robbins-pathology-chapter-2-trans.pdf

CHAPTER 2: CELLULAR RESPONSES TO STRESS AND TOXIC Functional Derangements and Clinical Manifestations INSULTS: ADAPTATION, INJURY, AND DEATH  functional consequences of these c...

CHAPTER 2: CELLULAR RESPONSES TO STRESS AND TOXIC Functional Derangements and Clinical Manifestations INSULTS: ADAPTATION, INJURY, AND DEATH  functional consequences of these changes  Functional abnormalities – the end results of genetic, biochemical, and structural INTRODUCTION TO PATHOLOGY changes in cells and tissues are functional abnormalities o Lead to clinical manifestations (symptoms and signs) of disease, as well as its progress (clinical course and outcome)  Pathology – devoted to the study of the structural, biochemical, and functional  Clinicopathologic correlations are very important in the study of the disease changes in cells, tissues, and organs that underlie disease  All forms of disease start with molecular or structural alterations in cells o Explain whys and whereof of the signs and symptoms manifested by  Rudolph Virchow – father of modern pathology; developed the concept of cellular patients; provide rational basis for clinical care and therapy basis of disease o Serves as the bridge between the basic sciences and clinical medicine  Cell and ECM injury leads to tissue and organ injury which determines the o Scientific foundation for all of medicine morphologic and clinical patterns of disease  General pathology – concerned with the common reactions of cells and tissues to injurious stimuli OVERVIEW: CELLULAR RESPONSE TO STRESS AND NOXIOUS STIMULI o Such reactions are not tissue specific (thus acute inflammation is response to bacterial infections produces same reactions in most tissues)  Normal cell is confined to narrow range of function and structure by its state of  Systemic pathology – examines the alterations and underlying mechanisms in metabolism, differentiation, and specialization; by constraints of neighboring cells; and organ specific diseases such as ischemic heart disease by the availability of metabolic substrates  Four aspects of disease process that form the core of pathology: o Homeostasis – steady state  Adaptations – reversible functional and structural responses to changes in Etiology or Cause  Cause of the disease physiologic states (e.g., pregnancy) and some pathologic stimuli (new and altered  Can be grouped into two classes: steady states are achieved to allow the cell survival and functioning)  Genetic – inherited mutations, disease-associated gene variants, polymorphisms  Adaptive response may consist of:  Acquired – infectious, nutritional, chemical, physical o Hypertrophy – increase in size and function  The idea that one etiologic agent is the cause of a disease is from the study of o Hyperplasia – increase in number of cells infections and inherited disorders caused by single genes o Atrophy – decrease in size and metabolic activity o Idea is not applicable to majority of diseases (e.g., atherosclerosis and o Metaplasia – change in the phenotype of cells cancer  multifactorial and cause by various external triggers)  When the stress is eliminated, the cell can recover to its original state without consequences Pathogenesis  biochemical and molecular mechanisms of its development If the limits of adaptive responses are exceeded or if cells are exposed to  Refers to the sequence of cellular, biochemical, and molecular events that injurious agents or stress, deprived of essential nutrients, or become follow the exposure of cells or tissues to an injurious agent compromised by mutations that affect essential cellular constituents, a o How mutations induced disease sequence of events follows that is termed cell injury.  Even when initial cause is known, it is many steps removed from expression of disease o Example: to understand cystic fibrosis it is essential to know the defective  Cell injury is reversible upto certain point  but if the stimulus persists or is severe gene product and the biochemical and morphological events leading to enough from the beginning  may lead to irreversible injury and ultimately cell formation of cysts and fibrosis in lungs, pancreas, etc. death  Stages of progressive impairment following different types of insults: o Adaptation Morphologic Changes  structural alterations induced in the cells and organs of the body o Reversible injury  Refer to the structural alterations in cells or tissues that are either o Cell death characteristic of a disease or diagnostic of an etiologic process  Example: ↑ hemodynamics  enlargement of heart muscle (adaptation)  may then  Morphology – determine the nature of the disease and to follow its progression; become injured (if blood supply to myocardium is not enough, heart muscle suffers diagnostic cornerstone reversible injury and eventually irreversible injury and cell death)  Limitations: morphologically identical lesions may arise by distinct molecular  Cell death – the end result of progressive cell injury; one of the most crucial events mechanisms in the evolution of disease in any tissue or organ o E.g., tumors  breast cancers that are indistinguishable morphologically may have widely different courses, therapeutic responses, and prognosis o Results from diverse causes (ischemia, infection, toxins)  Molecular analysis (e.g., next generation sequencing) – reveal genetic differences  Cell death is also a normal and essential process in embryogenesis, the development that predict the behaviour of tumor and responses of organs, and the maintenance of homeostasis abmrmtmd   Two principal pathways of cell death:  Striated cells (heart and skeletal muscles) have limited capacity for division  o Necrosis respond to increased metabolic demands mainly by undergoing hypertrophy o Apoptosis o Stimulus for hypertrophy in heart is usually chronic hemodynamic overload  Autophagy – adaptive cellular response triggered by nutrient deprivation; may also (may result from hypertension or faulty valves) culminate in death o Cells synthesize more proteins and number of myofilaments increases   Metabolic derangements in cells and sublethal, chronic injury may be associated with increase strength and work capacity intracellular accumulations of a number of substances, including proteins, lipids, and  Most common stimulus for hypertrophy of muscle is increased workload (e.g., body carbohydrates builders)  Calcium – often deposited at sites of cell death, resulting in pathologic calcification  Pregnancy  massive physiologic growth of uterus  Aging is also accompanied by characteristic morphologic and functional changes in o Hormone-induced enlargement of organ mainly from hypertrophy of muscle cells o Uterine hypertrophy is stimulated by estrogenic hormones acting on smooth  Three other processes that affect cells and tissues: muscle  ↑ synthesis of smooth muscle proteins and an increase in cell size o Intracellular accumulations o Pathologic calcifications Mechanisms of Hypertrophy o Cell aging  Hypertrophy is the result of increased production of cellular proteins  Hypertrophy of the heart may become mal-adaptive and can lead to heart failure, arrhythmias, and sudden death. Three basic steps in the molecular pathogenesis of cardiac hypertrophy:  The actions of mechanical sensors (triggered by ↑ workload), growth factors (TGF-B. IGF-1, FGF) and vasoactive agents (a-adrenergic agonist, endothelin 1, ANG II). Mechanical sensors induce production of growth factors and agonist.  