CELL INJURY, ADAPTATION, CELL INJURY, ADAPTATION, AND DEATHAND DEATH

Abdulmohsen Alkushi, MD, MSc, FRCPCAssociate Professor and Consultant Pathologist

PathologyPathology

• It is the bridge between clinical practice and basic science.

• Study of: – underlying causes of diseases (etiology)– Mechanisms of diseases (pathogenesis)– Structural and functional changes in cells, tissue, and

organs (morphology, clinical features, and complications).

– Molecular bases of diseases.

• General and systemic pathology.

The Cell and the Environment

• Adaptive Responses– Atrophy

– Hypertrophy

– Hyperplasia

– Metaplasia

• Cell Injury– Reversible

– Irreversible

• Irreversible Cell Injury:– Necrosis

– Apoptosis

Adaptive Responses

Irreversible Cell Injury

Causes of Cell Injury

• Hypoxia• Physical Agents• Chemicals and Drugs• Microbiologic Agents• Immunologic Reactions• Genetic Defects• Nutritional Imbalances• Aging

General Principles of Cell Injury

• The cellular response to injurious stimuli depends on:• the type of injury, its duration, and its severity.

• The consequences of cell injury depend on: • the type, state, and adaptability of the injured cell.

• Cellular function is lost far before cell death occur, and morphologic changes of cell injury lag behind both.

Cellular Adaptation to Injury

• Atrophy

• Hypertrophy

• Hyperplasia

• Metaplasia

Atrophy

• Definition: – Shrinkage in the size of the cell by loss of cell

substance.– When a sufficient number of cells are involved,

the entire tissue or organ diminishes in size or becomes atrophic.

– i.e decrease the size of an organ due to decrease the size of its cells

• Atrophy can be physiologic or pathologic.

Physiologic Causes of Atrophy

• Physiologic atrophy is common during early development. Some embryonic structures, such as the notocord or thyroglossal duct, undergo atrophy during fetal development.

• The uterus decreases in size shortly after parturition.

Pathologic Causes of atrophy

• Decreased workload (disuse atrophy).• Loss of innervation (denervation atrophy).• Diminished blood supply (ischemia).• Inadequate nutrition e.g. profound protein-calorie

malnutrition (marasmus).• Loss of endocrine stimulation.• Aging (senile atrophy).• Pressure atrophy.

Mechanism of Atrophy

• Atrophy represents a reduction in the structural components of the cell. In muscle, the cells contain fewer mitochondria and myofilaments and a lesser amount of endoplasmic reticulum.

• The biochemical mechanisms responsible for atrophy are incompletely understood but are likely to affect the balance between protein synthesis and degradation.

Brain Atrophy

Brain Atrophy

Renal Atrophy

Hypertrophy

• Hypertrophy refers to an increase in the size of cells and, with such change, an increase in the size of the organ. Thus, the hypertrophied organ has no new cells, just larger cells.

• In simple: increase the size of an organ due to increase the size of its cells.

• Hypertrophy can be physiologic or pathologic.

Causes of Hypertrophy

• Physiologic hypertrophy:– The massive physiologic growth of the uterus

during pregnancy.– hypertrophy of the breasts during lactation.– Skeletal muscular enlargement in athletes.

• Pathologic hypertrophy:– Cardiac enlargement due to hypertension or

faulty heart valves

Hypertrophy

Normal

Gingival Hypertrophy

Hyperplasia

• Hyperplasia constitutes an increase in the number of cells in an organ or tissue, which may then have increased volume.

• In simple: increase the size of an organ due to increase the

number of its cells.

• Hyperplasia and hypertrophy are frequently both occur together.

• Hyperplasia can be physiologic or pathologic.

Physiologic Hyperplasia

• Hormonal hyperplasia, examples: – proliferation of the glandular epithelium of the

female breast at puberty and during pregnancy – physiologic hyperplasia that occurs in the

pregnant uterus

• Compensatory hyperplasia, example, – the hyperplasia that occurs when a portion of

the liver is removed (partial hepatectomy).

