1A_General pathology_cellular pathology and inflammation.docx

**THEME 1A: GENERAL PATHOLOGY: CELLULAR PATHOLOGY AND INFLAMMATION.............................................................** **INTRODUCTION** Insight into molecular- and cellular processes (cellular pathology) in tissues and organs is essential for the understanding of the pathological change...

**THEME 1A: GENERAL PATHOLOGY: CELLULAR PATHOLOGY AND INFLAMMATION.............................................................** **INTRODUCTION** Insight into molecular- and cellular processes (cellular pathology) in tissues and organs is essential for the understanding of the pathological changes underlying disease. Both intracellular and extracellular changes can affect cell morphology and may alter cell function (and may even bring about cell death), resulting in changes in tissue/organ morphology and function. Knowledge about these changes has contributed to the identification and the classification of diseases, and is essential for diagnosis. Unraveling the underlying cause and pathogenetic mechanisms (see Table 1, page 4 of this module book) of such changes provides the basis for appropriate therapy. Inflammation is a local physiological response to tissue injury. It is not a disease, but usually the manifestation of the presence of a disease. The purpose of an inflammatory reaction is to eliminate the noxious (e.g. infectious) agents and allow repair of injured tissue. In contrast, inflammation may also cause disease, for example a swelling caused by an inflammatory response in the throat may compress the airway and cause suffocation. Inflammation is classified according to its time course and cell composition in acute inflammation (an initial and transient reaction) and chronic inflammation (a prolonged reaction). Chronic inflammation may follow an acute inflammation event but generally occurs as a primary reaction (chronic inflammation \"ab initio\"). Although the causes of inflammatory reactions differ, the inflammatory reaction invariably involves a vascular reaction and a cellular reaction, resulting in influx of leucocytes from blood into the tissue. Commonly, diseases which pathogenetic mechanism is based on an inflammatory response are described by the suffix \"-itis\" (e.g. inflammation of the appendix is called appendicitis). By examination of the morphological changes in the affected tissues and organs, the origin of a number of fundamental cell injury mechanisms and their effects on cells and tissue will be studied. The consequences of these changes will be related to the patient's clinical symptoms and signs. **OBJECTIVES** By the end of this week you will be able to: - Recognize and explain the different adaptive responses of cells (1). - Recognize the morphological alterations (and explain the biochemical process) that occur after reversible and irreversible cell injury (1). - Explain the differences between necrosis and apoptosis (1). - Describe the macroscopic and microscopic characteristics of acute and chronic inflammation (1). - Name at least four different causes that initiate an inflammatory reaction (2). - Explain the biochemical process that occur after ischemia (3). - Explain reperfusion injury (3). - Describe the three possible outcomes of acute inflammation (3). - Describe the essential differences between a granulomatous and a chronic inflammatory reaction (3). - Knowledge of the different cell types that appear in various inflammation reactions (3). - Know the mediators of inflammation (3). - Describe the events that occur with healing after inflammatory reactions (3). - Pathogenetic classification of the events in cellular injury, inflammation and repair according to the scheme on page 4 (3). *The number(s) between brackets refer to the five course objectives as listed in the General Introduction (see* *page 5).