Signals from cell membrane activate a complex of signal transduction pathways. Two biochemical pathways are involved in muscle hypertrophy o PI3K/AKT pathway – most important in physiologic hypertrophy o Signaling downstream of GPCRs – induced by many growth factors and vasoactive agents; though to be more important in pathologic hypertrophy  Signalling pathways activate transcription factors such as GATA4, NFAT, and MEF2. These transcription factors work co-ordinately to increase the synthesis of muscle proteins that are responsible for hypertrophy ADAPTATIONS OF CELLULAR GROWTH AND DIFFERENTIATION  Hypertrophy is also asso. w/ a switch of contractile proteins from adult to fetal  Adaptations – reversible changes in the size, number, phenotype metabolic neonatal forms activity, or functions of cells in response to changes in their environment o During muscle hypertrophy, the alpha isoform of heavy chain is replaced by the beta isoform  has slower, more energetically economical contraction HYPERTROPHY  Some genes expressed only during early development are reexpressed in hypertrophic cells, and the products participate in the cellular response to stress  Hypertrophy refers to an increase in the size of the cells, that results in an o E.g.,gene for ANF is expressed in both the atrium and the ventricle in the increase in the size of the affected organ embryonic heart. Cardiac hypertrophy is associated with ↑ ANF gene o Organ has no new cells, only larger cells o ANF – peptide hormone that causes salt secretion by the kidney, decreases  Due to synthesis and assembly of additional intracellular structural components blood volume and pressure  serves to reduce hemodynamic load  Cells capable of division may respond to stress by undergoing both hyperplasia and  Cardiac hypertrophy reaches a limit  enlargement is unable to cope with hypertrophy enlargement  regressive changes (lysis and loss of myofribrillar contractile o Non dividing (e.g., myocardial fibers) increased tissue mass is due to elements)  myocyte death  net result: cardiac failure (if stress is not relieved) hypertrophy o To prevent such consequences, several drugs that inhibit key signaling o Hypertrophy and hyperplasia may coexist and contribute to increase size pathways involving NFAT, GATA4, and MEF2 genes are in clinical trials  Hypertrophy can be physiologic or pathologic o Physiologic – caused by increased functional demand or by stimulation by hormones and growth factors abmrmtmd HYPERPLASIA ATROPHY  Hyperplasia is defined as an increase in the number of cells in an organ or  Atrophy is defined as a reduction in the size of an organ or tissue due to a tissue in response to a stimulus decrease in cell size and number; physiologic or pathologic  Hyperplasia can only take place if the tissue contains cells capable of dividing  increase number of cells; can be physiologic or pathologic Physiologic Atrophy  Common during normal development Physiologic Hyperplasia  Example: notochord and thryoglossal duct  atrophy during fetal development  Physiologic hyperplasia due to the action of hormones or growth factors occurs in  Uterus  decrease in size after parturition several circumstances: Pathologic Atrophy o When there is a need to increase functional capacity of hormone  Has several causes and can be: local or generalized sensitive organs o When there is need for compensatory increase after damage or resection Common Causes of Atrophy  Hormonal hyperplasia – eg, proliferation of glandular epithelium of female breast Decreased - Atrophy of disuse and enlargement (hypertrophy) of glandular epithelial cells workload - Example: immobilized fractured bone in a cast or complete bed rest  Compensatory hyperplasia – eg, liver regeneration (proliferation to its normal size) - Initial decrease in cell size is reversible once activity is resumed  Marrow undergo rapid hyperplasia in response to a deficiency of terminally - Prolonged disuse  skeletal muscle fibers ↓ in size differentiated blood cells - Muscle atrophy can be accompanied by ↑ bone resorption  lead to o Example: acute bleed or hemolysis  EPO are activated to stimulate RBC osteoporosis of disuse progenitors  ↑ RBC production upto 8x Loss of - Denervation atrophy inervation - Normal metabolism and function is dependent on nerve supply - Damage to nerves  atrophy of the muscle fibers supplied Pathologic Hyperplasia Diminished - Ischemia (↓ blood supply; e.g., occlusion)  Most forms of pathologic hyperplasia are caused by excessive or blood supply - Brain may go progressive atrophy in late adult life  due to ↓ blood inappropriate actions of hormones or growth factors acting on target cells supply from atherosclerosis  senile atrophy (also affect heart)  Endometrial hyperplasia – example of abnormal hormone-induced hyperplasia Inadequate - Marasmus (profound protein calorie malnutrition) – asso. w/ o Disturbed balance between estrogen and progesterone  ↑ estrogen  nutrition utilization of skeletal muscle proteins as a source of energy (other hyperplasia of endometrial glands reserves like adipose tissue has already been depleted)  results in o Common cause of abnormal menstrual bleeding cachexia (marked muscle wasting)  Benign prostatic hyperplasia – pathologic hyperplasia induced by androgens - Cachexia – also seen in patients w/ chronic inflammatory disease  These pathologic hyperplasia remains controlled; hyperplasia regresses if hormonal (CID) and cancer stimulation is eliminated - In CID, TNF is overproduced and is responsible for appetite  Pathologic hyperplasia constitutes a fertile soil for cancer suppression and lipid depletion  ends in muscle atrophy  Hyperplasia is a characteristic response to certain viral infections Loss of - Hormone responsive tissue (e.g., breast and reproductive o HPV – may cause skin warts & several mucosal lesions composed of endocrine organs) are dependent on endocrine stimulation for normal function masses of hyperplastic epithelium stimulation - Loss of estrogen stimulation after menopause results in physiologic atrophy of endometrium, vaginal epithelium, and breast Pressure - Tissue compression can cause atrophy Mechanisms of Hyperplasia - Enlarging benign tumor  atrophy in surrounding uninvolved tissue - This atrophy is due to ischemic changes caused by compromise of  Hyperplasia is the result of growth factor-driven proliferation of mature the blood supply by the pressure exerted by the expanding mass cells and, in some cases, by increased output of new cells from tissue stem  Fundamental cellular changes asso. w/ atrophy are identical in all of these settings cells o Initial response  decrease in cell size and organelles, which ↓ the  Example: after partial hepatectomy growth factors are produced in the liver that metabolic needs of the cell for its survival engage receptors on surviving cells and activate cell proliferation  In atropic muscle, cells contain fewer mitochondria and myofilaments and a reduced o If proliferative capacity of liver is compromised (e.g., hepatitis) hepatocytes amount of RER can instead regenerate from intrahepatic stem cells o New equilibrium is achieved  balance of metabolic demands, low levels of blood supply, nutrition or trophic stimulation  Atrophy caused by gradually reduced blood supply  progress to irreversible injury and death of cells (apoptosis) o cell death by apoptosis contributes to the atrophy of endocrine organs after hormonal withdrawal abmrmtmd Mechanism of Atrophy Mechanisms of Metaplasia  Atrophy results from decreased protein synthesis and increased protein  Metaplasia does not result from a change in the phenotype of an already degradation in cells differentiated cell type; instead it is the result of a reprogramming of stem o Protein synthesis decreases because of reduced metabolic activity cells that are known to exist in normal tissues, or of undifferentiated  The degradation of cellular proteins occurs mainly by the ubiquitin- mesenchymal xells present in connective tissue proteasome pathway o Precursor cells differentiate in a new pathway or lineage by signals from o Nutrient deficiency and disuse  activate ubiquitin ligase  attach ubiquitin cytokines, growth factors, and ECM components in the cell’s environment to cellular proteins  target proteins for degradation in proteasome  Direct link between transcription factor dysregulation and metaplasia is seen with o This pathway is responsible for the accelerated proteolysis in catabolic vitamin A (retinoic acid) deficiency or excess (both may cause metaplasia) conditions (e.g., cancer cachexia) o Retinoic acid regulates gene transcription directly through nuclear retinoid  Atrophy is also accompany by autophagy, marked by appearance of ↑ #s of receptors  influence differentiation of progenitors derived from tissue autophagic vacuoles stem cells o Autophagy (“self-eating”) – process where starved cells eat its own components to reduced nutrient demand to match the supply KEY CONCEPTS: Cellular Adaptation to Stress o Some cell debris resist digestion and seen in cytoplasm as membrane-bound Hypertrophy - Increased cell and organ size in response to increased workload residual bodies (e.g., lipofuscin granules  brown atrophy) - Induced by growth factors produced in response to mechanical stress or other stimuli - Occurs in tissues incapable of cell division METAPLASIA Hyperplasia - Increased cell numbers in response to hormones ad ther growth factors  Metaplasia is reversible change in which one differentiated cell type - Occurs in tissues whose cells are able to divide or contain (epithelial or mesenchymal) is replaced by another cell type abundant tissue stem cells  Often represents as an adaptive response  “one cell type that is sensitive to a Atrophy - Decreased cell and organ size as a result of decreased nutrient particular stress is replaced by another cell type that is better able to withstand the supply or disuse adverse environment” - Asso. w/ decreased synthesis of cellular building blocks and  Most common epithelial metaplasia is columnar to squamous (occurs in respiratory increased breakdown of cellular organelles Metaplasia - Change in phenotype of differentiated cells, often in response to tract in response to chronic irritation) chronic irritation, that makes cells better able to withstand the o Habitual cigar smoker’s trachea and bronci: ciliated columnar is replaced by stress stratified squamous cells - Usually induced by altered differentiation pathway of tissue stem o Secretory columnar epithelium (salivary gland, pancreas, bile duct) is cells replaced by stratified squamous epithelium (squamous metaplasia) - May result in reduced functions or increased propensity for o Vitamin A deficiency induces squamous metaplasia in the respiratory malignant transformation epithelium  Stratified squamous is able to survive under circumstances than columnar cells o However, important mechanisms of protection against infection are lost (e.g., mucus secretion and ciliary action)  Epithelial metaplasia is “double-edged sword”  The influences that predispose to metaplasia, if persistent, can initiate malignant transformation in metaplastic epithelium (common cause of cancer in respiratory tract is compose of squamous cells)  Metaplasia from squamous to columnar type also occur in Barrett esophagus  squamous is replaced by columnar cells under the influence of refluxed gastric acid o Cancer may arise in this areas and are typically glandular  Connective tissue metaplasia – formation of cartilage, bone, or adipose tissue (mesenchymal tissues) in tissues that do not contain these elements o Example: myositis ossificans – bone formation in muscle; occur after intramuscular hemorrhage; seen as adaptive response from cell or tissue injury abmrmtmd OVERVIEW OF CELL INJURY AND DEATH CAUSES OF CELL INJURY  Cell injury results when cells are severely stressed and no longer able to adapt or  May range from physical trauma to subtle cellular abnormalities such as mutation when cells are exposed to inherently damaging agents or from intrinsic abnormalities (e.g., lack of enzyme that impairs normal function)  Injury may progress through a reversible stage and culminate in cell death  Most injurious stimuli can be grouped into the following broad categories: Reversible Cell Injury - Hypoxia – deficiency of oxygen; causes cell injury by  Changes are reversible in early or mild forms if damaging stimulus are removed reducing aerobic oxidative respiration  Hallmarks of reversible injury: - Hypoxia is an extremely important and common cause of o Reduced oxidative phosphorylation with resultant depletion of energy stored cell injury and cell death in the form of ATP and; Oxygen Deprivation - Causes of hypoxia: o Cellular swelling caused by changes in ion concentration and water influx o Ischemia (reduced blood flow)  Intracellular organelles (mitochondria and cytoskeleton) may show alterations o Inadequate oxygenation of the blood due to cardiorespiratory failure o Decreased