Cellular Pathology

(VPM 151)

Lecture 2

(web)

Paul Hanna Jan 2018

CAUSES OF CELL INJURY

• common cause of cell injury and cell death

• hypoxia impairs oxidative respiration (energy production)

a) Deficient blood supply

• ischemia

= deficient blood supply from impeded arterial flow or reduced venous drainage

= hypoxia + ↓ delivery of nutrients + ↓ removal of metabolites

• cells may adapt to mild ischemia or die with severe ischemia

• infarction = localized area of ischemic necrosis

1) Hypoxia = Oxygen Deficiency

Human - Coronary artery with thrombus (top left) External surface of the heart, where a coronary artery has been cross-sectioned,

revealing a thrombus filling and completely occluding the lumen. Thrombi in coronary arteries are almost always due to endothelial damage

resulting from atherosclerosis. (top right ) Radiograph showing occlusion of a coronary artery and histiologic section showing almost

complete filling of the lumen of a coronary vessel with a thrombus.

Myocardial infarct, dog. Note pale area of necrosis of myocardium due to ishemia resulting from coronary artery thrombosis.

Tissue such as kidney and heart are prone to ischemic damage because

of limited collateral circulation and no dual blood supply (like lung and

liver). Note, ischemic necrosis in renal infarct (above and right)

b) Reduced oxygen-carrying capacity of the blood

• due to anemia = reduction in numbers / volume of rbc’s or quantity of Hb

• due to Hb dysfunction, eg nitrite poisoning → methemoglobinemia

CO poisoning → carboxyhemoglobinemia

1) Hypoxia = Oxygen Deficiency

2+

Nitrite converts hemoglobin (Fe2+ reduced) to

methemoglobin (Fe3+ oxidized), which does not bind or

transport oxygen, leading to hypoxia

Hemoglobin bonds to carbon

monoxide ~200 X stronger than

bonding to oxygen, so effectively,

carboxyhemoglobin will not

release the carbon monoxide, and

therefore hemoglobin will not be

available to transport oxygen

body.

McGraw-Hill

coreem.net

c) Interference with respiratory chain / oxidative phosphorylation

• eg, cyanide poisoning → blocks cytochrome oxidase / oxidative phosphorylation

1) Hypoxia = Oxygen Deficiency

www.towardsoneworld.eu/images/

O2

wikimedia.org

CAUSES OF CELL INJURY

• severity may be increased because of associated vascular injury

a) Direct mechanical trauma

b) Temperature extremes

c) Radiation

d) Electrocution

e) Sudden changes in atmospheric pressure

2) Physical Agents

Lacerations of hindlimb of sheep due to predation by dogs

Gangrene (ischemic necrosis) of distal limb due to frostbite. Note

other injurious stimuli can cause gangrene of extremities (eg

sepsis, ergot toxicity)

Widespread necrosis of the skin due to thermal burn

Tansy ragwort

(Jacobaea vulgaris)

Bracken fern

(Pteridium)

Yellow sweet clover

(Melilotus officinalis)

a) Inorganic poisons

b) Organic poisons

c) Manufactured chemicals

d) Physiologic compounds

e) Plant toxins

f) Animal toxins

g) Bacterial toxins / Mycotoxins

CAUSES OF CELL INJURY

3) Chemicals / Drugs / Toxins

a) Viruses / prions

b) Bacteria / rickettsia / chlamydia

c) Fungi

d) Protozoa

e) Metazoan parasites

CAUSES OF CELL INJURY

4) Infectious agents

CAUSES OF CELL INJURY

a) Immune / inflammatory response

• eg cells damaged as “innocent bystanders”

b) Hypersensitivity (allergic) reactions

• eg anaphylactic reaction

c) Autoimmune diseases

• reactions to self-antigens

5) Immunologic Reactions

CAUSES OF CELL INJURY

a) Cytogenetic disorders

• chromosomal aberrations

c) Multifactorial inheritance

• combined environmental factors and 2 or more mutated genes

b) Mendelian disorders (mutant genes)

• enzyme defects

• structural / transport protein defects

6) Genetic Abnormalities

Collagen dysplasia in a cat.

