstances, such as the use of high-dose irradiation that precedes bone marrow transplantation, exposure to whole-body irradiation is rare. The injurious effects of ionizing radiation vary with the dose, dose rate (a single dose can cause greater injury than divided or fractionated doses), and the differential sensitivity of the exposed tissue to radiation injury. Because of the effect on deoxyribonucleic acid (DNA) synthesis and interference with mitosis, rapidly dividing cells of the bone marrow and intestine are much more vulnerable to radiation injury than are tissues such as bone and skeletal muscle. Over time, occupational and accidental exposure to ionizing radiation can result in increased risk for the development of various types of cancers, including skin cancers, leukemia, osteogenic sarcomas, and lung cancer. Many of the manifestations of radiation therapy result from acute cell injury, dose-dependent changes in the blood vessels that supply the irradiated tissues, and fibrotic tissue replacement. The cell’s initial response to radiation injury involves swelling, disruption of the mitochondria and other organelles, alterations in the cell membrane, and marked changes in the nucleus. The endothelial cells in blood vessels are particularly sensitive to irradiation. During the immediate postirradiation period, only vessel dilatation takes place (e.g., the initial erythema of the skin after radiation therapy). Later or with higher levels of radiation, destructive changes occur in small blood vessels such as the capillaries and venules. Acute reversible necrosis is represented by such disorders as radiation cystitis, dermatitis, and diarrhea from enteritis. More persistent damage can be attributed to acute necrosis of tissue cells that are not capable of regeneration and chronic ischemia. Chronic effects of radiation damage are characterized by fibrosis and scarring of tissues and organs in the irradiated area (e.g., interstitial fibrosis of the heart and lungs after irradiation of the chest). Because the radiation delivered in radiation therapy inevitably travels through the skin, radiation dermatitis is common. There may be necrosis of the skin, impaired wound healing, and chronic radiation dermatitis.
Ultraviolet Radiation.
Ultraviolet radiation causes sunburn and increases the risk of skin cancers (). The degree of risk depends on the type of UV rays, the intensity of exposure, and the amount of protective melanin pigment in the skin. Skin damage induced by UV radiation is believed to be caused by reactive oxygen species and by damage to melaninproducing processes in the skin. UV radiation also damages DNA, resulting in the formation of pyrimidine dimers (i.e., the insertion of two identical pyrimidine bases into replicating DNA instead of one). Other forms of DNA damage include the production of single-stranded breaks and formation of DNA–protein cross-links. Normally, errors that occur during DNA replication are repaired by enzymes that remove the faulty section of DNA and repair the damage. The importance of the DNA repair in protecting against UV radiation injury is evidenced by the vulnerability of persons who lack the enzymes needed to repair UV-induced DNA damage. In a genetic disorder called xeroderma pigmentosum, an enzyme needed to repair sunlight-induced DNA damage is lacking. This autosomal recessive disorder is characterized by extreme photosensitivity and a greatly increased risk of skin cancer in sunexposed skin.2
Nonionizing Radiation.
Nonionizing radiation includes infrared light, ultrasound, microwaves, and laser energy. Unlike ionizing radiation, which can directly break chemical bonds, nonionizing radiation exerts its effects by causing vibration and rotation of atoms and molecules. All of this vibrational and rotational energy is eventually converted to thermal energy. Lowfrequency nonionizing radiation is used widely in radar, television, industrial operations (e.g., heating, welding, melting of metals, processing of wood and plastic), household appliances (e.g., microwave ovens), and medical applications (e.g., diathermy). Isolated cases of skin burns and thermal injury to deeper tissues have occurred in industrial settings and from improperly used household microwave ovens. Injury from these sources is mainly thermal and, because of the deep penetration of the infrared or microwave rays, tends to involve dermal and subcutaneous tissue injury.