Pathologic Hyperplasia

• Endometrial hyperplasia.

• Skin warts.

• Prostatic hyperplasia

Benign Prostatic Hyperplasia

Endometrial Hyperplasia

Metaplasia

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

Epithelial Metaplasia

• The most common epithelial metaplasia is columnar to squamous epithelium:– as occurs in the respiratory tract in response to chronic

irritation, in smokers.– Stones in the excretory ducts of the salivary glands,

pancreas, or bile ducts may cause replacement of the normal secretory columnar epithelium by stratified squamous epithelium.

– A deficiency of vitamin A induces squamous metaplasia in the respiratory epithelium.

– Uterine cervical metaplasia during reproductive age.

Epithelial Metaplasia

• Metaplasia from squamous to columnar:– as in Barrett oesophagus, in which the squamous

esophageal epithelium is replaced by intestinal-like columnar cells.

• Epithelial metaplasia is a two-edged sword and, in most circumstances, represents an undesirable change.

• It may induce, if persistent, cancer transformation in metaplastic epithelium.

Metaplasia

Pathologic calcification

• Pathologic calcification means abnormal deposition of calcium salts in tissues.

Pathologic Calcification

• There are two forms of pathologic calcification: – Dystrophic calcification:

• deposition occurs in nonviable or dying tissues, and it occurs with normal serum levels of calcium and in the absence of derangements in calcium metabolism.

– Metastatic calcification: • deposition of calcium salts in vital tissues, and it

almost always associated with some disturbance in calcium metabolism, leading to hypercalcemia.

Intracellular Accumulations

• Intracellular accumulation of abnormal amounts of various substances, this may be harmless or may cause cell injury.

• substances fall into three categories: – (1) a normal cellular constituent accumulated in excess,

such as water, lipid, protein, and carbohydrates.– (2) an abnormal substance, either exogenous, such as a

mineral or products of infectious agents, or endogenous, such as a product of abnormal synthesis or metabolism.

– (3) a pigment such as melanin.

Fatty Changes (Steatosis)

• It means an abnormal accumulations of triglycerides within parenchymal cells.

• Most commonly seen in the liver, but it is also seen in other organs such as heart, muscle, and kidney.

• Causes of fatty changes in the liver include:– Alcohol, diabetes mellitus, obesity, toxins (such

as CCl4) , protein malnutrition, and anoxia.

Liver Fatty Change (Steatosis)

Intracellular Accumulations

• Exogenous Pigments– Tattoos

• dyes phagocytosed by macrophages

Mechanism of Ischemic and Hypoxic Injury

Ischemic and Hypoxic Injury

Reversible Injury

• Decreased oxidative phosphorylation• reduced ATP

– increased cytosolic free calcium

• reduced activity of “sodium pump”– accumulation of sodium by cell

– isosmotic gain of water (swelling)

– diffusion of potassium from cell

Ischemic and Hypoxic Injury

Reversible Injury

• Increased anaerobic glycolysis• glycogen depletion

• lactic acid accumulation

• accumulation of inorganic phosphates

• reduced intracellular pH

Ischemic and Hypoxic InjuryReversible Injury

• Increased Cytosolic Calcium– Sources

• Mitochondria, endoplasmic reticulum, external to the cell.– Consequences (activates enzymes)

• ATPase– decreased ATP

• phospholipase– decreased phospholipids

• endonuclease– nuclear chromatin damage

• protease– disruption of membrane and cytoskeletal proteins

Ischemic and Hypoxic Injury

Reversible Injury • Detachment of ribosomes

– reduced protein synthesis

• Worsening mitochondrial function• Increasing membrane permeability• Cytoskeleton dispersion

– loss of microvilli– formation of cell surface blebs– Swelling of mitochondria, endoplasmic reticulum, and

entire cells.