* **LECTURE** \"Cellular pathology and Inflammation\". This lecture explains homeostasis and reversible and irreversible injury. Explains the different types of necrosis and the essential differences between apoptosis and necrosis. Deals with the basic forms of adaptation. Furthermore the differences between an acute and a chronic inflammatory reaction are explained. Explains the successive vascular and cellular processes that occur during an inflammatory reaction. and the most important chemical mediators of inflammatory reaction are presented and the lecture deals with the outcome of an acute inflammation and the characteristics of a granulomatous inflammatory reaction. It is partly a repetition of the course 'Immunology'. **ACTIVITIES** − Study the pages indicated in the reading list in advance. − Perform the SSA-test (Brightpace) before attending the work group. − Keep your course book (Robbins and Cotran Pathologic Basis of Disease) available the in the work group. − The workgroups are on campus with a tutor. **READING LIST AND STUDY GUIDELINES** Read the pages from Robbins and Cotran Pathologic Basis of Disease, Kumar, Abbas & Fausto, Elsevier Saunders, 10th edition 2020, that are indicated. - Chapter 2: pages 33-43. introduction to pathology...mechanisms of apoptosis. - Chapter 2: pages 49-66. Mechanisms of cell injury... cellular aging - Chapter 2: page 53. The orange marked text: Morphology of apoptosis - Chapter 2: pages 60-66. Intracellular accumulations... cellular aging - Chapter 3: pages 71-113. Overview of inflammation... phagocytosis and clearance **SSA: CELLULAR PATHOLOGY AND INFLAMMATION** *A. SSA-test* SH: 30 min. Answer the questions of the SSA-test (made available via Brightpace) after you have studied the indicated pages from Robbins and Cotran Pathologic Basis of Disease **WORK GROUP: CELLULAR PATHOLOGY AND INFLAMMATION AND MOLECULAR PATHOLOGY** *SH: 2 hrs* In this work group you will be provided with two case studies. You will prepare and discuss the provided questions and the case studies during the work group. Emphasis will be on recognizing structures and cells on microscopic slides. A laptop is required to obtain the work group assignments from Brightpace and the required histological images from a website. In addition, if necessary, you may wish to discuss any remaining questions about the SSA or the indicated pages from Robbins and Cotran Pathologic Basis of Disease, Chapter 2 and 3. **CELLULAR PATHOLOGY AND INFLAMMATION 26-03-20246 10.15-11.00...........................................................................** **OBJECTIVES OF CELLULAR PATHOLOGY AND INFLAMMATION** 1. Explain pathological processes by observation of morphological changes 2. Understand and apply the concepts of pathogenesis and etiology 3. Knowledge of the most important basis concepts and processes in cellular pathology, a.o. ischemia, atrophy and apoptosis 4. Recognize inflammation in a histological section (acute, chronic or granulomatous) 5. Know the most important hallmarks of these inflammations **HOMEOSTASIS** *The stages of cellular response to stress and injury* Afbeelding met tekst, schermopname, Lettertype, lijn Automatisch gegenereerde beschrijving **CELLULAR ADAPTATIONS** *This is an example of cellular adaptations. This is an example of hypertrophy* **CELLULAR ADAPTATIONS TO STRES** **Hypertrophy** -\> increase in cell- and organ size **Hyperplasia** -\> increase in cell number **Atrophy** -\> decrease in cell size and number **Metaplasia** -\> reversible change in cells Dell death (necrosis, apoptosis, autophagy) ***Dysplasia** -\>...* **CAUSE OF CELL INJURY** \> Physical agents (heat, cold, radiation, trauma) \> Oxygen deprivation (hypoxia, ischemia) \> anaerobic glycolysis \> Chemical agents and drugs \> Infectious agents \> Immunologic reactions \> Genetic derangements \> Nutritional imbalances **REVERSIBLE VERSUS IRREVERSIBLE INJURY** **Reversible:** Cellular swelling due to loss of fluid homeostasis - Hydrophic changes (liquid accumulation) - Fatty change (lipid vacuoles) **Irreversible:** Necrosis \> enzymatic digestion Apoptosis \> programmed cell death **ISCHEMIA** *When there is deprivation of oxygen, there won't be sufficient oxygen in the mitochondria for oxidative phosphorylation. So the oxidative phosphorylation decreases, as a result the ATP decreases. The cell will use anaerobic glycolysis (this yields less ATP). Anaerobic glycolysis has a biproduct of lactic acid, this lowers the pH value, and this affects proteins and chromatines. There will also be a change in ions and ion channel function. And also detachment of the ribosomes, which is important for protein synthesis.