Chemical Injury
Chemicals capable of damaging cells are everywhere around us. Air and water pollution contain chemicals capable of tissue injury, as do tobacco smoke and some processed or preserved foods. Some of the most damaging chemicals exist in our environment, including gases such as carbon monoxide, insecticides, and trace metals such as lead. Lead is a particularly toxic metal. Small amounts accumulate to reach toxic levels.6 There are innumerable sources of lead in the environment, including flaking paint, lead-contaminated dust and soil, lead-contaminated root vegetables, lead water pipes or soldered joints, pottery glazes, and newsprint. Children are exposed to lead through ingestion of peeling lead paint, by breathing dust from lead paint (e.g., during remodeling), or from playing in contaminated soil.7 Lead crosses the placenta, exposing the fetus to lead levels comparable to those of the mother. The toxicity of lead is related to its multiple biochemical effects.2 It has the ability to inactivate enzymes, compete with calcium for incorporation into bone, and interfere with nerve transmission and brain development. The major targets are the red blood cells, the gastrointestinal tract, the kidneys, and the nervous system. Some of the manifestations of lead toxicity include anemia, acute abdominal pain, signs of kidney damage, and cognitive deficits and neuropathies resulting from demyelination of cerebral and cerebellar white matter and death of cortical nerve cells. Chemical agents can injure the cell membrane and other cell structures, block enzymatic pathways, coagulate cell proteins, and disrupt the osmotic and ionic balance of the cell. Corrosive substances such as strong acids and bases destroy cells as the substances come into contact with the body. Other chemicals may injure cells in the process of metabolism or elimination. For example, carbon tetrachloride (CCl4) causes little damage until it is metabolized by liver enzymes to a highly reactive free radical (CCl3˙). Carbon tetrachloride is extremely toxic to liver cells.
Drugs.
Many drugs—alcohol, prescription drugs, over-thecounter drugs, and street drugs—are capable of directly or indirectly damaging tissues. Ethyl alcohol can harm the gastric mucosa, liver (28), developing fetus (4), and other organs. Antineoplastic (anticancer) and immunosuppressant drugs can directly injure cells. Other drugs produce metabolic end-products that are toxic to cells. Acetaminophen, a commonly used analgesic drug, is detoxified in the liver, where small amounts of the drug are converted to a highly toxic metabolite. This metabolite is detoxified by a metabolic pathway that uses a substance (i.e., glutathione) normally present in the liver. When large amounts of the drug are ingested, this pathway becomes overwhelmed and toxic metabolites accumulate, causing massive liver necrosis.
Injury From Biologic Agents
Biologic agents differ from other injurious agents in that they are able to replicate and can continue to produce their injurious effects. These agents range from submicroscopic viruses to the larger parasites. Biologic agents injure cells by diverse mechanisms. Viruses enter the cell and become incorporated into its DNA synthetic machinery. Certain bacteria elaborate exotoxins that interfere with cellular production of ATP. Other bacteria, such as the gram-negative bacilli, release endotoxins that cause cell injury and increased capillary permeability.
Injury From Nutritional Imbalances
Nutritional excesses and nutritional deficiencies predispose cells to injury. Obesity and diets high in saturated fats are thought to predispose persons to atherosclerosis. The body requires more than 60 organic and inorganic substances in amounts ranging from micrograms to grams. These nutrients include minerals, vitamins, certain fatty acids, and specific amino acids. Dietary deficiencies can occur in the form of starvation, in which there is a deficiency of all nutrients and vitamins, or because of a selective deficiency of a single nutrient or vitamin. Iron-deficiency anemia, scurvy, beriberi, and pellagra are examples of injury caused by the lack of specific vitamins or minerals. The protein and calorie deficiencies that occur with starvation cause widespread tissue damage.
Mechanisms of Cell Injury The mechanisms by which injurious agents cause cell injury and death are complex. Some agents, such as heat, produce direct cell injury; other factors, such as genetic derangement, produce their effects indirectly through metabolic disturbances and altered immune responses. There seem to be at least three major mechanisms whereby most injurious agents exert their effects: free radical formation, hypoxia and ATP depletion, and disruption of intracellular calcium homeostasis.
Free Radical Injury
Many injurious agents exert their damaging effects through a reactive chemical species called a free radical.2,8–10 Free radical injury is rapidly emerging as a final common pathway for tissue damage by many injurious agents. In most atoms, the outer electron orbits are filled with paired electrons moving in opposite directions to balance their spins. A free radical is a highly reactive chemical species arising from an atom that has a single unpaired electron in an outer orbit (Fig. 2-4). In this state, the radical is highly unstable and can enter into reactions with cellular constituents, particularly key molecules in cell membranes and nucleic acids. Moreover, free radicals can establish chain reactions, sometimes thousands of events long, as the molecules they react with in turn form free radicals.