Ischemic and Hypoxic InjuryIrreversible Injury

• Mitochondrial changes– severe vacuolization– amorphous calcium-rich densities

• Extensive plasma membrane damage.• Prominent swelling of lysosomes.• Massive influx of calcium (on reperfusion).• Continued loss of cell proteins, coenzymes,

ribonucleic acids and other metabolites• Leakage of enzymes measured in serum

Ischemic and Hypoxic Injury

Irreversible Injury• Injury to lysosomal membranes

– leakage of degradative enzymes• activation of acid hydrolases due to reduced intracellular pH

with degradation of cell components

• Prominent leakage of cellular enzymes.• Influx of macromolecules from interstitium• “Myelin figures”-whorled phospholipid masses

Ischemic and Hypoxic Injury

Mechanisms of Irreversible Injury

• Phenomena characterizing irreversibility– inability to reverse mitochondrial dysfunction.– profound disturbances in membrane function is

a central factor.

Ischemic and Hypoxic Injury

• Potential causes of membrane damage– progressive loss of membrane phospholipids

• activation of phospholipase• reduced synthesis of phospholipids

– cytoskeletal abnormalities• activation of proteases • cell swelling

– toxic oxygen radicals.

• Ultimately a massive influx of calcium

Free Radical-Induced Cell Injury

• Definition Of Free Radicals– Extremely unstable, highly reactive chemical

species with a single unpaired electron in an outer orbital

• Examples Of Free Radicals– superoxide anion radical (O2

.-), hydrogen

peroxide (H2O2 ), and hydroxyl ions (OH. )

Sources of Free Radicals1. Absorption of radiant energy (e.g.,

ultraviolet light, x-rays).

2. Enzymatic metabolism of exogenous chemicals or drugs (e.g., carbon tetrachloride [CCl4].

3. The reduction-oxidation reactions that occur during normal metabolic processes.

Sources of Free Radicals

4. Transition metals such as iron and copper donate or accept free electrons during intracellular reactions and catalyze free radical formation.

5. Nitric oxide (NO), chemical mediator generated by endothelial cells, macrophages, neurons, and other cell types, can act as a free radical and can also be converted to highly reactive nitrite species.

Reversible and Irreversible Cell Injury

• Mechanisms and general pathways of cell injury.

• Morphology of reversible cell injury.

• Irreversible cell injury:– Necrosis– Apoptosis

Morphology of Cell Injury

Morphology of Reversible Cell Injury

• Cellular swelling (hydropic change, vacuolar degeneration)– Earliest change– Gross: organ pallor, increased weight– Microscopic: small, clear cytoplasmic vacuoles

Reversible Cell InjuryCellular Swelling

Irreversible Cell InjuryNecrosis

• Necrosis refers to a spectrum of morphologic changes that follow cell death in living tissue.

• The morphologic appearance of necrosis is the result of two concurrent processes:– Enzymatic digestion of cell.

• Autolysis and Heterolysis.

– Denaturation of proteins.

Morphology of Necrosis

• Microscopic changes:– Cytoplasmic features

• Cytoplasmic eosinophilia (more pink staining) and glassy homogenous cytoplasm.

– Nuclear chnages• karyolysis

• pyknosis

• karyorrhexis

Nuclear Changes

• Karyolysis:– Fading of the basophilia of the chromatin, due to lyses

of DNA by DNase activity.

• Pyknosis:– Nuclear shrinkage and increased nuclear basophilia.

The DNA condenses into a solid, shrunken basophilic mass.

• Karyorrhexis:– The pyknotic or partially pyknotic nucleus undergoes

fragmentation.

Necrosis

Necrosis

Nuclear Changes

Morphological Types of Necrosis

1. Coagulative necrosis – It implies preservation of the basic outline of the

necrotic cells for days. The affected tissues exhibit a firm texture.

– the injury or the subsequent increasing intracellular acidosis denatures not only structural proteins but also enzymic proteins and so blocks the proteolysis of the cell.

