CELL INJURY, ADAPTATION, and DEATH

Normal cell is in a steady state Normal cell is in a steady state “Homeostasis”“Homeostasis”Change in Homeostasis due to stimuli - Change in Homeostasis due to stimuli - InjuryInjuryInjury - Reversible / IrreversibleInjury - Reversible / IrreversibleAdaptation / cell deathAdaptation / cell death

CELLULAR ADAPTATION TO STRESS

Adaptations are reversible changes in the number, size, phenotype, metabolic activity or functions of cells in response to changes in their environment

Physiologic adaptations are responses of cells to normal stimulation by hormones or endogenous chemical mediators

Pathologic adaptations are responses to stress that allow cells to modulate their structure and function and thus escape injury

Hypertrophyi

s an increase in the size of cells & consequently an increase in the size of an organ.

the enlargement is due to an increased synthesis of

structural proteins & organellesO

ccurs when cells are incapable of dividing Types:

a) physiologicb) pathologic

Causes:a) increased functional demandb) hormonal stimulation

Physiologic Hypertrophy of the Uterus During Pregnancy

Gravid Uterus Normal Uterus

Small spindle-shaped uterine Large, plump hypertrophiedsmooth muscle cells from a smooth muscle cells from anormal uterus gravid uterus

Heart hypertrophy in hypertension

Hyperplasia i

s an increase in the number of cells in an organ or tissuea

n adaptive response in cells capable of replicationa

critical response of connective tissue cells in wound healing Types:

a) physiologic hyperplasia1) hormonal

ex. Proliferation of glandular epithelium of the female breast at puberty & during pregnancy2) compensatory – hyperplasia that occurs when a portion

of a tissue is removed or diseased e.g. partial resection of a liver > mitotic activity 12 hours

laterb) pathologic hyperplasia

Caused by excessive hormonal or growth factor

stimulation

HYPERPLASIA OF THE PROSTATE GLAND

Atrophy

Shrinkage in the size of the cell by the loss of cell substance R

esults from decreased protein synthesis and increased protein degradation in cells

Is accompanied in many situations by increased autophagy with resulting Increases in autophagic vacoules

Causes:D

ecreased workloadL

oss of innervationD

iminished blood supplyI

nadequate nutritionL

oss of endocrine stimulationA

ging (senile atrophy)

Atrophy of the brain in an82-year-old man

Normal brain of a 25-year-oldman

ATROPHY IN OSTEOPOROSIS

Metaplasiaa

reversible change in which one adult cell type ( epithelial or mesenchymal) is replaced by another adult cell type.

is cellular adaptation whereby cells sensitive to a particular stress are replaced by other cell types better able to withstand the adverse environment

Epithelial metaplasia Examples

Squamos change that occurs in the respiratory epithelium in habitual cigarette smokers ( normal columnar epithelial cells of trachea & bronchi are replaced by stratified squamos epithelial cells

Vitamin A deficiency

Chronic gastric reflux, the normal stratified squamos epithelium of the lower esophagus may undergo metaplasia to gastric columnar epithelium

A.Schematic diagram of columnar to squamos epithelialB. Metaplastic transformation of esophageal epithelium

Mesenchymal metaplasia Ex. Bone formed in soft tissue particularly in foci of injury

METAPLASIA-ESOPHAGUSMETAPLASIA-ESOPHAGUS

METAPLASIA-LUNGSMETAPLASIA-LUNGS

CELLULAR INJURY

Cell Injury- pertains to the sequence of events when cells have no adaptive response or the limits of adaptive capability are exceeded

Types of Cell Injury1. Reversible Injury- injury that persists within certain limits, cells

return to a stable baseline

2. Irreversible Injury- when the stimulus causing the injury persists and is severe enough from the beginning that the affected cells diea. necrosis

b. apoptosis

Causes of Cell Injury1. Hypoxia

Causes:a. Ischemiab. Inadequate oxygenation of the bloodc. Reduction in the oxygen-carrying capacity of the blood

2. Chemical Agents a. glucose, salt or oxygen b. poisons c. environmental toxins d. social “stimuli” e. therapeutic drugs