Chain reactions may branch, causing even greater damage. Uncontrolled free radical production causes damage to cell membranes, cross-linking of cell proteins, inactivation of enzyme systems, or damage to the nucleic acids that make up DNA. Free radical formation is a by-product of many normal cellular reactions in the body, including energy generation, breakdown of lipids and proteins, and inflammatory processes. For example, free radical generation is the main mechanism for killing microbes in phagocytic white blood cells. Molecular oxygen (O2), with its two unpaired outer electrons, is the most common source of free radicals. During the course of normal cell metabolism, cells process energy-producing oxygen into water; in some reactions, a superoxide radical is formed. Lipid oxidation (i.e., peroxidation) is another source of free radicals. Exogenous sources of free radicals include tobacco smoke, certain pollutants and organic solvents, hyperoxic environments, pesticides, and radiation. Some of these compounds and certain medications are metabolized to free radical intermediates that cause oxidative damage to target tissues. Although the effects of these reactive free radicals are wide ranging, three types of effects are particularly important in cell injury: lipid peroxidation, oxidative modification of proteins, and DNA effects (Fig. 2-5). Destruction of the phospholipids in cell membranes, including the outer plasma membrane and those of the intracellular organelles, results in loss of membrane integrity. Free radical attack on cell proteins, particularly those of critical enzymes, can interrupt vital processes throughout the cell. DNA is an important target of the hydroxyl free radical. Damage can involve single-stranded breaks in DNA, modification of base pairs, and cross-links between strands. In most cases, various DNA repair pathways can repair the damage. However, if the damage is extensive, the cell dies. The effects of free radical-mediated DNA changes have also been implicated in aging and malignant transformation of cells.
■ FIGURE ■ Oxygen molecule and generation of free radical
Effects of free radical cell damage. Under normal conditions, most cells have chemical mechanisms that protect them from the injurious effects of free radicals. These mechanisms commonly break down when the cell is deprived of oxygen or exposed to certain chemical agents, radiation, or other injurious agents. Free radical formation is a particular threat to tissues in which the blood flow has been interrupted and then restored. During the period of interrupted flow, the intracellular mechanisms that control free radicals are inactivated or damaged. When blood flow is restored, the cell is suddenly confronted with an excess of free radicals that it cannot control. Scientists continue to investigate the use of free radical scavengers to protect against cell injury during periods when protective cellular mechanisms are impaired. Defenses against free radicals include vitamin E, vitamin C, and β-carotene.11 Vitamin E is the major lipid-soluble antioxidant present in all cellular membranes. Vitamin C is an important water-soluble cytosolic chain-breaking antioxidant; it acts directly with superoxide and singlet oxygen radicals. β-carotene, a pigment found in most plants, reacts with singlet oxygen and can also function as an antioxidant.
Hypoxic Cell Injury
Hypoxia deprives the cell of oxygen and interrupts oxidative metabolism and the generation of ATP. The actual time necessary to produce irreversible cell damage depends on the degree of oxygen deprivation and the metabolic needs of the cell. Well-differentiated cells, such as those in the heart, brain, and kidneys, require large amounts of oxygen to provide energy for their special functions. For example, brain cells begin to undergo permanent damage after 4 to 6 minutes of oxygen deprivation. A thin margin can exist between the time involved in reversible and irreversible cell damage. One study found that the epithelial cells of the proximal tubule of the kidney in the rat could survive 20 but not 30 minutes of ischemia.
Hypoxia can result from an inadequate amount of oxygen in the air, respiratory disease, ischemia (i.e., decreased blood flow caused by circulatory disorders), anemia, edema, or inability of the cells to use oxygen. Ischemia is characterized by impaired oxygen delivery and impaired removal of metabolic end-products such as lactic acid. In contrast to pure hypoxia, which affects the oxygen content of the blood and affects all of the cells in the body, ischemia commonly affects blood flow through small numbers of blood vessels and produces local tissue injury. In cases of edema, the distance for diffusion of oxygen may become a limiting factor. In hypermetabolic states, the cells may require more oxygen than can be supplied by normal respiratory function and oxygen transport.