oxygen-carrying capacity of blood (as in anemia or carbon monoxide poisoning or after severe blood loss) - Trauma, extremes of temperature (burns and deep cold), Cell Death Physical agents sudden changes in atmospheric pressure, radiation,  Injury becomes irreversible with continuous damage  cell cannot recover and dies electric shock  Two principal types of cell death: necrosis and apoptosis - Simple chemicals (glucose or salt) in hypertonic concentrations may cause cell injury directly or by - “accidental” and unregulated form of cell death due to damage deranging electrolyte balance in cells to cell membranes and loss of ion hemostasis - Oxygen at high concentration is toxic - When membrane damage is severe  lysosomal enzymes Chemical agents and - Poisons (arsenic, cyanide or mercuric salts) in trace enter the cytoplasma and digest the cell  give rise to drugs amounts may damage cells to cause death Necrosis morphologic changes or necrosis - Daily companions: environmental and air pollutants, - Cellular contents leak through the damaged plasma membrane insecticides, herbicides into extracellular space where they elicit reaction  - Industrial/Occupational hazards: carbon monoxide, inflammation asbestos - Necrosis is the pathway of cell death in many commonly - Recreational drugs (alcohol) and ever-increasing variety of encountered injuries (e.g., resulting from ischemia, exposure therapeutic drugs to toxins, various infections, trauma) Infectious agents - Range from viruses to tapeworms - When the cell’s DNA or proteins are damaged beyond repair, - Rickettsiae, bacteria, fungi, and parasites cell kill itself by apoptosis Immunologic - Immune system serves an essential function in defense - Characterized by: reactions against pathogens but may also cause cell injury Apoptosis o Nuclear dissolution - Autoimmune diseases o Fragmentation of cell w/o complete loss of - Genetic defects may cause cell injury due to deficiency of membrane integrity functional proteins o Rapid removal of the cellular debris o Enzyme defects in inborn errors of metabolis - Cellular contents do not leak out  no inflammatory Genetic o Accumulation of damaged DNA or misfolded reaction derangements proteins - Highly regulated process driven by a series of genetic o Both trigger cell death when damage is beyond pathways repair - “programmed cell death” - Polymorphism (DNA sequence variants) can also influence the susceptibility of cell to injury by chemicals and  Whereas necrosis is always pathologic process, apoptosis serves many normal environmental insults functions and is not necessarily associated with cell injury - Major cause of cell injury  Necrosis in some cases is also a form of programmed cell death  necroptosis Nutritional - Protein-calorie deficiencies cause number of deaths imbalances (among under privileged populations) - Can be self-imposed (anorexia nervosa) - Nutritional excess also cause cell injury (atherosclerosis and obesity) abmrmtmd MORPHOLOGIC ALTERATIONS IN CELL INJURY REVERSIBLE INJURY  All stresses and noxious influences exert their effects first at the molecular or  Two features of reversible cell injury can be recognized under the light biochemcical level microscope:  There is a time lag between stress and morphologic changes of cell injury or death o Cellular swelling o The duration of this delay may vary with the sensitivity of methods used to o Fatty change detect changes  Cellular swelling – appears when cells are incapable of maintaining ionic  Histochemical or Ultrastructural techniques  changes may be seen in minutes to homeostasis; result of failure of energy-dependent ion pumps in the plasma hours after injury membrane  Light microscopy or gross examination – changes take longer time (hours to days) to  Fatty change – occurs in hypoxic injury and various forms of toxic or metabolic be seen injury  Morphologic manifestations of necrosis take more time to develop than those or o Manifested by the appearance of lipid vacuoles in the cytoplasm reversible damage (Example: ischemia  cell swelling occur in minutes  progress to o Seen mainly in cells involved in and dependent on fat metabolism, such as irreversibility within an hour or two) hepatocytes and myocardial cells  Light microscopic changes of cell death may not be seen until 4 to 12 hours after onset of ischemia MORPHOLOGY  FIGURE 2-8 (sequential changes in cell injury to cell death) Cellular swelling – first manifestation of almost all forms of injury to cells  Difficult to see in LM; more apparent at the level of whole organ Features of Necrosis and Apoptosis  When it affects many cells, it causes pallor, increased turgor, increased weight of the organ; Swelling of cells is reversible Feature Necrosis Apoptosis  On microscopic exam: small clear vacuoles may be seen in cytoplasm  represent Cell size Enlarged (swelling) Reduced (shrinkage) distended and pinched-off segments of the ER Nucleus Pyknosis  karyorrhexis  Fragmentation into nucleosome-size o This pattern of non lethal injury is called hydropic change or vacuolar karyolysis fragments degeneration Plasma Disrupted Intact; altered structure (orientation of  Cells may also increased eosinophilic staining  becomes more pronounced with membrane lipids) progression to necrosis  Ultractructural changes of reversible injury include: Cellular Enzymatic digestion; may leak out Intact; may be released in apoptotic 1. Plasma membrane alterations  blebbing, blunting, and loss of microvilli contents of cell bodies 2. Mitochondrial changes  including swelling anf the appearance of small Adjacent Frequent No amorphous densities inflammation 3. Dilation of the ER with detachment of polysomes; intracytoplasmic myelin Physiologic or Invariably pathologic (culmination Often physiologic, means of figures may be present pathologic of irreversible cell injury) eliminating unwanted cells; may be 4. Nuclear alterations  disaggregation of granular and fibrillar elements role pathologic after injury (DNA damage)  Reversible injury is characterized by: NECROSIS o Generalized swelling of cell and organelles o Blebbing of the plasma membrane  The morphologic appearance of necrosis as well as necroptosis is the result o Detachment of ribosomes from the ER of denaturation of intracellular proteins and enzymatic digestion of the o Clumping of nuclear chromatin lethally injured cell  These morphologic changes are associated with:  Necrotic cells are unable to maintain membrane integrity and their contents leak out o Decreased generation of ATP  cause inflammation o Loss of cell membrane integrity  Enzymes that digest necrotic cells are derived from the lysosomes of the dying cells o Defects in protein synthesis themselves and from the lysosomes of leukocytes o Cytoskeletal damage  Digestion of cellular contents and response take hours to