3. Physical agents- trauma, extremes of temperature, radiation, electric shock, & sudden changes in atmospheric pressure

4. Infectious agents

5. Immunologic reactions Example: anaphylactic reaction to a foreign protein or a drug reaction to self antigens

6. Genetic defects Examples are genetic malformations associated with Down Syndrome, sickle cell anemia & inborn errors of metabolism

7. Nutritional Imbalances

All stresses & noxious influences exert their effects first at the molecular or biochemical level

Cellular function is lost far before cell death occurs and the morphologic changes of cell injury (or death) lag far behind both

Ultrastructural Changes of Reversible Cell injury

1. Alteration in plasma membrane reflecting disturbance in ion and volume regulation induced by loss of ATP

2. Mitochondrial changes

3. Endoplasmic reticulum changes

4. Nuclear alterations

MORPHOLOGY OF CELL AND TISSUE INJURY

•Cellular swelling

•Formation of cytoplasmic blebs

•Blunting and distortion of microvilli

•Deterioration and loosening of intercellular attachments

PLASMA MEMBRANE ALTERATIONS

Early it appears condensed as a result of loss of matrix protein following loss of ATPFollowed by swelling due to ionic shiftsAmorphous densities which correlate with the onset of irreversibilityFinally, rupture of membrane followed by progressing increased calcification

Mithochondrial Changes

•Dilation•Detachment of ribosomes and dissociation of polysomes with decreased protein synthesis•Progressive fragmentation and formation of intracellular aggregates of myelin figures

Endoplasmic Reticulum Changes

• Disaggregation of granular and fibrillar elements

Nuclear Alterations

Two Patterns of Morphologic Change Correlating toReversible Injury that can be recognized under the lightMicroscope: cellular swelling and fatty change

Cellular Swelling● Is the result of failure of energy-dependent ion pumps in the plasma membrane leading to an inability to maintain ionic & fluid homeostasis● first manifestation of almost all forms of injury to cells•microscopically small, clear vacoules may be seen within the cytoplasm•sometimes called hydropic change or vacoular degeneration•swelling of cells is reversible

Hydropic degeneration: kidneyCloudy swelling & hydropic change reflect failure of membrane ion pumps, due to lack of ATP, allowing cells to accumulate fluid

Fatty Change* occurs in hypoxic injury & various forms of toxic( alcohol & halogenated hydrocarbons like chloroform ) or

metabolic injury like diabetes mellitus & obesity•manifested by the appearance of lipid vacoules in the cytoplasm•principally encountered in cells participating in and

involved in fat metabolism e.g. hepatocytes & myocardial cells•also reversible

Morphologic Alterations in Reversible Cell Injury

Cell swelling

Fatty change

Plasma membrane blebbing and loss of microvilli

Mitochondrial swelling

Dilation of the ER

Eosinophilia (due to decreased cytoplasmic RNA)

NECROSISR

efers to a series of changes that accompany cell death, largely resulting from the degradative action of enzymes on lethally injured cells

The enzymes responsible for digestion of the cell are derived either from the: 1) Lysosomes of the dying cells themselves or from 2) lysosomes of leukocytes that are recruited as part of the inflammatory reaction to the dead cells

Morphologic alterations in Necrosis✔Increased eosinophilia (pink staining from eosin dye)✔Myelin figures ( whorled phospholipid masses)✔Nuclear changes assume one of three patterns all due to breakdown of DNA & chromatin:

1) Karyolysis2) Pyknosis characterized by nuclear shrinkage and

increased basophila 3) Karyorrhexis – fragmentation and dissolution✔Breakdown of plasma membrane and organellar Membranes✔Leakage and enzymatic digestion of cellular contents

Patterns of Tissue Necrosis

Coagulative Necrosis➢A form of tissue necrosis in which the component cells are dead but the basic tissue architecture is preserved for at least several days➢ It is characteristics of infarcts ( areas of ischemic necrosis) in all solid organs except the brain