Hypoxia also serves as the ultimate cause of cell death in other injuries. For example, toxins from certain microorganisms can interfere with cellular use of oxygen, and a physical agent such as cold can cause severe vasoconstriction and impair blood flow. Hypoxia literally causes a power failure in the cell, with widespread effects on the cell’s functional and structural components.
As oxygen tension in the cell falls, oxidative metabolism ceases, and the cell reverts to anaerobic metabolism, using its limited glycogen stores in an attempt to maintain vital cell functions. Cellular pH falls as lactic acid accumulates in the cell. This reduction in pH can have profound effects on intracellular structures. The nuclear chromatin clumps and myelin figures, which derive from destructive changes in cell membranes and intracellular structures, are seen in the cytoplasm and extracellular spaces.
One of the earliest effects of reduced ATP is acute cellular swelling caused by failure of the energy-dependent sodium/ potassium (Na+/K+) ATPase membrane pump, which extrudes sodium from and returns potassium to the cell. With impaired function of this pump, intracellular potassium levels decrease, and sodium and water accumulate in the cell. The movement of fluid and ions into the cell is associated with dilatation of the endoplasmic reticulum, increased membrane permeability, and decreased mitochondrial function.
To this point, the cellular changes caused by ischemia are reversible if oxygenation is restored. However, if the oxygen supply is not restored there is a continued loss of essential enzymes, proteins, and ribonucleic acid through the hyperpermeable membrane of the cell. Injury to the lysosomal membranes results in leakage of destructive lysosomal enzymes into the cytoplasm and enzymatic digestion of cell components. Leakage of intracellular enzymes through the permeable cell membrane into the extracellular fluid is used as an important clinical indicator of cell injury and death. These enzymes enter the blood and can be measured by laboratory tests.
Impaired Calcium Homeostasis
Calcium functions as a messenger for the release of many intracellular enzymes. Normally, intracellular calcium levels are kept extremely low compared with extracellular levels. These low intracellular levels are maintained by energy-dependent, membrane-associated calcium/magnesium (Ca2+/Mg2+) ATPase exchange systems.2 Ischemia and certain toxins lead to an increase in cytosolic calcium because of increased influx across the cell membrane and the release of calcium stored in the mitochondria and endoplasmic reticulum. The increased calcium level activates a number of enzymes with potentially damaging effects. The enzymes include the phospholipases responsible for damaging the cell membrane, proteases that damage the cytoskeleton and membrane proteins, ATPases that break down ATP and hasten its depletion, and endonucleases that fragment chromatin.
REVERSIBLE CELL INJURY AND CELL DEATH
The mechanisms of cell injury can produce sublethal and reversible cellular damage or lead to irreversible injury with cell destruction or death (Fig. 2-6). Cell destruction and removal can involve one of two mechanisms: apoptosis, which is designed to remove injured or worn-out cells, or cell death and necrosis, which occurs in irreversibly damaged cells.
Reversible Cell Injury
Reversible cell injury, although impairing cell function, does not result in cell death. Two patterns of reversible cell injury can be observed under the microscope: cellular swelling and fatty change. Cellular swelling occurs with impairment of the energy-dependent Na+/K+ ATPase membrane pump, usually as the result of hypoxic cell injury. Fatty changes are linked to intracellular accumulation of fat. When fatty changes occur, small vacuoles of fat disperse throughout the cytoplasm. The process is usually more ominous than cellular swelling, and although it is reversible, it usually indicates severe injury. These fatty changes may occur because normal cells are presented with an increased fat load or because injured cells are unable to metabolize the fat properly. In obese persons, fatty infiltrates often occur within and between the cells of the liver and heart because of an increased fat load. Pathways for fat metabolism may be impaired during cell injury, and fat may accumulate in the cell as production exceeds use and export. The liver, where most fats are synthesized and metabolized, is particularly susceptible to fatty change, but fatty changes may also occur in the kidney, the heart, and other organs.