* **REPERFUSION INJURY \> RESTORATION OF THE BLOODFLOW** *Ischemic injury can be repaired, but this can result in reperfusion injury. You will get a high dose of oxygen and this results in reactive oxygen species. reactive oxygen species can cause cellular damage.* *So you will have both damage due through the reactive oxygen species, and also acidosis (cause by a low pH).* **CELL INJURY: MECHANISMS** *This is a summery of the types of cellular damages:* *Mitochondrial damage is cause by lack of oxygen, the ATP decreases and the ROS increases. The change in pH will cause increased entry of Ca2+, this increases the mitochondrial permeability and activation of multiple cellular enzymes. Membrane damage is irreversible. Protein misfolding is also cause by decrease in pH.*  **MITOCHONDRIAL DAMAGE** *Mitochondrial damage can result in necrosis, but it can also result in apoptosis. The apoptosis is cause by leakage of mitochondrial proteins.* Afbeelding met tekst, schermopname, ontwerp Automatisch gegenereerde beschrijving **GENERATION AND REMOVAL OF ROS** *The ROS can be removed by SOD enzymes.*  **TYPES OF NECROSIS** *You should be able to recognize this in the tissue, see figures in book.* \> **Coagulative necrosis** -\> The tissue retains it morphology, but the cells are dead. You don't see nuclei. More pink \> **Liquefactive necrosis** = colliquative -\> pus \> **Gangrenous necrosis** -\> looks like mummified tissue \> **Caseous necrosis** -\> the material formed by necrosis cannot be discarded by the body, it is encapsulated \> **Fat necrosis** -\> (fat destruction \> calcium soap formation, saponification) \> **Fibrinoid necrosis** -\> immune mediated, antigen-antibody complexes **INTRACELLULAR ACCUMULATIONS** *Cellular adaptations, there can be accumulation of substances. This can occur due to:* 1. ***Abnormal metabolisms*** 2. ***Proteins are not properly folded or*** 3. ***Lack of enzymes needed for homeostasis*** 4. ***Ingestion of indigestible materials*** **PATHOLOGIC CALCIFICATION** *Another cellular adaptation:* **Dystrophic calcification** -\> deposition of calcium at injured or necrotic location **Metastatic calcification** -\> Occurs in normal tissue, hypercalcemia cause calcium deposition (PTH ↑) **INFLAMMATION** *Hallmarks of inflammation:* 1. Heat (calor) 2. Redness/erythema (rubor) 3. Swelling (tumor) 4. Pain (dolor) 5. Loss of function (function laesa) **COMPONENTS OF INFLAMMATION** **Vascular -** Initial vasoconstriction, followed by a longer period of vasodilation **Cellular** - Stasis, with transmigration of leukocytes and erythrocytes into extravascular tissue **Mediators** - Plasma-derived, produced in liver (complement, factor XII) \- Cell-derived (histamine, cytokines) **HISTOLOGICAL CHANGES OVER TIME\ ***First you have edema, swelling due to the expansion of interstitial fluid volume in a tissue* Early in inflammation -\> there are primary Neutrophilic granulocytes Later on in inflammation -\> there are monocytes/macrophages  **MEDIATORS OF INFLAMMATION** *The mediators of inflammation organize the attraction of cells to the inflammation* - Generated from cells or plasma proteins - Are produced in response to various stimuli - Cascade mechanism to amplify effects - Vary in range of cellular targets - After activation, usually short-lived - Mediators can stimulate each other (e.g. complement \> histamine) **PRINICPAL MEDIATORS OF INFLAMMATIONS** Afbeelding met tekst, schermopname, Website, Webpagina Automatisch gegenereerde beschrijving **CYTOKINES INVOLVED IN INFALLAMTION** - Cytokines are produced by many different cell types esp. immune cells - \+ 35 different cytokines: different types for acute and chronic inflammation - TNF family and IL-1 \> leucocyte recruitment - Chemokines \> chemoattractants for leukocytes - IFNγ \> activation of macrophages (chronic) **OUTCOMES OF ACUTE INFLAMMATION** *When the acute inflammation is cleared and the tissue is repaired, there is resolution. When the tissue is 'repaired' with non-functional cells it is termed fibrosis. If the tissue is not repaired there can be chronic inflammation.