A wedge-shaped kidneyInfarct (yellow) with preservation of the outlines

Liquefactive Necrosis ➢ Seen in focal bacterial or occassionally fungal infections because microbes stimulate the accumulation of Inflammatory cells and the enzymes of leukocytes digest ( “liquefy”) the tissue➢This necrosis is characteristic of hypoxic death of cells witnin the CNS➢Associated with suppurative inflammation (accumulation of pus)➢The areas undergoing necrosis are transformed into a Semi-solid consistency or state (liquid viscuous mass) Example: abcess

Liquefactive necrosis. An infarct in the brain, showing dissolution of tissue

Caseous Necrosis➢ Encountered most often infoci of tuberculous infection➢Characterized by a cheesy yellow-white appearance of the area of necrosis➢It is often enclosed within a distinctive inflammatory border

A tuberculous lung with a largearea of caseous necrosiscontaining yellow-white andcheesy debris

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➢Occurs in acute pancreatitis

Fat necrosis in aqcute pancreatitis. The areas of white chalky deposits represent foci of fat necrosis with calcium soapformation (saponification) at sites of lipid breakdown in the mesentery

Fibrinoid necrosis ➢A special form of necrosis usually seen in immune reactions involving blood vessels➢This pattern of necrosis is prominent when complexes of antigens and antibodies are deposited in the walls of Arteries➢Deposits of these immune complexes together with fibrin that has leaked out of vessels result in a bright pink and amorphous appearance called 'fibrinoid”

Fibrinoid necrosis in an arteryin a patient with PolyarteritisNodosa. The wall of the arteryshows a circumferential brightpink area of necrosis withprotein deposition andinflammation

Gangrenous Necrosis➢ This is not a distinctive pattern of cell death➢It is usually applied to a limb, generally the lower leg, that has lost its blood supply involving multiple tissue layers➢Types:✔Wet gangrene✗Occurs in naturally moist areas like mouth, bowels lungs✗Characterized by numerous bacteria ✔Dry gangrene✗begins at the distal part of the limb due to ischemia and often occurs in the toes and feet of elderly patients due to arteriosclerosis✗This is mainly due to arterial occlusion✗There is limited putrefaction and bacteria fail to survive

SUBCELLULAR RESPONSES TO INJURY

Autophagy

Refers to lysosomal digestion of the cell's own components

It is thought to be a survival mechanism in times of nutrient deprivation

Organelles are enclosed in vacoules that fuse with lysosomes

Heterophagy a cell usually a macrophage ingests substances from the outside for intracellular destruction

Hypertrophy of Smooth Endoplasmic Reticulum

Cells exposed to toxins that are metabolized in the SER show hypertrophy, a compensatory mechanism to maximize removal of the toxins

Mitochondrial Alterations* alterations in size, number, shape & functionEx. Mitochondria assume extremely large & abnormal shapes (megamitochondria) in hepatocytes in various nutritional deficiencies & alcoholic liver disease Cellular hypertrophy > # of mitochondria in cells Atrophy < # of mitochondria

Cytoskeletal Abnormalities some drugs & toxins interfere with the assembly & functions of Cytoskeleton filaments or result in abnormal

accumulations of filaments

General Principles Relevant To Most Forms Of Cell Injury• The cellular response to injurious stimuli depends on the type of injury, its duration, and its severity• The consequences of an injurious stimulus depend on the type , status , adaptability , and genetic makeup of the injured cell•Cell injury results from functional & biochemical abnormalities in• one or more of several essential cellular components

The most important target of injurious stimuli are: 1) cell membrane integrity, critical to cellular ionic and osmotic homeostasis 2) mitochondrial, the site of adenosine triphosphate (ATP) generation 3) protein synthesis 4) integrity of the genetic apparatus 5) cytoskeleton