Cell death
In each cell line, the control of cell number is regulated by a balance of cell proliferation and cell death. Cell death can involve apoptosis or necrosis. Apoptotic cell death involves controlled cell destruction and is involved iormal cell deletion and renewal. For example, blood cells that undergo constant renewal from progenitor cells in the bone marrow are removed by apoptotic cell death. Necrotic cell death is a pathologic form of cell death resulting from cell injury. It is characterized by cell swelling, rupture of the cell membrane, and inflammation.
Apoptosis.
Apoptosis, from Greek apo for “apart” and ptosis for “fallen,” means fallen apart. Apoptotic cell death, which is equated with cell suicide, eliminates cells that are worn out, have been produced in excess, have developed improperly, or have genetic damage. Iormal cell turnover, this process provides the space needed for cell replacement.
The process, which was first described in 1972, has become one of the most vigorously investigated processes in biology.13 Apoptosis is thought to be involved in several physiologic and pathologic processes. Current research is focusing on the genetic control mechanisms of apoptosis in an attempt to understand the pathogenesis of many disease states such as cancer and autoimmune disease. Apoptotic cell death is characterized by controlled autodigestion of cell components. Cells appear to initiate their own death through the activation of endogenous enzymes. This results in cell shrinkage brought about by disruption of the cytoskeleton, condensation of the cytoplasmic organelles, disruption and clumping of nuclear DNA, and a distinctive wrinkling of the cell membrane.2 As the cell shrinks, the nucleus breaks into spheres, and the cell eventually divides into membranecovered fragments. During the process, membrane changes occur, signaling surrounding phagocytic cells to engulf the cell fragments and complete the degradation process (Fig. 2-7). Apoptosis is thought to be responsible for several normal physiologic processes, including programmed destruction of cells during embryonic development, hormone-dependent involution of tissues, death of immune cells, cell death by cytotoxic T cells, and cell death in proliferating cell populations. During embryogenesis, in the development of a number of organs such as the heart, which begins as a single pulsating tube and is gradually modified to become a four-chambered pump, apoptotic cell death allows the next stage of organ development. It also separates the webbed fingers and toes of the developing embryo:
■ FIGURE ■ Outcomes of cell injury: reversible cell injury, apoptosis and programmed cell removal, cell death and necrosis.
The control of immune cell numbers and destruction of autoreactive T cells in the thymus have been credited to apoptosis. Cytotoxic T cells and natural killer cells are thought to destroy target cells by inducing apoptotic cell death.
■ FIGURE ■ Apoptotic cell removal: (A) shrinking of the cell structures, (B and C) condensation and fragmentation of the nuclear chromatin, (D and E) separation of nuclear fragments and cytoplasmic organelles into apoptotic bodies, and (F) engulfment of apoptotic fragments by phagocytic cell.
Apoptotic cell death occurs in the hormonedependent involution of endometrial cells during the menstrual cycle and in the regression of breast tissue after weaning from breast-feeding. Apoptosis appears to be linked to several pathologic processes. For example, suppression of apoptosis may be a determinant in the growth of cancers.
Apoptosis is also thought to be involved in the cell death associated with certain viral infections, such as hepatitis B and C, and in cell death caused by a variety of injurious agents, such as mild thermal injury and radiation injury.
Apoptosis may also be involved ieurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis (ALS). The loss of cells in these disorders does not induce inflammation; although the initiating event is unknown, apoptosis appears to be the mechanism of cell death.
Several mechanisms appear to be involved in initiating cell death by apoptosis. As in the case of endometrial changes that occur during the menstrual cycle, the process can be triggered by the addition or withdrawal of hormones. In hepatitis B and C, the virus seems to sensitize the hepatocytes to apoptosis.
Certain oncogenes and suppressor genes involved in the development of cancer seem to play an active role in stimulation or suppression of apoptosis. Injured cells may induce apoptotic cell death through increased cytoplasmic calcium, which leads to activation of nuclear enzymes that break down DNA.
In some instances, gene transcription and protein synthesis, the events that produce new cells, may be the initiating factors. In other cases, cell surface signaling or receptor activation appears to be the influencing force.
■ FIGURE ■ Examples of apoptosis: (A) separation of webbed fingers and toes in embryo, (B) development of neural connections, (C) removal of cells from intestinal villa, and (D) removal of senescent blood cells.