– Characteristic of hypoxic cell death except in the brain, myocardial and renal infarct are good examples

Coagulative Necrosis

Coagulative Necrosis

Morphological Types of Necrosis

2. Liquefactive necrosis:– This is a type of necrosis characterized by liquefactive

process resulting in complete digestion of the dead cells.

– It is characteristic of focal bacterial or fungal infections, because these agents constitute powerful stimuli to the accumulation of inflammatory cells that lead to complete digestion of dead cells.

– Hypoxic death of cells within the central nervous system often evokes liquefactive necrosis.

– Brian infraction and abscess are good examples of liquefactive necrosis.

Liquefactive Necrosis

Necrosis

Liquefactive NecrosisCoagulative Necrosis

Morphological Types of Necrosis

3. Gangrenous Necrosis:– This is not a distinctive pattern of cell death, the term

is still commonly used in surgical clinical practice.

– It is usually applied to a limb, generally the lower leg, that has lost its blood supply and has undergone coagulation necrosis.

– When bacterial infection is superimposed, coagulative necrosis is modified by the liquefactive action of the bacteria and the attracted leukocytes (wet gangrene).

Gangrenous Necrosis

Morphological Types of Necrosis

4. Caseous necrosis: – This a distinctive form of coagulative

necrosis, is seen most often in foci of tuberculous infection

– The term caseous is derived from the gross appearance (white and cheesy) of the area of necrosis.

– Unlike coagulative necrosis, the tissue architecture is completely obliterated

Caseous Necrosis

Morphological Types of Necrosis

5. Fat necrosis:– Focal area of fat destruction appears as white,

chalky areas grossly– shadowy outlines of necrotic fat cells with

basophilic calcium deposits– Example: after acute pancreatitis due to action

of pancreatic lipases

Fat Necrosis

Fate of Necrotic Tissue

• Ultimately, in the living patient, most necrotic cells and their debris disappear by a combined process of enzymic digestion and fragmentation, with phagocytosis of the particulate debris by leukocytes.

• If necrotic cells and cellular debris are not completely destroyed and reabsorbed, they attract calcium salts and other minerals and become calcified. This is called dystrophic calcification

Irreversible Cell InjuryApoptosis

• Defintion of Apoptossis:– Literally means “Falling away from”

– It is programmed and active cell death.

– It is a distinctive and important mode of cell death, Its development follows a cascades of events start at initiation phase and ends at execution phase and involves activations of several integrated genes, genes products, and intracellular enzymes called caspases.

• It can be physiologic and pathologic.

Physiologic Examples of Apoptosis

• The programmed destruction of cells during embryogenesis, including implantation, organogenesis, and developmental involution.

• Hormone-dependent involution in the adult, such as endometrial cell breakdown during the menstrual cycle, ovarian follicular atresia in the menopause, the regression of the lactating breast after weaning, and prostatic atrophy after castration.

• Cell deletion in proliferating cell populations, such as intestinal crypt epithelia.

Physiologic Examples of Apoptosis

• Death of neutrophils during an acute inflammatory response.

• Deletion of autoreactive T cells in the thymus.

• Death of immune cells, both B and T lymphocytes after cytokine depletion.

• Cell death induced by cytotoxic T cells

Physiologic Apoptosis

• In fact, failure of certain cells to undergo physiologic apoptosis may result in abnormal development, autoimmune diseases, and uncontrolled tumor proliferation.

Pathologic Examples of Apoptosis

• Cell death in tumors.

• Pathologic atrophy in organs after duct obstruction, such as occurs in the pancreas, parotid gland, and kidney.

• Cell injury in certain viral diseases, as in viral hepatitis.

• Cell death produced by a variety of injurious stimuli that are capable of producing necrosis, but when given in low doses. For example, heat, radiation, cytotoxic anticancer drugs, and hypoxia can induce apoptosis if the insult is mild, but large doses of the same stimuli result in necrotic cell death.

Morphology of Apoptosis

• The following morphologic features, some best seen with the electron microscope, characterize cells undergoing apoptosis:

1. Cell shrinkage: • The cell is smaller in size; the cytoplasm is dense; and the

organelles, although relatively normal, are more tightly packed.