MECHANISMS OF CELL INJURYMECHANISMS OF CELL INJURY

➢ ATP depletion: failure of energy-dependent functions ATP depletion: failure of energy-dependent functions reversible Injury necrosisreversible Injury necrosis➢Mitochondrial damage: ATP depletion failure of energy-Mitochondrial damage: ATP depletion failure of energy- dependent cellular functions ultimately necrosis;dependent cellular functions ultimately necrosis; under some conditions, leakage of proteins that causes apoptosisunder some conditions, leakage of proteins that causes apoptosis➢Influx of calcium: activation of enzymes that damage cellularInflux of calcium: activation of enzymes that damage cellular components and may also trigger apoptosiscomponents and may also trigger apoptosis➢Accumulation of reactive oxygen species: covalent modifications ofAccumulation of reactive oxygen species: covalent modifications of cellular proteins, lipids, nucleic acidscellular proteins, lipids, nucleic acids➢Increased permeability of cellular membranes: may affect plasmaIncreased permeability of cellular membranes: may affect plasma membrane, lysosomal membranes, mitochondrial membranes;membrane, lysosomal membranes, mitochondrial membranes; typically culminates in necrosistypically culminates in necrosis➢Accumulations of damaged DNA and misfolded proteins triggersAccumulations of damaged DNA and misfolded proteins triggers apoptosis apoptosis

Accumulation of Oxygen-Derived Free radicals (Oxidative Stress)

Free radicals are chemical species with single unpaired electron in an outer orbital. In such a state the radicals are extremely unstable & readily react with inorganic or organic chemicals.

Free radicals may be generated within cells by• Reduction-oxidation (redox) reactions• Nitric oxide (NO)• Absorption of radiant energy (e.g. ultraviolet light, x-rays)• Enzymatic metabolism of exogenous chemicals (e.g. carbon

tetrachloride)• Inflammation, because free radicals are produced by leukocytes

that enter tissues

Mechanisms that remove Free radicals●Action of superoxide dismutases (SODS)●Glutathione (GSH) peroxidase●Catalase present in perixisomes●Endogenous or exogenous antioxidants (e.g. vitamins E, A and C, and beta-Carotene may either block the formation of free radicals or scavenge them once they have formed●Iron and Copper can catalyze the formation of Reactive Oxygen Species (ROS)

APOPTOSIS (“FALLING OFF”)➢Is a pathway of cell death that is induced by a tightly regulated suicide program in which cells destined to die activate enzymes capable of degrading the cells own nuclear DNA and nuclear and cytoplasmic proteins➢It differs from necrosis in the following characteristics

1) Plasma membrane of the apoptotic cell remains intact

2) Has no leakage of cellular contents3) Does not elicit an inflammatory reaction in the host

➢Sometimes coexist with necrosis➢Apoptosis induced by some pathologic stimuli may progress to necrosis

Causes of Apoptosis

Apoptosis in Physiologic Situations Death by apoptosis is a normal phenomenon that

serves to eliminate cells that are no longer needed and to maintain a steady number of various cell populations in tissuesProgrammed destruction of cells during embryogenesis, Including implantation, organogenesis, developmental involution, and metamorphosis

Involution of hormone- dependent tissues upon hormone deprivation such as endometrial cell breakdown during the menstrual cycle and regeression of the lactating breast after Weaning

Cell loss in proliferating cell populations, such as intestinal Crypt epithelia

Death of cells that have served their useful purpose, such as neutrophils in an acute inflammatory response and Lymphocytes at the end of an immune response

Elimination of potentially harmful self-reactive lymphocytes Either before or after they have completed their maturation

Cell death induced by cytotoxic T lymphocytes, a defense mechanism against viruses and tumors that serves to kill eliminate virus-infected and neoplastic cells

Apoptosis in Pathologic SituationsApoptosis eliminates cells that are genetically altered or

Injured beyond repair without eliciting a severe host reaction,thus keeping the damage as contained as possible DNA damage

Radiation, cytotoxic anticancer drugs, extremes of temperature and even hypoxia can damage DNA either directly or via production of free radicals

Accumulation of misfolded proteins✔These may arise because of mutations in the genes encoding these proteins or because of extrinsic factors such as free radicals✔Excessive accumulation of these proteins in the ER leads to a condition called ER stress

Cell injury in certain infections particularly viral infections

Pathologic atrophy in parenchymal organs after duct obstruction such as in pancreas, parotid gland and kidney

Morphologic Alterations in Apoptosis●Nuclear chromatin condensation●Formation of apoptotic bodies ( fragments of nuclei and cytoplasm)