*  **CHRONIC INFLAMMATION** Prolonged duration of inflammation - Infiltrate \* - tissue injury - tissue repair, connective tissue → Occurrence: - Persistent infections \> microorganisms - Hypersensitivity \> autoimmune disease, allergy - Prolonged toxic exposure \> atherosclerosis, liver cirrhosis Cells in chronic inflammation: - Macrophages - T- and B lymphocytes - Eosinophils \> parasites - Mast cells \> allergy **ACUTE VERSUS CHRONIC INFLAMMATION**  **TISSUE REPAIR** - Regeneration or scar formation (fibrosis) - Depends on the tissue, stem cells, growth factors - Liver tissue is especially able to regenerate Scar formation = Replacement w. connective tissue (fibrosis) Angiogenesis Granulation tissue formation Remodeling of connective tissue \>scar Mediated by TGF-β **GRANULOMATOUS INFLAMMATION: INTERACTION BETWEEN MACROPHAGES AND LYMPHOCYTES** *Granulomatous inflammation occur for example after tuberculosis. This is a type of inflammation which cannot be cleaned up by the body. You often see giant cells, with multiple nuclei, these are macrophages that collect other cells but are not able to get rid of it.* **CHAPTER 2: PAGES 33-43. INTRODUCTION TO PATHOLOGY...MECHANISMS OF APOPTOSIS..............................................** **INTRODUCTION TO PATHOLOGY**..................................................................................................................................................... The four **aspects of a disease process** that form the core of pathology are causation (etiology), biochemical and molecular mechanisms (pathogenesis), the associated structural (morphologic changes) and functional alterations in cells and organs, and the resulting clinical consequences (clinical manifestations). - **Etiology** is the initiating cause of a disease, this can be genetic or environmental. - **Pathogenesis** refers to the sequence of molecular, bio-chemical, and cellular events that lead to the development of disease. - **Morphologic changes** refer to the structural alteration in cells or tissues that are characteristic of a disease and hence diagnostic of an etiologic process. - **Clinical manifestations** **OVERVIEW OF CELLULAR RESPONSES TO STRESS AND NOXIOUS STIMULI**.............................................................................. **Adaptations** are reversible functional and structural responses to changes in physiologic state and some pathologic stimuli. The adaptive response may consists of: - **Hypertrophy**: an increase in the size and functional activity of cells. - **Hyperplasia**: an increase in cell number - **Atrophy**: a decrease in the size and metabolic activity of cells - **Metaplasia**: a change in the phenotype of cells If the limits of adaptive responses are exceeded or if cells are exposed to damaging insults, deprived of critical nutrients, or compromised by mutations that affect essential cellular functions, a sequence of events follow that is termed cell injury. Cell injury can be reversible or persistent/irreversible. **Processes that affect cells and tissues:** - Cellular adaptations - Intracellular accumulations - Pathologic calcification - Cell aging **Causes of cell injury** - **Oxygen deprivation** (if minor -\> adaptations, atrophy. But if it is more severe -\> cell injury and cell death) - **Physical agents** (mechanical trauma/extreme temp/sudden change atmosphericpressure/radiation/electric shock) - **Chemical agents and drugs** - **Infectious agents** - **Immunologic reactions** - **Genetic abnormalities** - **Nutritional imbalances** **The progression of cell injury and death** Afbeelding met tekst, schermopname, lijn, Lettertype Automatisch gegenereerde beschrijving **REVERSIBLE CELL INJURY**.................................................................................................................................................................. **Reversible cell injury** is characterized by functional and structural alterations in early stages or mild forms of injury, which are correctable if the damaging stimulus is removed. Two features in reversibly injured cells: - **Swelling of the cell**... and the organelles, blebbing of the plasma membrane, detachment of ribosomes from the endoplasmic reticulum, and clumping of nuclear chromatin. Swelling results from influx of water. This is usually caused by failure of the NA+K+ pumps due to ATP depletion resulting from oxygen deficiency. - **Fatty chance** occurs in organs involved in lipid metabolism. It result when toxic injury disrupts metabolic pathways and leads to rapid accumulation of lipid vacuoles. - Other