2. Chromatin condensation: • This is the most characteristic feature of apoptosis. The

nucleus itself may break up, producing two or more fragments.

Morphology of Apoptosis

3. Formation of cytoplasmic blebs and apoptotic bodies:• The apoptotic cell first shows extensive surface

blebbing, then undergoes fragmentation into a number of membrane-bound apoptotic bodies composed of cytoplasm and tightly packed organelles, with or without a nuclear fragment.

4. Phagocytosis of apoptotic cells or bodies:• by adjacent healthy cells, either parenchymal cells

or macrophages.

Morphology of Irreversible Cell Injury

Apoptosis

H & E section

Electron Microscopy

Mechanism of Apoptosis• Apoptosis is the endpoint of an energy-dependent cascade

of molecular events, initiated by certain stimuli, and consisting of four separable but overlapping components: 1. Signaling pathways that initiate apoptosis.

2. Control and integration, in which intracellular positive and negative regulatory molecules inhibit, stimulate, or forestall apoptosis and thus determine the outcome.

3. A common-execution phase consisting of the actual death program and accomplished largely by the caspase family of proteases.

4. Removal of dead cells by phagocytosis

Mechanism of Apoptosis

Mechanism of Apoptosis

• Genes and gene products that control apoptosis:– Bcl-2 family of proteins are of two types:

• Pro-apoptotic proteins (promote apoptosis) by increasing mitochondrial membrane permeability and releasing an apoptotic trigger, cytochrome c, from mitochondria into the cytosol. e.g. Bax, and Bad.

• Anti-apoptotic proteins (inhibit apoptosis) by preventing increased mitochondrial membrane permeability e.g. Bcl-2, and Bcl-x.

– p53 promote apoptosis in cells that have DNA damage and failed to be repaired

Bcl-2 Family of Proteins

• Pro-apoptotic protiens:– Bax

– Bad

• Anti-apoptotic proteins:– Bcl-2

– Bcl-x

Example of Apoptosis:DNA Damage-Mediated Apoptosis

• Exposure of cells to radiation or chemotherapeutic agents induces apoptosis by a mechanism that is initiated by DNA damage and that involves the tumor-suppressor gene p53.

• p53 accumulates when DNA is damaged and arrests the cell cycle (at the G1 phase) to allow additional time for repair.

• If the repair process fails, p53 triggers apoptosis via Bax.

Dysregulated apoptosis (" too little or too much" )

• Two groups of disorders may result from such dysregulation:

1. Disorders associated with inhibited apoptosis and increased cell survival. Here, low rate of apoptosis may prolong survival of abnormal cells, these accumulated cells can give rise to.

a) Cancers

b) Autoimmune disorders.

Dysregulated apoptosis (" too little or too much" )

2. Disorders associated with increased apoptosis and excessive cell death. These diseases are characterized by a marked loss of normal or protective cells and include:

a) neurodegenerative diseases, such as in the spinal muscular atrophies

b) ischemic injury, such as in myocardial infarction and stroke

c) virus-induced lymphocyte depletion, such as occurs in acquired immune deficiency syndrome (AIDS).

Intrinsic Molecular Programs of Aging

– When somatic cells replicate, a small section of the telomere is not duplicated, and telomeres become progressively shortened .

– Shortened telomeres are proposed to signal a growth checkpoint allowing cells to become senescent.

– Conversely in immortal cancer cells, telomerase is reactivated, and telomeres are not shortened.

Intrinsic Molecular Programs of Aging

• Normal human fibroblasts have limited numbers of divisions.

• Cells from children undergo more number of divisions than cells from older people.

• In contrast, cells from patients with Werner's syndrome, a rare disease characterized by premature aging, have markedly reduced number of divisions.

• After a fixed number of divisions, all cells become arrested in a terminally nondividing state, known as cellular senescence.

Intrinsic Molecular Programs of Aging

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