Necrosis.
Necrosis refers to cell death in an organ or tissue that is still part of a living person.2 Necrosis differs from apoptosis in that it involves unregulated enzymatic digestion of cell components, loss of cell membrane integrity with uncontrolled release of the products of cell death into the intracellular space, and initiation of the inflammatory response. In contrast to apoptosis, which functions in removing cells so they can be replaced by new cells, necrosis often interferes with cell replacement and tissue regeneration.
NECROSIS:
With necrotic cell death, there are marked changes in the appearance of the cytoplasmic contents and the nucleus. These changes often are not visible, even under the microscope, for hours after cell death. The dissolution of the necrotic cell or tissue can follow several paths. The cell can undergo liquefaction (i.e., liquefactioecrosis); it can be transformed to a gray, firm mass (i.e., coagulatioecrosis); or it can be converted to a cheesy material by infiltration of fatlike substances (i.e., caseous necrosis). Liquefactioecrosis occurs when some of the cells die but their catalytic enzymes are not destroyed. An example of liquefactioecrosis is the softening of the center of an abscess with discharge of its contents. During coagulatioecrosis, acidosis develops and denatures the enzymatic and structural proteins of the cell. This type of necrosis is characteristic of hypoxic injury and is seen in infarcted areas. Infarction (i.e., tissue death) occurs when an artery supplying an organ or part of the body becomes occluded and no other source of blood supply exists. As a rule, the infarct’s shape is conical and corresponds to the distribution of the artery and its branches. An artery may be occluded by an embolus, a thrombus, disease of the arterial wall, or pressure from outside the vessel. Caseous necrosis (i.e., soft, cheeselike center) is a distinctive form of coagulatioecrosis. It is most commonly associated with tubercular lesions and is thought to result from immune mechanisms.
Gangrene.
The term gangrene is applied when a considerable mass of tissue undergoes necrosis. Gangrene may be classified as dry or moist. In dry gangrene, the part becomes dry and shrinks, the skin wrinkles, and its color changes to dark brown or black. The spread of dry gangrene is slow, and its symptoms are not as marked as those of wet gangrene. The irritation caused by the dead tissue produces a line of inflammatory reaction (i.e., line of demarcation) between the dead tissue of the gangrenous area and the healthy tissue (Fig. 2-9). Dry gangrene usually results from interference with arterial blood supply to a part without interference with venous return and is a form of coagulatioecrosis. In moist or wet gangrene, the area is cold, swollen, and pulseless. The skin is moist, black, and under tension. Blebs form on the surface, liquefaction occurs, and a foul odor is caused by bacterial action. There is no line of demarcation between the normal and diseased tissues, and the spread of tissue damage is rapid. Systemic symptoms are usually severe, and death may occur unless the condition can be arrested. Moist or wet gangrene primarily results from interference with venous return from the part. Bacterial invasion plays an important role in the development of wet gangrene and is responsible for many of its prominent symptoms. Dry gangrene is confined almost exclusively to the extremities, but moist gangrene may affect the internal organs or the extremities. If bacteria invade the necrotic tissue, dry gangrene may be converted to wet gangrene. Gas gangrene is a special type of gangrene that results from infection of devitalized tissues by one of several Clostridium bacteria. These anaerobic and spore-forming organisms are widespread iature, particularly in soil; gas gangrene is prone to occur in trauma and compound fractures in which dirt and debris are embedded. Some species have been isolated in the stomach, gallbladder, intestine, vagina, and skin of healthy persons. The bacteria produce toxins that dissolve the cell membranes, causing death of muscle cells, massive spreading edema, hemolysis of red blood cells, hemolytic anemia, hemoglobinuria, and renal toxicity.16 Characteristic of this disorder are the bubbles of hydrogen sulfide gas that form in the muscle. Gas gangrene is a serious and potentially fatal disease. Because the organism is anaerobic, oxygen is sometimes administered in a hyperbaric chamber.
■ FIGURE ■ Gangrenous toes.