alterations... **Irreversible cell injury** - **Inability to reverse mitochondrial dysfunction** (lack of oxidative phosphorylation and ATP generation) - **Profound disturbance in membrane function** **Morphology of cellular swelling:**  **CELL DEATH**....................................................................................................................................................................................... There are two principle types of cell death, necrosis and apoptosis, which differ in their mechanisms, morphology, and roles in physiology and disease. **Necrosis:** - Necrosis can be caused by loss of oxygen supply (ischemia), exposure to microbial toxins, burns and other forms of chemical and physical injury. - Necrosis is characterized by denaturation of cellular proteins, leakage of cellular contents through damaged membranes, local inflammation (macrophages for example recognize DAMPs), and enzymatic digestion of the lethally injured cell. - Intracellular proteins leak into the blood, circulating proteins serve as a biomarkers to asses and quantify necrotic damage. **Morphologic features are**: eosinophilia; nuclear shrinkage, fragmentation, and dissolution; breakdown of plasma membrane and organellar membranes; abundant myelin figures; and leakage and enzymatic digestion of cellular contents **Morphology of necrotic cells:** - **Increased eosinophilia in H&E stains** (attributable in part to the loss of cytoplasmic RNA and in part to accumulation of denatured cytoplasmic proteins which bind the red dye eosin). - **Glassy homogeneous appearance** (...relative to normal cells, mainly as a result of the loss of glycogen particles). - **Vacuolated cytoplasm** (when enzymes have digested the cell's organelles). - Dead cells may be replaced by large whorled phospholipid precipitates called myelin figures, which are either phagocytosed by other cells or further degraded into fatty acids; calcification of such fatty acid residues results in deposition of **calcium-rich precipitates**. Electron microscopy: - Discontinuities in plasma and organelle membranes - Dilation of mitochondria (with the appearance of large amorphous densities, intracytoplasmic myelin figures, amorphous debris, and aggregates of fluffy material representing denatured protein) - Nuclear changes: The basophilia of the chromatin may fade (karyolysis); nuclear shrinkage and increased basophilia (pyknosis, also in apoptosis); fragmentation of pyknotic nucleus (karyorrhexis)  Afbeelding met tekst Automatisch gegenereerde beschrijving **\ ** **Morphology of necrosis in tissue:** **Coagulative necrosis** -\> Dead tissue is preserved for some days. The affected tissue has a firm texture. Intensely eosinophilic cells with indistinct or reddish nuclei may persist for days or weeks. Ultimately, the necrotic cells are broken down by the action of lysosomal enzymes derived from infiltrating leukocytes, which also remove the debris of the dead cells by phagocytosis. A localized area of coagulative necrosis is called an infarct.  **Liquefactive necrosis** -\> in contrast to coagulative necrosis, is characterized by digestion of the dead cells, resulting in transformation of the tissue into a viscous liquid. It is seen in focal bacterial or, occasionally, fungal infections, because microbes stimulate the accumulation of leukocytes and the liberation of enzymes from these cells. The necrotic material is frequently creamy yellow because of the presence of leukocytes and is called pus. **Gangrenous necrosis** is not a specific pattern of cell death, but the term is commonly used in clinical practice. It is usually applied to a limb, generally the lower leg, that has lost its blood supply and has undergone necrosis (typically coagulative necrosis) involving multiple tissue planes. **Caseous necrosis** is encountered most often in foci of tuberculous infection. The term caseous (cheeselike) is derived from the friable white appearance of the area of necrosis. On microscopic examination, the necrotic area appears as a structureless collection of fragmented or lysed cells and amorphous granular debris enclosed within a distinctive inflammatory border; this appearance is characteristic of a focus of inflammation known as a granuloma. **Fat necrosis** -\> refers to focal