develop o DNA damage o No detectable changes if myocardial infarct caused sudden death  Persistent of excessive injury  “point of no return” ; irreversible injury and death o Earliest histologic evidence of myocardial necrosis does not become  Severe mitochondrial damage with depletion of ATP and rupture of lysosomal and apparent until 4-12 hours later plasma membranes are associated with necrosis  Because of the loss of plasma membrane integrity, cardiac-specific enzymes and  Necrosis also occur in: proteins are rapidly released from necrotic muscle and can be detected in the blood as o Ischemia; Exposure to toxins early as 2 hours after myocardial necrosis o Various infections; Trauma abmrmtmd  pancreatitis  pancreatic enzymes leak out of acinar cells and liquefy the membranes of fat cells in the peritoneum  released lipase split the triglyceride esters within fat cells - Fatty acids combine with calcium to produce chalky-white areas (fat saponification) - Histologic exam: necrosis takes the form of foci shadowy outlines of necrotic fat cells, with basophilic calcium deposits, surrounded by an inflammatory reaction Fibrinoid - Special form of necrosis usually seen in immune reactions involving necrosis blood vessels - Occurs when complexes of antigens and antibodies are deposited in the wallsof arteries - Deposits of “immune complexes” together with fibrin result in a bright pink and amorphous appearance in H&E  called “fibrinoid” Patterns of Tissue Necrosis (fibrin-like)  Most necrotic cells disappear due to enzymatic digestion and phagocytosis by WBC  When large number of cells die the tissue or organ is said to be necrotic  If not destroyed  they provide a nidus for deposition of calcium and become  Necrosis of tissue has severe distinct morphological patterns  important to recognize calcified  dystrophic calcification because they provide clues about the underlying cause KEY CONCEPTS: Morphologic Alterations in Injured Cells and Tissues Coagulative - Architecture of dead tissues is preserved for a span of some days  Reversible cell injury: cellular swelling, fatty change, PM blebbing and loss of necrosis - Affected tissue exhibit a firm texture microvilli, mitochondrial swelling, ER dilation, eosinophilia (↓ cytoplasmic RNA) - The injury denatures not only structural proteins but also enzymes  Necrosis: ↑ eosinophilia, nuclear shrinkage, fragmentation, dissolution; breakdown of and so blocks the proteolysis of the dead cells  as a result, PM and organelle membranes; abundant myelin figures; leakage and enzymatic eosinophilic, anucleate cells may persist for days or weeks digestion of cellular contents - Ultimately the necrotic cells are removed by phagocytois of the  Pattern of tissue necrosis: under different conditions, necrosis in tissues may assume cellular debris by infiltrating leukocytes and by digestion of dead specific patterns: coagulative, liquefactive, gangrenous, casseous, fat, fibrinoid cells by lysosomal enzymes of leukocytes - Ischemia caused by obstruction in a vessel may lead to coagulative necrosis of the supplied tissue in all organs except the brain MECHANISMS OF CELL INJURY - A localized area of coagulative necrosis is called an infarct Liquefactive - Characterized by digestion of the dead cells  results in  Several principles that are relevant to most forms of cells injury: necrosis transformation of the tissue into a liquid viscous mass - Seen in focal bacterial or fungal infections because microbes stimulate the accumulation of leukocytes and the liberation of  The cellular response to injurious stimuli depends on the nature of the enzymes from these cells injury, its duration, and its severity o Small doses of chemical toxin or brief period of ischemia may induce Gangrenous - Not a specific pattern of cell death reversible injury necrosis - Usually applied to a limb that has lost its blood supply and has  The consequence of cell injury depend on the type, state, and adaptability undergone necrosis (coagulative necrosis) involving multiple tissue of the injured cell planes o Nutritional and hormonal status and metabolic needs  important in - When bacterial infection is superimposed there is more liquefactive necrosis because of the actions of degradative enzymes in the response to injury bacteria and the attracted leukocytes  give rise to wet gangrene o Striated muscle in leg can be placed at rest and preserved when deprived of Caseous - Encountered most often in foci of tuberculosis infection its blood supply; not so the striated muscle of the heart necrosis - “caseous” (cheese-like) derived from the friable white appearance o Exposure of two individual cells to a toxin may have no effect in one cell of the area of necrosis and cell death in another  due to polymorphism - Microscopic exam: necrotic area appears as a structureless collection  Cell injury results from different biochemical mechanisms acting on several of fragmented or lysed cells and amorphous granular debris essential cellular components (Figure 2-16) enclosed within a distinctive inflammatory border  this appearance is characteristic of a focus of inflammation known as granuloma o Cellular components that are most frequently damaged by injurious stimuli Fat necrosis - Does not denote a specific pattern of necrosis include: mitochondria, cell membranes, the machinery of protein synthesis - Refers to focal areas of fat destruction, typically resulting from and packaging, and DNA release of activated pancreatic lipase into pancreas and peritoneal o Any injurious stimulus may simultaneously trigger multiple interconnected cavity mechanisms that damage cells  difficult to ascribe cell injury to a single - Occurs in the calamitous abdominal emergency known as acute biochemical derangement abmrmtmd DEPLETION OF ATP MITOCHONDRIAL DAMAGE  Reduction in ATP levels is fundamental cause of necrotic cell death  Mitochondria are critical players in cell injury and cell death by all pathways  ATP depletion and decreased ATP synthesis  asso. w/ hypoxic and chemical (toxic) o They supply life-sustaining energy by producing ATP injury  Mitochondria can be damaged by : (they are sensitive to hypoxia and toxins)  ATP is produced in two ways: o Increases of cytosolic Ca2+ o Major pathway is oxidative phosphorylation of ADP  results in o Reactive oxygen species reduction of oxygen by the electron transfer system of mitochondria o Oxygen deprivation o Glycolytic pathway  can generate ATP in the absence of oxygen using  Mutations in mitochondrial genes are the cause of some inherited diseases glucose derived from body fluids or from hydrolysis of glycogen  Three major consequence of mitochondrial damage:  Major causes of ATP depletion are: o Reduced supply of oxygen and nutrients  Formation of a high-conductance channel in the mitochondrial membrane called