Lecture 2 Cell Adaptation, Injury & Death

Dr. Nabila Hamdi

MD, PhD

ILOs

• Describe the major mechanisms whereby most injurious agents exert their effects

• Understand the examples of cell injury stated in this lecture, including their causes and mechanisms

• Describe cell changes that occur with atrophy, hypertrophy, hyperplasia and metaplasia, and state general conditions under which these changes occur

• State and discuss patterns of reversible/irreversible cell injury

• Differentiate cell death associated with apoptosis and necrosis

• Compare the pathogenesis of dystrophic and metastatic calcifications

• Understand autophagy

• Define intracellular accumulations and cite examples

• Understand the process of aging

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Outline I. Overview

II. Cellular Adaptation to Injury

III. Causes of Cell Injury

IV. Morphology of Cell & Tissue Injury 1. Reversible injury

2. Necrosis/Apoptosis

3. Patterns of Tissue Necrosis

V. Mechanisms of Cell Injury 1. General Biochemical Mechanisms

2. Ischemic & Hypoxic Injury

3. Ischemia/Reperfusion Injury

4. Free Radical-Induced Cell Injury

5. Chemical Injury

VI. Autophagy

VII. Intracellular Accumulations

VIII. Pathologic Calcifications

IV. Ageing

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Causes of Cell Injury

Cell Injury

Oxygen Deprivation Hypoxia: low O2 cc Ischemia: reduced blood supply

Chemical Agents poisons, air pollutants, insecticides, CO, ethanol, therapeutic drugs…

Infectious Agents

Immunologic Reactions autoimmune reactions, allergic reactions…

Genetic Defects Hb S Sickle cell anemia Enzyme metabolic diseases

Nutritional Imbalances Protein-calorie insufficiency Vitamin deficiencies Obesity, animal fat…

Physical Agents Trauma, radiation, electric shock, extremes of temperatures…

Aging Cellular senescence

Viruses, rickettsiae, bacteria, fungi, protozoans and worms

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Morphology of Cell & Tissue Injury

Reversible cell injury: • In early stages or mild forms of injury the functional and morphologic

changes are reversible if the damaging stimulus is removed.

• The injury has typically not progressed to severe membrane damage and nuclear dissolution.

Cell death: • With continuing damage, the injury becomes irreversible, at which time

the cell cannot recover and it dies.

• There are two types of cell death, necrosis and apoptosis, which differ in their mechanisms, morphology, and roles in disease and physiology.

• Necrosis is the major pathway of cell death in many commonly encountered injuries, such as those resulting from ischemia, exposure to toxins, various infections, and trauma.

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Cellular features of necrosis and apoptosis

Pyknosis/

Karyorrhexis/ karyolysis

Nucleus:

Phospholipid-rich densities

Detachment of ribosomes

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Apoptosis 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. Fragments of the apoptotic cells then break off, giving the appearance that is responsible for the name (apoptosis, "falling off")

Physiologic eliminates cells that are no longer needed • Destruction of cells during embryogenesis • Endometrial cell breakdown during the menstrual cycle • Regression of the lactating breast after weaning • Neutrophils in an acute inflammatory response • Lymphocytes at the end of an immune response • Elimination of potentially harmful self-reactive lymphocytes, to prevent autiommunity.

Pathologic eliminates cells that are genetically altered or injured beyond repair • DNA damage: Radiation, cytotoxic anticancer

drugs, hypoxia • Accumulation of misfolded proteins (gene

mutation or extrinsic factor) leading to “ER stress”

• Cell injury in certain infections (mainly, viral)

Morphology of Cell & Tissue Injury

Apoptosis

Inducing apoptosis of cancer cells is a desired effect of chemotherapeutic agents, many of which work by damaging DNA. 7

Pattern of Tissue Necrosis Coagulative Necrosis

• Coagulative necrosis is 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 characteristic of infarcts (areas of ischemic necrosis) in all solid organs except the brain.

