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Cell Injury, Adaptation and Death

Medical Biologist Faculty of Education Ishik University 100M Road Erbil-Iraq Tel.: 07504095454
Research Fellow Manchester Fungal Infection Group The University of Manchester Institute of Inflammation and Repair Manchester, UK M13 9NT Tel. 07927133678
GBD Expert Global Burden of Disease IHME Institute for Health Metrics and Evaluation University of Washington Seattle, WA 98121, USA
Dr. Karzan Mohammad PhD. MSc. BSc.

Overview of Cell Injury

Cells actively control the composition of their immediate environment and intracellular milieu Under physiological stresses or pathological stimuli (“injury”), cells can undergo adaptation to achieve a new steady state that would be compatible with their viability in the new environment. If the injury is too severe (“irreversible injury”), the affected cells die.

Causes of Cell Injury

Hypoxia and ischemia •“Chemical” agents “Physical” agents InfectionsImmunological reactions Genetic defects Nutritional defects A g i n g Hypoxia (oxygen deficiency) and ischemia (blood flow deficiency) “Chemical” agents: including drugs and alcohol “Physical” agents: including trauma and heat Infections Immunological reactions: including anaphylaxis and loss of immune tolerance that results in autoimmune disease Genetic defects: hemoglobin S in SS disease, inborn errors of metabolism Nutritional defects: including vitamin deficiencies, obesity leading to type II DM, fat leading to atherosclerosis Aging: including degeneration as a result of repeated trauma, and intrinsic cellular senescence

Mechanisms of Cell Injury: General Principles

Cell response to injury is not an all-or-nothing phenomenon Response to a given stimulus depends on the type, status, and genetic make-up of the injured cell Cells are complex interconnected systems, and single local injuries can result in multiple secondary and tertiary effects Cell function is lost far before biochemical and subsequently morphological manifestations of injury become detectable Cell response to injury is not an all-or-nothing phenomenon: The stronger and the longer the stimulus, the larger the damage Response to a given stimulus depends on the type, status, and genetic make-up of the injured cell: Contrast ischemia in skeletal muscle (tolerates 2 hours) versus cardiac muscle (tolerate 20 minutes); contrast neurons and glia. Cells are complex interconnected systems, and single local injuries can result in multiple secondary and tertiary effects: Cyanide indirectly affects osmotic regulation by loss of function ofNa/KATPase Cell function is lost far before biochemical and subsequently morphological manifestations of injury become detectable: This has big implications for the use of pathology as gold standard for evaluation of new technologies that could detect changes before they are morphologically apparent

General Biochemical Mechanisms

Loss of energy (ATP depletion, O2depletion)Mitochondrial damageLoss of calcium homeostasisDefects in plasma membrane permeabilityGeneration of reactive oxygen species (O2•, H2O2, OH•) and other free radicalsCytosolicfree calcium is 10,000 times lower than extracellularcalcium or sequestered intracellular calcium. Increase in cytosoliccalcium can result in activation of phospholipases(membrane damage), proteases, ATPases, and endonucleases. Increase in cytosoliccalcium, oxidative stress, and lipid breakdown products result in formation of high-conductance channels in inner mitochondrial membranes (“mitochondrial permeability transition”) that result in loss of proton gradient. CytochromeC can also leak into the cytosol, activating apoptotic pathways

Loss of energy (ATP depletion, O2depletion)

Loss of calcium homeostasis

Free Radicals
chemical with single unpaired electron in an outer orbital react with other molecules, resulting in chemical damage Free radicals initiate autocatalytic reactions;

Intracellular Sources of Free Radicals

Normal redoxreactions generate free radicalsNitric oxide (NO) can act as a free radical.Ionizing radiation (UV, X-rays) can hydrolyze water into hydroxyl (OH•) and hydrogen (H•) free radicalsFree radical generation is a “physiological” antimicrobial reaction

Neutralization of Free Radicals

Spontaneous decaySuperoxidedismutase(SOD):2O2•+ 2H →O2+ H2O2Glutathione (GSH):2OH•+ 2GSH →2H2O + GSSGCatalase:2H2O2→O2+ H2OEndogenous and exogenous antioxidants (Vitamins E, A, C and β-carotene)

If not adequately neutralized, free radicals can damage cells by three basic mechanisms: 1.Lipid peroxidation of membranes 2.DNA fragmentation 3.Protein cross-linking: Sulfur mediated.


“Reperfusion” Damage If cells are reversibly injured due to ischemia, restoration of blood flow can paradoxically result in accelerated injury.Reperfusion damage is a clinically important process that significantly contributes to myocardial and cerebral infarctionsExact mechanisms are unclear, but–Restoration of flow may expose compromised cells to high concentrations of calcium, and–Reperfusion can result in increase free radicals production from compromised mitochondria and the circulating inflammatory cells

Chemical Injury

Direct damage such as binding of mercuric chloride to sulfhydrylgroups of proteins•Generation of toxic metabolites such as conversion of CCl4 to CCl3•free radicals in the SER of the liver


Cellular Adaptation to Injury
- Via, receptor binding, signal transduction, gene transcription or protein synthesis•The most common morphologically apparent adaptive changes are–Atrophy(decrease in cell size)–Hypertrophy(increase in cell size)–Hyperplasia(increase in cell number)–Metaplasia(change in cell type)

Atrophy

Shrinkage in cell size by loss of cellular substance. Entire organ can become atrophic. Causes of atrophy include decreased workload, pressure, diminished blood supply or nutrition, loss of endocrine stimulation, and aging. Mechanisms of atrophy are not specific, but atrophic cells usually contain increased autophagic vacuoles

Brain atrophy

Hypertrophy
• Increase in cell size by gain of cellular substance.• Entire organ can become hypertrophic• Hypertrophy is caused either by increased functional demand or by specific endocrine stimulations.• Not only the size, but also the phenotype of individual cells can be altered in hypertrophy• With increasing demand, hypertrophy can reach a limit beyond which degenerative changes and organ failure can occur

Cardiac Hypertrophy

Hyperplasia
Hyperplasia constitutes an increase in the number of indigenous cells in an organ or tissue Pathological hyperplasia if typically the result of excessive endocrine stimulation Hyperplasia is often a predisposing condition to neoplasia

Metaplasia

Metaplasia is a “reversible” change in which one adult cell type is replaced by another adult cell typeMetaplasia is a cellular adaptation in which indigenous cells are replaced by cells that are better suited to tolerate a specific abnormal environmentBecause of metaplasia, normal protective mechanisms may be lostPersistence of signals that result in metaplasia often lead to neoplasia


Barrett’s EsophagusIntestinal Metaplasia of the Esophagus normal stratified squamous epithelium lining of the esophagus by simple columnar epithelium with goblet cells (which are usually found lower in the gastrointestinal tract).

Cell Death