o Mitochondrial damage mitochondrial permeability transition pore o Actions of some toxins (e.g., cyanide) o Opening of channel leads to the loss of mitochondrial membrane potential  High energy phosphate in the form of ATP is required for virtually all synthetis and  leads to failure of oxidative phosphorylation and progressive ↓ of ATP  degradative processes within the cell (e.g., membrane transport, protein synthesis, ends in cell death lipogenesis, deacylation-reacylation reaction for phospholipid turnover) o Cyclophilin D – one of the structural components of mitochondrial  Depletion of ATP to 5%-10% of normal levels has widespread effects on many critical permeability transition pore (MPTP) cellular systems:  Targeted by cyclosporine (immunsuppressive drug)  Activity of the plasma membrane energy-dependent Na-pump is reduced  Cyclosporine reduces injury by preventing opening of the MPTP o Failure of this active transport system causes Na to enter and  Abnormal oxidative phosphorylation also leads to the formation of ROS which have accumulate inside cells and K to diffuse out (SIPO) many deleterious effect o Net gain of solute = isosmotic gain of water  cell swelling and  Mitochondria produce several proteins capable of activating apoptotic pathways dilation of ER o Includes cytochrome c and proteins that indirectly activate apoptosis-  Cellular energy ,metabolism is altered inducing enzymes called caspases o If supply of oxygen is reduced = oxidative phosphorylation ceases o Increased permeability of outer membrane results in leakage of these  ↓ ATP and ↑ AMP proteins in the cytosol and death by apoptosis o Changes stimulate phosphofructokinase and phosphorylase activities  lead to ↑ anaerobic glycolysis (generate ATP from glycogen)  rapid ↓ glycogen stores o Anaerobic glycolysis causes accumulation of lactic acid and INFLUX OF CALCIUM AND LOSS OF CALCIUM HOMEOSTASIS inorganic phosphates  ↓ intracellular pH  ↓ cellular enzymes activity  Calcium ions are important mediators or cell injury  Failure of Ca2+ pump leads to influx of Ca2+  has damaging effects  Depleting calcium protects cells from injury induced by harmful stimuli  Prolonged depletion of ATP  structural disruption of the protein synthetic  Cytosolic calcium – maintained at 0.1umol apparatus occurs  manifested as detachment of ribosomes and  Extracellular levels 0 1.3 mmol dissociation of polysomes with reduction in protein synthesis  Most intracellular calcium is sequestered in mitochondria and ER  In cells deprived of oxygen or glucose, proteins become misfolded  Ischemia and certain toxins cause an increase in cytosolic calcium concentration o Accumulation of misfolded proteins in ER triggers a cellular o Initially due to release of Ca2+ from intracellular stores and later due to reaction called “unfolded protein response” that may increased influx across the plasma membrane accumulate in cell inury and even death  Increased intracellular Ca2+ causes cell injury by several mechanisms:  Ultimately, there is irreversible damage to mitochondrial and lysosomal o Accumulation of Ca2+ in mitochondria  opens MPTP and fails production membranes, and the cell undergoes necrosis of ATP o ↑ cytosolic calcium activates enzymes with harmful effects on cells  Phospholipase – cause membrane damage  Proteases – breakdown both membrane and cytoskeletal proteins  Endonuclease – responsible for DNA and chromatin fragmentation  ATPase – hastening ATP depletion o ↑ intracellular Ca2+ levels result in the induction of apoptosis by direct activation of caspases and by increasing mitochondrial permeability abmrmtmd ACCUMULATION OF OXYGEN-DERIVED FREE RADICALS (OXIDATIVE STRESS)  Series of enzymes acts as free radical-scavenging systems (breaks down H2O2 and O2-); these enzymes are located near the sites of generation of the oxidants and  Cell injury induced by free radicals, particularly ROS, is an important includes: mechanism of cell damage in many pathologic conditions, such as: o Catalase – present in peroxisomes; decomposes H2O2 o chemical and radiation injury, o Superoxide dismutase (SOD) – found in many cell types; convert O2- to o ischemia-reperfusion injury (induced by restoration of blood flow in H2O2 ischemic tissue)  Includes manganese SODs (mitochondria) o cellular aging and microbial killing phagocytes  Copper-zinc SODs (cytosol)  Free radicals – chemical species that have single unpaired electron in outer orbit  Glutathione peroxidise – protects against injury by catalyzing free radical o Unpaired electrons are highly reactive; “attack” and modify adjacent breakdown molecules (inorganic or organic chemicals – CHON, CHO, Lipids, nucleic o Intracellular ration of oxidized glutathione (GSSG) to reduced glutathione acids) (GSH) is a reflection of the oxidative state of the cell and is an important o Some reactions are catalytic  molecules that react with free radicals are indicator of the cell’s ability to detoxify ROS themselves converted to free radicals  Reactive oxygen species (ROS) – type of oxygen-derived free radicals o Produced normally inc ells during mitochondrial respiration and energy generation  degraded and removed by cellular defense system o Do not cause damage at low concentrations o ↑ production or ↓ scavenging of ROS  cause oxidative stress o Oxidative stress – cell injury, cancer, aging and degenerative diseases o ROS are also produced by leukocytes (neutrophils and macrophage) Generation of Free Radicals Free radicals may be generated within cells in several ways  The reduction-oxidation reactions that occur during normal metabolic processes Pathologic Effects of Free Radicals  Absorption of radiant energy  Rapid bursts of ROS are produced in activated leukocytes during inflammation Effects of ROS and other free radicals are wide-ranging. Three actions relevant to cell injury:  Transition metals (iron and copper) donate or accept free electrons during intracellular and catalyze free electrons during intracellular reactions and catalyze free radical  Lipid peroxidation in membranes formation (Fenton Reaction) o Presence of O2  peroxidation of lipids  Nitric Oxide – important chemical mediator generated by endothelial cells, o Oxidative damage is initiated when the double bonds in unsaturated fatty macropahges, neurons and other cell types acids of membrane lipids are attacked by O2 derived free radicals (.OH) o Act as free radical and can also be converted to highly reactive peroxynitrite o Lipid free radical interactions yield peroxides  unstable and reactive  anion as well as NO2 and NO3 autocatalytic chain reactions ensues (called propagation) extensive membrane damage Removal of Free Radicals  Oxidative modification of proteins o Free radicals promote oxidation of amino acid side chains, formation of Free radicals are unstable and decay