In summary, cell injury can be caused by a number of agents, including physical agents, chemicals, radiation, and biologic agents. Among the physical agents that generate cell injury are mechanical forces that produce tissue trauma, extremes of temperature, electricity, radiation, and nutritional disorders. Chemical agents can cause cell injury through several mechanisms: they can block enzymatic pathways, cause coagulation of tissues, or disrupt the osmotic or ionic balance of the cell. Ionizing radiation affects cells by causing ionization of molecules and atoms in the cell, by directly hitting the target molecules in the cell, or by producing free radicals that interact with critical cell components. Biologic agents differ from other injurious agents in that they are able to replicate and continue to produce injury. Among the nutritional factors that contribute to cell injury are excesses and deficiencies of nutrients, vitamins, and minerals. Injurious agents exert their effects largely through generation of free radicals, production of cell hypoxia, or unregulated intracellular calcium levels. Partially reduced oxygen species called free radicals are important mediators of cell injury in many pathologic conditions. They are an important cause of cell injury in hypoxia and after exposure to radiation and certain chemical agents. Lack of oxygen underlies the pathogenesis of cell injury in hypoxia and ischemia. Hypoxia can result from inadequate oxygen in the air, cardiorespiratory disease, anemia, or the inability of the cells to use oxygen. Increased intracellular calcium activates a number of enzymes with potentially damaging effects. Injurious agents may produce sublethal and reversible cellular damage or may lead to irreversible cell injury and death. Cell death can involve two mechanisms: apoptosis or necrosis. Apoptosis involves controlled cell destruction and is the means by which the body removes and replaces cells that have been produced in excess, developed improperly, have genetic damage, or are worn out. Necrosis refers to cell death that is characterized by cell swelling, rupture of the cell membrane, and inflammation.
Cell degeneration
Degeneration is a type of nonlethal cell damage that generally occurs in the cytoplasm and that doesn’t affect the nucleus. Degeneration usually affects organs with metabolically active cells, such as the liver, heart, and kidneys, and is caused by these problems:
· increased water in the cell or cellular swelling
· fatty infiltrates
· atrophy
· autophagocytosis (that is, the cell absorbs some of its own parts)
· pigmentation changes
· calcification
· hyaline infiltration
· hypertrophy
· dysplasia (related to chronic irritation)
· hyperplasia.
When changes in cells are identified, prompt health care can slow degeneration and prevent cell death. An electron microscope can help identify cellular changes, and thus diagnose a disease, before the patient complains of any symptoms. Unfortunately, many cell changes remain unidentifiable even under a microscope, making early detection of disease impossible.
Cell aging
During the normal process of aging, cells lose both structure and function. Atrophy, a decrease in size or wasting away, may indicate loss of cell structure. Hypertrophy or hyperplasia is characteristic of lost cell function.
FACTORS THAT AFFECT CELL AGING
Cell aging can be affected by the intrinsic and extrinsic factors listed below.
INTRINSIC FACTORS
· Congenital
· Degenerative
· Immunologic
· Inherited
· Metabolic
· Neoplastic
· Nutritional
· Psychogenic
EXTRINSIC FACTORS
Physical agents
· Chemicals
· Electricity
· Force
· Humidity
· Radiation
· Temperature
Infectious agents
· Bacteria
· Fungi
· Insects
· Protozoa
· Viruses
· Worms
Signs of aging occur in all body systems. Examples include diminished elasticity of blood vessels, bowel motility, muscle mass, and subcutaneous fat. Cell aging can slow down or speed up, depending on the number and extent of injuries and the amount of wear and tear on the cell. The cell aging process limits the human life span (of course, many people die from disease before they reach the maximum life span of about 110 years). A number of theories attempt to explain the reasons behind cell aging.
When a cell dies, enzymes inside the cell are released and start to dissolve cellular components. This triggers an acute inflammatory reaction in which white blood cells migrate to the necrotic area and begin to digest the dead cells. At this point, the dead cells — primarily the nuclei — begin to change morphologically in one of three ways:
· pyknosis, in which the nucleus shrinks, becoming a dense mass of genetic material with an irregular outline.
· karyorrhexis, in which the nucleus breaks up, strewing pieces of genetic material throughout the cell.
· karyolysis, in which hydrolytic enzymes released from intracellular structures called lysosomes simply dissolve the nucleus.
