4a. Cell Injury II_Hazen-Martin_NOTES.pdf

Cell Injury 2 Debra Hazen-Martin, PhD Office: 792-2906 Email: [email protected] Cell Injury 2: Cell Injury, Death, and Patterns of Tissue Necrosis Outline: I. Examples of Cell Injury with Defined Etiologies A. Chemical Injury B. Hypoxia C. Ischemia and Reperfusion II. Reversible vs. Irreversible...

Cell Injury 2 Debra Hazen-Martin, PhD Office: 792-2906 Email: [email protected] Cell Injury 2: Cell Injury, Death, and Patterns of Tissue Necrosis Outline: I. Examples of Cell Injury with Defined Etiologies A. Chemical Injury B. Hypoxia C. Ischemia and Reperfusion II. Reversible vs. Irreversible Injury A. Key morphological characteristics III. Cell Death A. Necrosis and Apoptosis B. Review Distinguishing Characteristics IV. Tissue Necrosis A. Biochemical Processes B. Types of Necrosis V. Pathologic Calcification A. Types B. Causes Suggested Reading: Robbins Basic Pathology by Kumar, Abbas, and Aster Chapter 2 Objectives: 1) Describe and contrast the features of a liver cell subjected to limited carbon tetrachloride exposure as compared to the same type of cell subjected to prolonged carbon tetrachloride exposure. 2) Define and differentiate between ischemia and hypoxia. 3) Describe possible causes of ischemic hypoxia as compared to hypoxia without ischemia. 4) Describe the morphologic changes you would expect in epithelial cells subjected to hypoxia. 5) Explain mechanisms by which reperfusion of an ischemic tissue with evidence of hypoxia might cause additional injury to the tissue. 6) Describe common subcellular changes that account for hydropic change (acute cell swelling) seen collectively at the light microscopic level. 7) Review the morphologic indicators of reversible vs. irreversible injury. 1 Cell Injury 2 8) Describe the 2 biochemical processes that occur in tissue necrosis (enzymatic digestion and protein denaturation) and identify which process prevails in different named forms of necrosis. 9) Describe the morphology and common tissue sites of coagulation, liquefactive, caseous, gangrenous and fat necrosis. 10) Describe the differences between dystrophic and metastatic calcification. I. Examples of Cell Injury with Defined Etiologies: The following examples serve to illustrate how the 3 basic biochemical mechanisms of cell injury (described in the last lecture) are incorporated into actual cells/tissues subjected to particular stresses. You will see that these 3 mechanisms seldom occur in isolation. A. Chemical Injury: Chemicals may injure cells by direct combination with an organelle or molecule. An example of this is taxol or vincristine’s (2 cancer chemotherapeutic agents) ability to directly bind to tubulin of microtubules to either stabilize or dissociate those structures preventing mitosis. Another example is the ability of CN to directly block cytochrome oxidase activity. The second type of chemical interaction involves conversion of the chemical to a reactive and sometimes more toxic metabolite by the cell’s own metabolic machinery. An example of this type of conversion is the interaction of carbon tetrachloride and the P450 system of oxidases in the SER of the liver as described below. 2 Cell Injury 2 Carbon tetrachloride in the liver: The P450 oxidases convert CCl4 to CCl3 which then attacks surrounding phospholipid molecules. The free radical form as a metabolite is more toxic than the original molecule. The common substance acetaminophen (Tylenol) is treated similarly. Eqtn Location Early events(3) Rapid breakdown of the ER proceeds with swelling of the SER and loss of ribosomes from the RER with concomitant loss of protein synthesis. Without production of apoprotein, triglycerides accumulate and the liver becomes “fatty”. The products of lipid peroxidation continue to damage membranes and, if the insult continues, selective permeability is lost and the cell swells. Plasma membrane and mitochondrial membrane damage result in calcium influx and depletion of ATP. Combined, these events often result in irreversible injury and cell death with continued exposure. 3 Cell Injury 2 Defs B. Hypoxia and Ischemia: Ischemia is the loss of blood supply due to impeded arterial flow or reduced venous drainage. Hypoxia is a lack of oxygen. Ischemia is a common cause of hypoxia, but not the only cause. Ischemia occurs when coronary arteries are blocked resulting in myocardial infarct. Blockage of cerebral arteries results in stroke. Other non-ischemic events including respiratory insufficiency, anemia, and NonCO poisoning may cause hypoxia also resulting in a decrease in oxidative phosphorylation. ischemic events(3) When hypoxia is caused by ischemia with vascular blockage there is also loss of glycolytic substrates normally carried by circulation of blood to the tissue. Loss of glycolytic substrates prevents anaerobic production of ATP as well. So the consequences and tissue damage are more severe. Events of injury: When blood supply is lost, tissue oxygen levels fall slowing the production of ATP by oxidative phosphorylation. Glycolytic pathways are up regulated, lowering intracellular pH due to accumulation of lactic acid. (This occurs only if the cells of the tissues affected contain stored glycogen.) The loss of ATP reduces the activity of both Na+K+- ATPase and Ca++,Mg++-ATPase. Increased concentrations of intracellular sodium cause movement of water into the cytoplasm and the cell swells. Similarly, increased calcium activates damaging enzymes. 