spontaneously. Mechanisms to remove free radicals and covalent protein-protein cross –lins and oxidation of the protein backbones minimize injury: o Oxidative modification of proteins damages the active sites of enzymes, disrupt the conformation of structural proteins, and enhance the  Antioxidants – block or inactivate free radicals proteasomal degradation of unfolded or misfolded proteins o Ex: vitamin E and A; ascorbic acid; glutathione in cytosol  Lesions in DNA  Free iron and copper can catalyze formation of ROS o Free radicals are capable of causing single and double-strand breaks in o Reactivity of these metals is minimized by their binding to storage and DNA, cross-linking of DNA strands. And formation of adducts transport proteins (e.g., transferrin, lactoferrin, ferritin, ceruloplasmin)  o Oxidative DNA damage has been implicated in cell aging and in malignant prevents these metals from participating in reactions that generate ROS transformation of cells abmrmtmd DEFECTS IN MEMBRANE PERMEABILITY Reversible vs. Irreversible Injury  Early loss of selective membrane permeability, leading ultimately to overt  “point of no return” – damage becomes irreversible membrane damage, is a consisitent feature of most forms of cell injury  Two phenomena consistently characterized irreversibility (except apoptosis) o the inability to reverse mitochondrial dysfunction – lack of oxidative  Mechanisms and pathologic consequences of membrane damage: phosphorylation and ATP generation o profound disturbances in membrane function Mechanisms of Membrane Damage  Injury to lysosomal membranes results in the enzymatic dissolution of the injured cell that is characteristic of necrosis In ischemic cells, membrane defects may be result of : ↓ ATP and calcium-mediated activation of  Leakage of intracellular proteins through the damage cell membrane and ultimately phospholipases. Plasma membrane can also be damages by: bacterial and viral toxins, lytic into the circulation provides a means of detecting tissue-specific cellular injury and complement components, physical and chemical agents. necrosis using serum samples ROS - Cause injury to cell membranes by lipid peroxidation Mechanisms of Cell Injury ↓ Phospholipid - Consequence of defective mitochondrial function or hypoxia   ATP depletion: failure of energy-dependent functions  reversible injury  necrosis Synthesis both decrease the production of ATP  Mitochondrial damage: ATP depletion  failure of energy-dependent cellular - Affect cellular membranes and mitochondria functions  ultimately, necrosis; under some conditions, leakge of mitochondrial ↑ Phospholipid - Due to activation of Ca-dependent phospholipases by ↑ levels proteins that cause apoptosis Breakdown of cytosolic and mitochondrial Ca2+  Influx of calcium: activation of enzymes that damage cellular components and may - Phospholipid breakdown  lead to accumulation of lipid also trigger apoptosis breakdown products (FFA, acyl carnitine, lysosphospholipids   Accumulation of ROS: covalent modification of cellular proteins, lipids, nucleic acids detergent effect on membranes)  Increased permeability of cellular membranes: may affect plasma membrane, - May also insert in lipid bilayer  changes in permeability and lysosomal membranes, mitochondrial membranes; typically culminates in necrosis electrophysiologic alterations  Accumulation of damaged DNA and misfolded proteins: triggers apoptosis Cytoskeletal - Activation of protease  damage in cytoskeleton abnormalities CLINICOPATHOLOGIC CORRELATIONS: SELECTED EXAMPLES OF CELL INJURY AND Consequences of Membrane Damage NECROSIS The most important part of membrane damage during cell injury are: mitochondrial membrane, Common and clinically significant forms of cell injury that culminate in necrosis: plasma membrane, membranes of lysosomes. ISCHEMIC AND HYPOXIC INJURY Mitochondrial - Results in opening of MPTP  lead to ↓ ATP generation and membrane damage release of proteins that trigger apoptotic death  Ischemia is the most common type of cell injury in clinical medicine and it Plasma membrane - Results in loss of osmotic balance and influx of fluids and results from hypoxia induced by reduced blood flow, most commonly due to damage ions, as well as loss of cellular components a mechanical arterial obstruction (also venous drainage) - Cells may also leak metabolites vital for reconstitution of ATP furthering the depletion of energy stores  Anaerobic glycolysis continue and exhaust glycolytic substrates  stops anaerobic Injury to lysosomal - Results in leakage of enzymes into cytoplasma energy generation membranes - Activates the acid hydrolase in the acidic intracellular pH of  Glycolysis is inhibited by accumulation of metabolites that would be washed out by injured cell flowing blood - Lysosomes contain RNAses, DNAses, proteases,  Ischemia tends to cause more rapid and severe cell and tissue injury than does phosphatises, glucosidases  activation leads to digestion of hypoxia in the absence of ischemia proteins, RNA, DNA, glycogen and cells die by necrosis  Ischemia – deficient blood supply  Hypoxia – deficient oxygen supply DAMAGE TO DNA AND PROTEINS Mechanisms of Ischemic Cell Injury  Cells have mechanisms that repair damage to DNA, but if DNA damage is  ↓ oxygen tension in cells  ↓ oxidative phosphorylation; ↓ATP generation too seveer to be corrected (e.g., after exposure to DNA damaging drugs,  ↓ ATP  failure of Na-pump  K out; Na and water in  cell swelling radiation, or oxidative stress), the cell initiates a suicide program that  Influx of calcium  harmful effects  loss of glycogen and ↓ protein synthesis results in death by apoptosis  If oxygen is restored, all of these disturbances are reversible  If ischemia persists, irreversible injury and necrosis ensue abmrmtmd   Irreversible injury is associated with: Activation of - Contribute to ischemia-reperfusion injury o Severe swelling of mitochondria Complement - IgM  deposit in ischemic tissues and when blood flow is o Extensive damage to plasma membrane (give rise to myelin figures) System resumed, complement proteins bind to the deposited antibodies  activated and cause cell injury and inflammation o Swelling of lysosomes  Large, flocculent, amorphous densities develop in the mitochondrial matrix  indications of irreversible injury in the myocardium; seen 30-40mins after ischemia CHEMICAL (TOXIC) INJURY  Massive influx of calcium then occurs if ischemic zone is reperfused  Death is mainly by necrosis, but apoptosis also contributes  Chemical injury remains a frequent problem in clinical medicine and is a o Apoptotic pathway is activated by the release of pro-apoptotic molecules major limitation to drug therapy from leaky mitochondria  Liver – frequent target of drug toxicity  Cell’s components are degraded  widespread leakage of cellular