areas of fat destruction, typically resulting from release of activated pancreatic lipases into the substance of the pancreas and the peritoneal cavity. On histologic examination, the necrotic areas contain the shadowy outlines of necrotic fat cells, basophilic calcium deposits, and an inflammatory reaction. **Fibrinoid necrosis** -\> a special form of vascular damage usually seen in immune reactions involving blood vessels. It typically occurs when complexes of antigens and antibodies are deposited in the walls of arteries. Deposits of these immune complexes, together with plasma proteins that has leaked out of vessels, result in a bright pink and amorphous appearance in H&E stains called "fibrinoid" (fibrin-like) by pathologists. **Morphology apoptosis** *In H&E-stained tissue, the apoptotic cell appears as a round or oval mass of intensely eosinophilic cytoplasm with fragments of dense nuclear chromatin. The absence of an inflammatory response can also make it difficult to detect apoptosis by light microscopy.* - **Cell shrinking**, Cell size is reduced, the cytoplasm is dense and eosinophilic, and the organelles, although relatively normal, are more tightly packed. This contrasts with necrosis, in which an early feature is cell swelling, not shrinkage - **Chromatin condensation**. This is the most characteristic feature of apoptosis. The chromatin aggregates peripherally, under the nuclear membrane, into dense masses of various shapes and sizes (see Fig. 2.12B). The nucleus itself may break up into two or more fragments. - **Formation of cytoplasmic blebs and apoptotic bodies**. The apoptotic cell first shows extensive surface membrane blebbing, which is followed by fragmentation of the dead cells into membrane-bound apoptotic bodies composed of cytoplasm and tightly packed organelles, with or without nuclear fragments. - Phagocytosis of apoptotic cells or cell bodies, usually by macrophages. The apoptotic bodies are rapidly ingested by phagocytes and degraded by the phagocyte's lysosomal enzymes. **CHAPTER 2: PAGES 49-66. MECHANISMS OF CELL INJURY... CELLULAR AGING......................................................................** **MECHANISMS OF CELL INJURY**......................................................................................................................................................... **Principles relevant to most forms of cell injury:** - The cellular response to injurious stimuli depends on the nature of the injury, its duration, and its severity. - The consequences of cell injury depend on the type, state, and adaptability of the injured cell. - Any injurious stimulus may simultaneously trigger multiple interconnected mechanisms that damage cells. **Cell injury results from abnormalities in one or more essential cellular components:** - **MITOCHONDRIAL DAMAGE** - **MEMBRANE DAMAGE** - **DAMAGE TO DNA** Afbeelding met tekst, schermopname, Lettertype, diagram Automatisch gegenereerde beschrijving **Mitochondrial damage** Mitochondria are critical player in all pathways leading to cell injury and death. - Three major consequences of mitochondrial damage: - ATP depletion - Reduced supply of oxygen and nutrients - Mitochondrial permeability transition pore - Effects of ATP depletion: - Reduced activity of NA+K+ pomp -\> swelling - Altered cellular energy metabolism - Reduction in protein synthesis - Necrosis - Incomplete oxidative phosphorylation results in the formation of ROS. - Leakage of mitochondrial proteins BAX and BAK and apoptosis. **Membrane damage** Early loss of selective membrane permeability, leading ultimately to overt membrane damage, is a consistent feature of most forms of cell injury (except apoptosis). - Mechanisms that contribute to membrane damage - ROS - Decreased phospholipid synthesis - Increased phospholipid breakdown - Cytoskeletal abnormalities - Damage to different cellular membranes has diverse effects on cells: - Mitochondrial membrane damage -\> permeability transition pore, decrease ATP and release apoptotic proteins - Plasma membrane damage -\> loss of osmotic balance and influx of fluids and ions, loss of cellular contents. - Injury to lysosomal membranes -\> leakage of enzymes that degrade RNA, DNA, proteins, phosphoproteins, and glycogen and push cell into necrosis **Damage to