4 Cell Injury 2 Fig Mit damage events(2) Mitochondrial changes and damage ensues with swelling of the matrix and disruption of cristae. The changes above are likely reversible. Necrosis vs apoptosis Calcium-induced proteases and continued loss of oxygen could cause further damage to the mitochondrial membrane. Opening of mitochondrial permeability transition pore may trigger loss of the proton gradient or release Cytochrome c which may trigger necrotic or apoptotic death of the cell, an irreversible event. 5 Cell Injury 2 Process(3) This electron micrograph illustrates the resulting damage to multiple cellular compartments. Loss of ATP-dependent ion pumps results in nonselective movement of ions and fluid into the cell. Cell swelling includes multiple membrane bound compartments. The picture is one of acute cellular swelling. At the light level this would be described as hydropic change. Calcium-induced proteases and phospholipases attack the proteins and lipids of all of the cell membranes. RER becomes distended and ribosomes are shed with loss of protein synthesis. Calcium-induced proteases and phospholipases attack the proteins and lipids of the plasmalemma and dissociate the cytoskeleton from the membrane, resulting in membrane blebs and loss of microvilli as seen in the micrograph below. Cell junctions and other cell-to-cell contacts are damaged. Endonucleases attack the DNA of the nucleus and chromatin clumps. If the nuclear damage does not progress beyond chromatin clumping, the cell may survive. 6 Cell Injury 2 C. Ischemia and Reperfusion Mec One might imagine that restoration of blood flow and normal oxygen levels to the previously ischemic tissue would allow reversal of injury and repair. However, the sudden input of oxygen and influx of white blood cells may promote additional free radical damage. The increase of oxygen in an area of injury promotes continued reactive oxygen species in damaged mitochondria. Furthermore, the WBC’s will also produce free radicals in the process of their normal phagocytotic activity. The mitochondrial events noted in this side lead to irreversible injury and cell death. 7 Cell Injury 2 II. Reversible vs. Irreversible Injury A. Key morphological characteristics Fig(4) Nuclear changes that progress beyond mild chromatin clumping will all lead to cell death. Cells undergoing necrotic death may exhibit a fading nucleus termed karyolysis. Others may have darkening chromatin indicating chromatin condensation progressing to pyknosis and karyorrhexis, the breaking up of the nucleus. In cells undergoing apoptotic death, these nuclear events are very distinctive. Endoplasmic reticulum changes - It is somewhat difficult to define clear boundaries between reversible and irreversible injury when evaluating other organelles. When RER is damaged and or swollen with influx of water, ribosomes do not remain attached. The loss of RER and apoprotein production in cells that metabolize lipids leads to fatty change which may be reversed. Loss of RER has been associated with irreversible injury due to an inability to repair. 8 Cell Injury 2 Lysosomes: The lysosomal system is very active in the injured cell and autophagosomes may be more apparent as the cell attempts to digest damaged organelles or misfolded proteins. Cellular stress may lead to activation of autophagy genes (Atg) resulting in formation of the autophagosome. Certain membrane components cannot be completely digested and remain as residual bodies. Current research focuses on tissue-specific patterns in Atg gene activation to determine whether this activation is protective, leading to cell repair or a precursor to cell death, triggering apoptosis. The unregulated release of hydrolytic lysosomal enzymes secondary to general membrane damage is clearly an event that leads to cell death as seen in the overview below. 9 Cell Injury 2 III. Cell Death A. Necrosis and Apoptosis It is very important that you have a clear understanding of the differences between cellular necrosis and apoptosis. Apoptosis occurs in regulated events and usually involves isolated cells. The process does not elicit an inflammatory response. Necrosis almost always results in inflammation. In your study of disease, you will see much more evidence of necrotic cell death. B. Review Distinguishing Characteristics of Apoptosis 10 Cell Injury 2 IV. Tissue Necrosis A. Biochemical Processes Tissue necrosis are morphological changes that follows irreversible injury and accompany cell death. Two biochemical processes occur in necrosis. 