A, A wedge-shaped kidney infarct (yellow) with preservation of the outlines. B, Microscopic view of the edge of the infarct, with normal kidney (N) and necrotic cells in the infarct (I). The necrotic cells show preserved outlines with loss

of nuclei, and an inflammatory infiltrate is present (difficult to discern at this magnification). 8

Pattern of Tissue Necrosis Liquefactive Necrosis

• Liquefactive necrosis is seen in focal bacterial or, occasionally, fungal infections.

• Occurs in hypoxic death of cells within the central nervous system (obscure reasons?)

• Liquefaction completely digests the dead cells, resulting in transformation of the tissue into a liquid viscous mass (inflammatory cells)

Liquefactive necrosis: An infarct in the brain, showing dissolution of the tissue. 9

Pattern of Tissue Necrosis Gangrenous Necrosis

It is usually applied to a limb, generally the lower leg, that has lost its blood supply and has

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

leukocytes (wet gangrene).

Amputated diabetic leg The affected skin is dark red to black and there is a large area of ulceration

http://studentbsmu.blogspot.com/2011/11/gangrenous-necrosis-lower-limb.html

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Pattern of Tissue Necrosis Caseous Necrosis

Caseous necrosis is encountered most often in foci of tuberculous infection. The term "caseous" (cheese-like) is derived from the friable yellow-white appearance of the area of necrosis.

A tuberculous lung with a large area of caseous necrosis containing yellow-white and cheesy debris 11

Pattern of Tissue Necrosis Fat Necrosis

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, typically in

acute pancreatitis

Fat necrosis in acute pancreatitis The areas of white chalky deposits represent foci of fat necrosis with calcium soap formation (saponification) at sites of lipid breakdown in the mesentery 12

Mechanisms of Cell Injury General Biochemical Mechanisms

The principal biochemical mechanisms and sites of damage in cell injury ATP, adenosine triphospate; ROS, reactive oxygen species

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Mechanisms of Cell Injury ATP Depletion

Depletion of ATP to less than 5% to 10% of normal levels has widespread effects

on many critical cellular systems

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Mechanisms of Cell Injury Mitochondrial Damage

Mitochondrial permeability transition pores

(high-conductance channels)

Failure of oxidative phosphorylation

Abnormal oxidative phosphorylation

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Mechanisms of Cell Injury Calcium Influx

ATP-dependent calcium

transporters (Ischemia/Toxins)

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Mechanisms of Cell Injury Oxidative Stress

A. In all cells, superoxide (O2•) is generated during mitochondrial respiration by the electron transport chain and may be converted to H2O2 and the hydroxyl (•OH) free radical or to peroxynitrite (ONOO−)

(Superoxide dismutase)

B. In leukocytes (mainly neutrophils and macrophages), the phagocyte oxidase enzyme in the phagosome membrane generates superoxide, which can be converted to other free radicals.

Pathways of production of reactive oxygen species (ROS) Oxygen-derived free radicals 17

Mechanisms of Cell Injury Oxidative Stress

The generation, removal, and role of reactive oxygen species (ROS) in cell injury

• 2 GSH (glutathione) + H2O2 → GS-SG + 2 H2O • Catalase: 2H2O2 → O2 + 2H2O

• Endogenous or exogenous antioxidants (vitamins E, A, and C and β-carotene) may either block the formation of free radicals or scavenge them once they have formed.

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Mechanisms of Cell Injury Defects in Membrane Permeability

The most important sites of membrane damage during cell injury are the mitochondrial membrane, the plasma membrane, and membranes of lysosomes. 19

Ischemic and Hypoxic Injury

Ischemia is the most common cause of cell injury in clinical medicine

Ischemia injures tissues faster than does hypoxia !

Why?

Anaerobic glycolysis (less efficient)

Anaerobic glycolysis No delivery of substrates

ATP depletion

Loss of function (60 sec) + Reversible changes

Coronary occlusion

If ischemia persists

Irreversible injury and necrosis

• Severe swelling of mitochondria • Damage to plasma membranes • ROS accumulation • Influx of calcium

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Ischemia/Reperfusion Injury

Oxygen paradox

1. ROS generation : • mitochondrial damage leads to incomplete reduction of oxygen • action of oxidases in leukocytes, endothelial cells, or parenchymal cells • antioxidant defense mechanisms are compromised by ischemia

2. Inflammation : • increased influx of leukocytes and plasma proteins • The products of activated leukocytes • Activation of the complement system

Under certain circumstances, the restoration of blood flow to ischemic but viable tissues (myocardial and cerebral ischemia) results, paradoxically, in the death of cells that are

not otherwise irreversibly injured.