DNA** Damage to nuclear DNA activates sensors that trigger p53-dependent pathways. **OXIDATIVE STRESS: ACCUMULATION OF OXYGEN-DERIVED FREE RADICALS** Cell injury induced by free radicals, particularly ROS, is an important mechanism of cell damage in many pathologic conditions. Free radicals are chemical species that have a single unpaired electron in an outer orbit, they are highly reactive and attack and modify adjacent molecules. Afbeelding met tekst, schermopname, diagram, Lettertype Automatisch gegenereerde beschrijving **Generation of free radicals:** - The reduction-oxidation reactions that occur during **normal metabolic processes**. - **Absorption of radiant energy** (e.g. x-rays) - **Inflammation**, rapid bursts of ROS are produced in activated leukocytes during inflammation. - **Enzymatic metabolism of exogenous chemicals or drugs** - **Transition metals**, such as iron and copper donate or accept free electrons - **Nitric oxide** (NO), a chemical mediator generated by endothelial cells, macrophages, neurons and other cell types. **Removal of free radicals:** Free radicals are inherently unstable and generally decay spontaneously. In addition, cells have developed multiple non-enzymatic and enzymatic mechanisms to remove free radicals and thereby minimize injury: - **Antioxidants** -\> they block free radical formation or inactivate (scavengers) free radicals. - Transition metals (iron, copper) are normally bound to **storage and transport proteins** (e.g. transferrin) - **Enzymes** that act as radical-scavenging systems and break them down (e.g. catalase) **Pathologic effect of free radicals:** The effects of ROS and other free radicals are wide-ranging, but three reactions are particularly relevant to cell injury: - **Lipid peroxidation in membranes** - **Oxidative modification of proteins** - **Lesions in DNA** (ss-breaks, ds-breaks, cross-linking DNA strands, adduct formation)  **DISTURBANE IN CALCIUM HOMEOSTASIS** Calcium ions normally serve as second messengers in several signaling pathways, but if released into the cytoplasm of cells in excessive amounts, are also an important source of cell injury. - Ischemia and certain toxins cause excessive increase in cytosolic Ca2+ - Accumulation of Ca2+ in mitochondria results in opening of the mitochondrial permeability transition pores, and failure of ATP generation. - Increased cytosolic Ca2+ activated a number of enzymes (e.g. proteases, endonucleases, ATPases) **ENDOPLASMIC RETICULUM STRESS: THE UNFOLDED PROTEIN RESPOSNE** The accumulation of misfolded proteins in the ER can stress adaptive mechanisms and trigger apoptosis. - Metabolic alterations, genetic mutations in proteins or chaperons, viral infection, chemical insults, deprivation of glucose and oxygen can all increase misfolding of proteins. - Unfolded or misfolded proteins accumulate in the ER and trigger the 'unfolded protein response', this reduces the misfolded proteins. - When the cell is unable to cope with the accumulation of misfolded proteins, the cell activates caspases and induces apoptosis -\> ER stress. **KEY CONCEPTS** **Mechanisms of cell injury:** - ATP depletion: failure of energy-dependent functions → reversible injury → necrosis - Mitochondrial damage: ATP depletion → failure of energydependent cellular functions → ultimately, necrosis; under some conditions, leakage of mitochondrial proteins that cause apoptosis - Increased permeability of cellular membranes: may affect plasma membrane, lysosomal membranes, mitochondrial membranes; typically culminates in necrosis - Accumulation of damaged DNA and misfolded proteins: triggers apoptosis - Accumulation of ROS: covalent modification of cellular proteins, lipids, nucleic acids - Influx of calcium: activation of enzymes that damage cellular components and may also trigger apoptosis - Unfolded protein response and ER stress: Accumulation of misfolded proteins in the ER activates adaptive mechanisms that help the cell to survive, but if their repair capacity is exceeded they trigger apoptosis **\ ** **CLINICAOPATHOLOGIC CORRELATIONS: SELECTED EXAMPLES OF CELL INJURY AND DEATH..............................................** **HYPOXIA AND ISCHEMIA** *Ischemia, the most common cause of cell injury in clinical medicine, results from hypoxia induced by reduced blood flow, most often due to a mechanical arterial obstruction.