The fundamental event in apoptosis is the activation of enzymecalled caspases

Two Major Pathways in the Initiation of Apopotosis1) Mitochondrial ( intrinsic) pathway

Triggered by loss of survival signals, DNA damage and accumulation of misfolded proteins (ER stress)

2) Death receptor (extrinsic) pathwayResponsible for the elimination of self-reactive

lymphocytes and damage by cytotoxic T lymphocytes

INTRACELLULAR ACCUMULATIONS

THREE MAIN PATHWAYS OF ABNORMAL INTRACELLULARACCUMULATIONS●A normal substance is produced at abnormal or an increased rate, but metabolic rate is inadequate to remove it Example. Fatty change in the liver

●A normal or abnormal endogenous substance accumulates because of genetic or acquired defects in its folding, packaging, transport or secretion Example. Accumulation of of proteins in anti-trypsin deficiency●An abnormal exogenous substance is deposited and Accumulates because the cell has neither the enzymatic Machinery to degrade the substance nor the ability to transport It to other sites. Example. Accumulation of carbon or silica particles

Fatty Change (Steatosis)

✔Refers to any abnormal accumulation of triglycerides within parenchymal cells

✔Most often seen in the liver but may also occur in the heart, Skeletal muscle, kidney and other organs

✔May be caused by toxins, protein malnutrition, diabetes mellitus, obesity and anoxia

✔Alcohol abuse and diabetes associated with obesity are the most common causes of fatty liver

Fatty Liver

Cholesterol and Cholesteryl Esters

✔ Result of defective catabolism and excessive intake

✔Present in lipid vacoules of smooth muscle cells and macrophages in atherosclerosis (hardening of the aorta)

✔Give atherosclerotic plaques their characteristic yellow color and contibute to the pathogenesis of the lesion

✔ Xanthomas are hypercholesterolemic tumurous masses found in the connective tissue of the skin or tendons

Proteins ✔ Less common than lipid accumulations✔Occur because excess are presented to the cells or because the cells synthesize excessive amounts✔Examples: 1) Nephrotic syndrome there is heavy protein leakage across the glomerular filter due to a much larger reabsorption of albumin 2) accumulation of newly synthesized imunoglobulins in RER of some plasma cells forming rounded, eosinophilic Russell bodies 3) Mallory body or “ alcoholic hyalin” is an eosinophilic cytoplasmic inclusion in liver cells highly characteristic of alcoholic liver disease 4) Neurofibrillary tangle found in the brain in Alzheimer disease

Protein reabsorption droplets in the renal tubular epithelium

Glycogen ✔Accumulations of these are associated with abnormalities in the metabolism of either glucose or glycogen✔Ex. 1) In poorly controlled diabetes mellitus, glycogen accumulates in renal tubular epithelium, cardiac myocytes, and β cells of Islets of langerhans 2) Glycogen storage diseases or glycogeneses are Genetic disorders where glycogen accumulates in macrophages of patients with defects in lysosomal enzymes

Pigments➢ colored substances that are either exogenous or endogenous ● Exogenous – coming from outside the body

1) Carbon ( ex. Coal dust)➔Most common air pollutant➔Aggregates of the pigment blacken the draining lymph nodes and pulmonary parenchyma (Anthracosis)➔Heavy accumulations may induce emphysema or a fibroblastic reaction that can result in a serious lung disease called coal workers pneumoconiosis

● Endogenous – synthesized within the body itself 1) Lipofuscin or “wear-and -tear pigment or lipochrome✔an insoluble brownish-yellow granular intracellular material that accumulates in the heart, liver, & brain as a function of age or atrophy✔represents complexes of lipid & protein that derive from the free radical-catalyzed peroxidation of polyunsaturated lipids✔it is not injurious to the cell but is important as a marker of past free-radical injury✔the brown pigment when present in large amounts, imparts an appearance to the tissue that is called brown atrophy

The pale golden brown finely granular pigment seen here in nearly all hepatocytes is lipochrome (lipofuscin).