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Chemical/Toxic Injury

Some chemicals act directly • Mercuric chloride poisoning • Antineoplastic chemotherapeutic agents

Other chemicals must be first converted to reactive toxic metabolites (P-450 in SER) • Carbon tetrachloride CCl4 CCl3

• Liver toxicity "fatty liver“ within 2 hours!

• Acetaminophen overdose Acute liver failure

Direct binding to critical molecular component or cellular organelle

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Autophagy (“self-eating”)

Lysosomal Catabolism

Lipofuscin

• Autophagy refers to lysosomal digestion of the cell’s own components.

• It is a survival mechanism in times of

nutrient deprivation.

• With time, starved cell eventually can no longer cope by devouring itself; at this stage, autophagy may also signal cell death by apoptosis.

• Autophagy is also involved in the clearance of misfolded proteins, for instance, in neurons and hepatocytes

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Intracellular Accumulations

• Fats (fatty liver)

• Cholesterol and cholesterol esters (foam cells in atherosclerosis)

• Glycogen (diabetes mellitus or glycogen storage

diseases)

• Pigments

- Exogenous: carbon (an example is coal dust)

- Endogenous: lipofuscin ("wear-and-tear pigment“)

Figure: Mechanisms of intracellular accumulation. (1) Abnormal metabolism, as in fatty change in the liver. (2) Mutations causing alterations in protein folding and transport, so that defective molecules accumulate intracellularly. (3) A deficiency of critical enzymes responsible for breaking down certain compounds, causing substrates to accumulate in lysosomes, as in lysosomal storage diseases. (4) An inability to degrade phagocytosed particles, as in carbon pigment accumulation.

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Intracellular Accumulations

Fatty liver (Steatosis)

A, The possible mechanisms leading to accumulation of triglycerides in fatty liver. Defects in any of the steps of uptake, catabolism, or secretion can lead to lipid accumulation. B, High-power detail of fatty change of the liver. In most cells the well-preserved nucleus is squeezed into the displaced rim of cytoplasm about the fat vacuole. (Courtesy of Dr. James Crawford, Department of Pathology, University of Florida School of Medicine, Gainesville, Florida.)

Alcohol

CCl4

Malnutrition

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Pathologic Calcification Dystrophic

Calcification of the aortic valve (important cause of aortic stenosis in the elderly)

A view looking down onto the unopened aortic valve in a heart with calcific aortic stenosis

Dystrophic calcification is the abnormal deposition of calcium salts in dead or dying tissues, in the absence of calcium metabolic derangements

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Pathologic Calcification Metastatic

Increased secretion of PTH (primary parathyroid tumors or production of PTH-related

protein by other malignant tumors)

Destruction of bone (accelerated turnover, immobilization, tumors)

Vitamin D-related disorders (vitamin D intoxication)

Renal failure (phosphate retention leads to secondary hyperparathyroidism)

Hypercalcemia

deposition in normal tissues

Metastatic calcification is the deposition of calcium salts in normal tissues and almost always reflects some derangement in calcium metabolism

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Aging

Telomeres are short repeated sequences of DNA present at the linear ends of chromosomes that are important for ensuring the complete replication of chromosome ends and for protecting the ends from fusion and degradation. When somatic cells replicate, a small section of the telomere is not duplicated, and telomeres become progressively shortened. The ends of chromosomes cannot be protected and are seen as broken DNA, which signals cell cycle arrest. The lengths of the telomeres are normally maintained by nucleotide addition mediated by an enzyme called telomerase.

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References

ROBBINS Basic Pathology 9th Edition ROBBINS Basic Pathology 8th Edition Basic Pathology 7th Edition, by Kumar, Cotran and Robbins Source of the cover cell image http://timothyjoseph.net/kill-or-be-killed/

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