* !! During hypoxia the blood flow is maintained and de production of anaerobic glycolysis continuous, but during ischemia the delivery of substrates for glycolysis is compromised. **Mechanisms of ischemic cell injury:** Oxidative phosphorylation fails -\> ATP decreases **Ischemia-reperfusion injury** Restoration of blood flow to ischemic tissues can promote recovery of cells if they are reversibly injured but can also paradoxically exacerbate cell injury and cause cell death, due to: \> **Oxidative stress**, reoxygenation increases reactive oxygen and nitrogen species (free radicals) \> **Intracellular calcium overload** \> **Inflammation**, causes additional tissue injury. \> Activation of **complement system** **Chemical (toxic) injury** Chemical injury remains a frequent problem in clinical medicine and is a major limitation to drug therapy. Chemicals induce cell injury by one of two general mechanisms: - **Direct toxicity** - **Conversion to toxic** **metabolites** **KEY CONCEPTS** Examples of cell injury - Mild ischemia: Reduced oxidative phosphorylation → ATP depletion → failure of Na pump → influx of sodium and water → organelle and cellular swelling (reversible) - Severe/prolonged ischemia: Severe swelling of mitochondria, calcium influx into mitochondria and into the cell with rupture of lysosomes and plasma membrane. Death by necrosis and apoptosis due to release of cytochrome c from mitochondria. - Reperfusion injury follows restoration of blood flow to ischemic tissues and is caused by oxidative stress due to release of free radicals from leukocytes and endothelial cells. Blood brings calcium that overloads reversibly injured cells with consequent mitochondrial injury, as well as oxygen and leukocytes, which generate free radicals and cytokines. Complement may be activated locally by IgM antibodies deposited in ischemic tissues. - Chemicals may cause injury directly or by conversion into toxic metabolites. The organs chiefly affected are those involved in absorption or excretion of chemicals or others such as liver, where the chemicals are converted to toxic metabolites. Direct injury to critical organelles such as mitochondria or indirect injury from free radicals generated from the chemicals/toxins is involved. **\ ** **ADAPTATIONS OF CELLULAR GROWTH AND DIFFERENTIATION**................................................................................................. Adaptations are reversible changes in the size, number, phenotype, metabolic activity, or function of cells in response to changes in their environment. **Hypertrophy** -\> Hypertrophy is an increase in the size of cells that results in an increase in the size of the affected organ. In many sites, hypertrophy and hyperplasia coexist. Hypertrophy is the result of increased cellular protein production. **Pathologic hypertrophy**, cells that have limited division increase is size in response to increased metabolic demands (such as muscle cells). **Physiological hypertrophy**, such as the cells in the uterus that increase in size in response to hormone signaling. **Hyperplasia** -\> hyperplasia is an increase in the number of cells in an organ or tissue in response to a stimulus. Hyperplasia can only take place if the tissue contains cells capable of dividing. Also, hyperplasia is a characteristic response to certain viral infections. **Physiological hyperplasia**, due to the action of hormones or growth factors occurs when there is a need to increase functional capacity of hormone sensitive organs, or when there is need for compensatory increase after damage or resection. **Pathological hyperplasia**, most are caused by excessive or inappropriate action of hormones or growth factors acting on target cells. pathological hyperplasia constitutes a fertile soil in which cancerous proliferation may eventually arise. **Atrophy** -\> **CHAPTER 2: PAGE 53. THE ORANGE MARKED TEXT: MORPHOLOGY OF APOPTOSIS.............................................................** / **CHAPTER 2: PAGES 60-66. INTRACELLULAR ACCUMULATIONS... CELLULAR AGING..............................................................** / zie hierboven **CHAPTER 3: PAGES 71-113. OVERVIEW OF INFLAMMATION... PHAGOCYTOSIS AND CLEARANCE....................................**