2) Melanin✔An endogenous, brown-black pigment synthesized exclusively by melanocytes when the enzyme tyrosinase catalyzes tyrosine to (DOPA) dihydroxyphenylalanine located in the epidermis✔Acts as a screen against harmful ultraviolet radiation✔Basal keratinocytes in the skin can accumulate the pigment (e.g. in freckles)

Melanin pigment in melanoma

3) Hemosiderin✔ A hemoglobin-derived granular pigment that is golden yellow to brown and accumulates in tissues when there is a local or systemic excess of iron

✔Iron is normally stored within cells in association with the protein apoferritin, forming ferritin micelles✔Iron can be identified by the Prussian blue reaction

The original red-blue color of hemoglobin is transformed to varying shades of green-blue by the local formation of biliverdin (green bile) and bilirubin (red bile) from the heme moiety✔The iron of hemoglobin accumulate as golden- yellow hemosiderin

✔Local excess of iron & consequently of hemosiderin result from hemorrhage Ex. Bruise

Hemosiderosis✔a condition where hemosiderin is deposited in many organs and tissues whenever there is systemic overload of iron✔It occurs in the following settings1)Increased absorption of dietary iron2)Impaired utilization of iron3)Hemolytic anemias4)Transfusions Hereditary Hemochromatosis✔A condition where there is extensive accumulations of iron with tissue injury like liver fibrosis, heart failure and diabetes mellitus✔Characterized principally by 1) the deposition of hemosiderin in the following organs (in decreasing order of severity):liver, pancreas, myocardium, pituitary, adrenal, thyroid, joints & skin 2) cirrhosis and 3) pancreatic fibrosis

PATHOLOGIC CALCIFICATION➢implies the abnormal deposition of calcium salts, together with small amounts of iron, magnesium, and other minerals

TYPESA. Dystrophic calcification✔deposition of calcium in dead or dying tissues✔occurs in the absence of calcium metabolic derangements ( with normal serum levels of calcium)✔Local deposits of calcium may occur in

1) necrotic tissue which is not absorbed●old infarcts●tuberculous foci●old collection of pus●dead parasites●acute pancreatic necrosis

2) Tissue undergoing slow degeneration●Hyaline areas in benign tumors●Fibroids●In arteries due to atheromatous degeneration or old age●Old thrombi●Diseased or abnormal heart valves

Pathogenesis➢Initiation ( or nucleation)➢Propagation

Both may be intracellular or extracellular with calcium phosphate as the end product

B. Metastatic Calcification✔ deposition of calcium salts in normal tissues✔almost always reflects some derangement in calcium metabolism ( hypercalcemia)

Four major causes of hypercalcemia1) Increased secretion of parathyroid hormone, due to either parathyroid tumors or production of parathyroid hormone- Related protein by other malignant tumors2) Destruction of bone due to the effects of accelerated turnover (e.g. Paget disease),immobilization, or tumors (increased bone catabolism associated with multiple myeloma, leukemia or diffuse skeletal metastases)3) Vitamin D-related disorders like vitamin D intoxication and sarcoidosis ( in which macrophages activate a vitamin D precursor4) Renal failure, in which phosphate retention leads to secondary hyperparathyroidism

CELLULAR AGING➢ results from combination of accumulating cellular damage (e.g., by free radicals), reduced capacity to divide (replicative senescence), and reduced ability to repair damaged DNA

Cellular senescence➢Aging of a person is intimately related to cellular aging

Mechanisms known or suspected to be responsible for cellular aging●DNA damage✔defective DNA repair mechanisms DNA repair may be activated by calorie restriction (known to prolong aging in model organisms)

●Replicative senescence✔Reduced capacity of cells to divide because of decreasing amounts of telomerase and progressive shortening of chromosomal ends (telomeres) Telomeres➔are short repeated sequences of DNA present at the linear ends of chromosomes➔Importance * for ensuring the complete replication of chromosome ends * and for protecting the ends from fusion & degradation ●Progressive accumulation of metabolic damage✔Repeated environmental exposure to radiation✔Progressive reduction of antioxidant defense mechanism Like Vit. E & glutathione peroxidase●Possible roles of growth factors

Telomerase in Ageing:

Germ cells

Somatic cells

WERNER SYNDROME➢A rare disease characterized by premature aging