1) enzymatic digestion 2) denaturation of proteins If the source of the hydrolytic enzymes is from the dead cells themselves the process is called autolysis. If the enzymes originate from inflammatory cells that invade the necrotic focus, the process is termed heterolysis. B. Types of Necrosis Types If enzymatic digestion prevails in a necrotic site, it will produce a liquefactive necrosis. Protein denaturation alone (often due to a drop in pH) produces coagulation necrosis. Other types of necrosis (gangrenous, caseous, and fat necrosis) may have elements of both types of tissue degradation as well as other distinctive features. Coagulation necrosis: This is the most common form of necrosis and occurs in a number of solid organs. Hypoxic death of cells in all tissues except brain will result in a coagulation necrosis. In this form the structural outline of elements of the tissue is preserved for days due to the denaturation of proteins which inactivates enzymes and coagulates structural proteins. 11 Cell Injury 2 Histologically, the necrotic tissue contains eosinophilic ghosts of structures or cells that are devoid of nuclear detail. The necrotic area may have an inflammatory infiltrate. In this cardiac infarct, cardiac myofibers are glassy pink, and have no cross striations or other characteristic cellular detail. In this slide, an infarcted area of the kidney exhibits coagulation necrosis. While you can see the presence/shape of tubules on the right side of the histological slide, the tubule is an eosinophilic cast of the former structure. Nuclei are not visible within the cells. Note the nuclei in cells surrounding the tubules are those of the inflammatory cell infiltrate Liquefactive necrosis: This type of necrosis occurs in any tissue where there is a focus of bacterial or fungal infection (abscess). The focus will attract inflammatory cells that contribute enzymes for digestion (pus). This process obliterates the structure of tissue. Liquefactive necrosis is occurs in the central nervous system with infections and in hypoxia. You should be able to explain this based on information from the first lecture. 12 Cell Injury 2 Liquefactive necrosis in the kidney due to a focal fungal infection will form an abscess. Structural detail is lacking within the abscess. The structure at the site of the abscess will not be restored and a scar or cyst will remain. Many of you will see remnants of this in your cadavers in the gross lab. Cerebral infarct followed by hypoxic injury to the brain results in foci of liquefactive necrosis. When the debris at the site of the infarct is resolved (if the patient survives) the structure is not restored. Xter(3) Normally this type of necrosis involves the distal region of the limb (most often the lower limb). Loss of blood supply initiates a coagulative necrosis with a superimposed bacterial infection (wet gangrene) 13 Cell Injury 2 Xter(5) Caseous necrosis: This type of necrosis is a form of coagulation necrosis at the focus of a tuberculous infection. It is seen in the lung and any other tissue affected by tuberculosis. The necrotic area is white and cheesy and devoid of structure (unlike typical coagulative necrosis) but surrounded by a ring of granulomatous inflammation. Once resolved, this leaves cavities at the site. Fat necrosis: Enzymatic fat necrosis may be seen in focal areas of the peritoneal cavity and is caused by release of pancreatic enzymes into the peritoneum. The pancreatic lipases attack the fat associated with the mesenteries and omentum hydrolyzing triglyceride esters to release fatty acids. The fatty acids combine with calcium to form chalky white areas (fat saponification). Histologically these calcium soaps appear as pale basophilic amorphic foci. Traumatic fat necrosis is seen in the breast. Outcome of necrosis: The release of intracellular products from necrotic cells may be detected in circulating blood and serum. Increases of some proteins in serum may provide useful clinical markers for specific organ damage. Ex: cardiac muscle – creatine kinase , troponin or liver – alkaline phosphatase 14 Cell Injury 2 V. Pathologic Calcification: A. Types and Causes of Calcification Calcification is an abnormal deposition of calcium salts and other types of minerals in tissues. There are 2 types of calcification, dystrophic and metastatic. Dystrophic calcification occurs in areas of necrosis despite normal circulating levels of calcium. Metastatic calcification occurs in normal tissues. Abnormally high levels of circulating calcium (hypercalcemia) due to endocrine dysfunction or a tumor causing bone destruction are the cause of this sort of calcium deposition. Dystrophic calcification Crystal formation is initiated as calcium combines with phosphate to form a hydroxyapatitelike deposit in the dead cells or mitochondria of the necrotic tissues. Propagation of this process may lead to heterotopic bone formation. Dystrophic calcification can contribute to further organ dysfunction. In this slide, the diseased aortic valves of the heart are calcified leading to calcific aortic stenosis and loss of function. 15