enzymes in the o Toxic liver injury – most frequent reason for terminating drug use extracellular space and there is entry of extracellular molecules from the interstitial  Chemicals induce cell injury by one of two general mechanisms cells into the dying cells  Finally, dead cells become replaced by large masses composed of phospholipids in the Direct Toxicity form of myelin figures  Some chemicals can injure cells directly by combining with critical molecular o Phagocytosed by WBC or degraded further into fatty acids components o Calcification of fatty acids  calcium soaps  Ex: mercuric chloride poisoning  mercury binds with sulfhydryl groups  cause ↑  Leakage on intracellular enzymes  important clinical indicators of cell death membrane permeability and inhibition of ion transport  Hypoxia-inducible factor-1  promotes new blood vessels, stimulates cell survival o Greatest damage is usually to the cells that use, absorb, excrete, or concentrate the chemicals (GIT and kidney) pathways, and enhance anaerobic glycolysis  Cyanide – poisons mitochondrial cytochrome oxidase  inhibits oxidative phosphorylation ISCHEMIA-REPERFUSION INJURY  Antineoplastic chemo agents and antibiotics  induce cell damge by direct cytotoxic effects  Restoration of blood flow to ischemic tissues can promote recovery of cells Conversion to Toxic Metabolites if they are reversibly injured, but can also paradoxically exacerbate the  Most toxic chemicals are not biologically active in their native form injury and cause cell death o Converted to reactive toxic metabolites and act on target molecules  Ischemia-reperfusion injury – process where reperfused tissues sustain loss of o Modification is usually accomplished by cytochrome P-450 mixed-function cells in addition to the cells that are irreversibly damaged at the end of ischemia oxidases in the sER of liver and other organs  Toxic metabolites cause membrane damage and cell injury mainly by formation of free o Clinically important: contributes to tissue damage during myocardial and radicals and subsequent lipid peroxidation, also direct covalent binding cerebral infarction and following therapies to restore blood flow  Ex: CCL4  converted to highly reactive free radical CCL3 by cyt450 o Reperfusion injury occurs when new damaging processes happen during o Causes lipid peroxidation and damages many cellular structure reperfusion , causing death of cells that might have recovered otherwise.  Ex: Acetaminophen  analgesic; converted to a toxic product in the liver  cause cell  Several mechanisms: injury Oxidative stress - New damage may be initiated during reoxygenation by ↑ ROS - Free radicals may be produced in reperfused tissue as a result of KEY CONCEPTS: Ischemic and Toxic Injury incomplete reduction of oxygen by damaged mitochondria, or  Mild Ischemia: ↓ oxidative phoaphorylation  ↓ ATP generation  failure of Na pump because of action of oxidases from leukocytes, endothelial or  influx of Na and H2O  organelle and cellular swelling (reversible) parenchymal cells  Severe/prolonged ischemia: severe swelling of mitochondria, Ca influx into - Cellular antioxidant defense mechanism may be compromised by mitochondria and into cell w/ rupture of lysosomes and PM. Death by necrosis and ischemia  favors accumulation of free radicals apoptosis due to release of cytC from mitochondria Intracellular - Intracellular and mitochondrial calcium overload begins during  Reperfusions injury follows blood flow into ischemic area is due to oxidative sress by Ca2+ overload acute ischemia  exacerbated during perfusion due to influx of the release of free radicals from WBC and endothelial cells. Blood brings Ca that calcium resulting from cell membrane damage and ROS overloads reversibly injured cells with mitochondrial injury. Influx of WBC generated mediated injury to SR free radicals and cytokines. Local activation of complement by IgM deposited in - Ca2+ overload favors opening of the MPTP with resultant ischemic tissues depletion of ATP  furthers cell injury  Chemicals may cause injury directly or by conversion to toxic metabolites. Direct Inflammation - Result of: “dangers signals” released from dead cells, cytokines injury to critical organelles such as mitochondria or indirect injury from free radicals released by macrophages; increased expression of adhesion generated from chemicals/toxins involved molecules by hypoxic parenchymal and endothelial cells  recruit circulating to reperfused tissue - Inflammation causes additional tissue injury abmrmtmd APOPTOSIS  Accumulation of misfolded proteins o May arise because of mutations in genes or because of extrinsic factors  Apoptosis is a pathway of cell death that is induced by a tightly regulated o Excessive accumulation in ER leads to ER stress  ends in apoptotic cell suicide program in which cells destined to die activate intrinsic enzymes death that degrade the cell’s own nuclear DNA and nuclear and cytoplasmic o Apoptosis by accumulation of misfolded proteins  basis of several proteins degenerative diseases of the CNS and other organs  Apoptotic bodies – fragmented apoptotic cells  Cell death in certain infections (particularly viral infections) o Contain portions of the cytoplasma and nucleus o Adenovirus, HIV or viral hepatitis o Plasma membrane is intact but they become “tasty” targets for phagocytes o Important host response  cytotoxic T cells  induce apoptosis of infected o Rapidly devoured before contents have leaked out  cell death does not cells  eliminate reservoir of infection elicit inflammation o Cytotoxic T cells also responsible for cell death in tumors and cellular  Apoptosis – programmed cell death; characterized by: rejection of transplants o Loss of membrane integrity  Pathologic atrophy in parenchymal organs after duct obstruction o Enzymatic digestion of cells o Pancreas, parotid glands, kidney o Leakage of cellular components o Host reaction MORPHOLOGIC AND BIOCHEMICAL CHANGES IN APOPTOSIS CAUSE OF APOPTOSIS  Best seen in electron microscope Apoptosis – serves to remove unwanted, aged, or potentially harmful cells; also a Cell shrinkage - Cell is smaller in size; cytoplasm is dense; organelles pathologic event when diseased cells become damaged beyond repair are tightly packed Chromatin condensation - Most characteristic feature of apoptosis Apoptosis in Physiologic Situations - Chromatin aggregates peripherally,under the nuclear membrane, into dense masses of shapes and sizes - Nucleus fragmentation  Death by apoptosis is a normal phenomenon that serves to eliminate cells that are no Blebbing and apoptotic - Apoptotic cell first shows extensive surface blebbing, longer needed, and to maintain a steady number of various cell populations in tissues. bodies then undergo fragmentation into membrane-bound  It is important in following physiologic situations