Note stretchable skin and

wounds from easily torn skin.

The collagen dysplasias have been best studied in humans, eg Ehlers-Danlos syndromes were at least 10 variants are recognized, based on

clinical, biochemical and molecular abnormalities. These many variants can be explained by the fact that the biosynthesis of collagen is a

complex process that can be disturbed by genetic errors that may affect any one of the numerous genes for the structural components of

collagen or enzymes necessary for post-transcriptional modifications of collagen.

CAUSES OF CELL INJURY

a) Deficiencies

• protein-calories (starvation)

• vitamins (A to E)

• minerals (eg copper, iron, selenium)

b) Overnutrition

• excess lipids / calories (obesity, diabetes, atherosclerosis, etc)

7) Nutritional Imbalances

Nutritional Myopathy (“white

muscle disease”)

• Note pallor of muscles (above) and on

closer examination white streaking

(left)

• note fragmentation, hyalinization (glassy acidophilic fibers with loss of striation)

and often basophilic discoloration (mineralization) of myofibers.

Nutritional Myopathy (“white muscle disease”)

a) Overworked cells

b) Underworked cells

CAUSES OF CELL INJURY

8) Workload Imbalances

• life time of damage → diminished capacity for homeostasis / adaptability

9) Aging

Malformation

Miscellaneous

Infectious

Immune

Nutritional

Neoplastic

Trauma

Toxicity

“Double MINT”

Frida Kahlo (1907 – 1954) was a Mexican painter, who has achieved great international popularity. She painted using vibrant

colors in a style that was influenced by indigenous cultures of Mexico as well as by European influences that include Realism,

Symbolism and Surrealism.

Cellular response to injurious stimuli is dependant on:

• type of injury

• duration of injury

• severity of injury

eg, low doses or brief durations reversible cell injury

high doses or longer intervals irreversible injury / cell death

b) Consequences of an injurious stimulus are dependent on:

• type of cell injured

• current status of the cell (nutritional, hormonal, metabolic, O2 requirement)

MECHANISMS OF CELL INJURY

General Considerations

SENSITIVITY CELL TYPE TIME (to irreversible cell injury)

HIGH Neurons ~ 3 to 5 min

INTERMEDIATE Cardiac myocyte ~ 30 min to 1 hrs

Hepatocyte

Renal epithelium

LOW Fibroblasts many hrs

Keratinocytes

Skeletal muscle

Tissue sensitivity to hypoxia

MECHANISMS OF CELL INJURY

General Biochemical Mechanisms

• certain injurious agents attack known specific molecular / biochemical sites:

Cyanide attacks cytochrome oxidase

Fluoroacetate blocks citric acid cycle

Enterotoxigenic E. coli elaborates toxin causing Cl secretion (+ Na/H2O)

Prions conversion of PrPc to PrPsc

MECHANISMS OF CELL INJURY

• other injurious agents can damage a variety of intracellular sites, some of which

are particularly vulnerable to injury:

Cell membranes

Mitochondria

Protein synthesis, folding & packaging

Genetic apparatus

MECHANISMS OF CELL INJURY

Figure 1-9 (Zachary) The process of acute cell swelling (hydropic degeneration).

www.au.dk/natrium-kalium-pumpen/

d) Interrelationship of structural and biochemical elements

eg, impairment of aerobic respiration disruption of energy dependant

membrane Na+/K+ pump ionic / osmotic imbalance cell swelling

1) ATP depletion

(Ca2+ pump)

Figure 2-17 (Robbin’s) Functional

and morphologic consequences of

decreased intracellular adenosine

triphosphate (ATP) during cell injury.

The morphologic changes shown here

are indicative of reversible cell injury.

Further depletion of ATP results in cell

death, typically by necrosis. ER,

Endoplasmic reticulum.

2) Free radical induced injury (Oxidative Stress)

• free radicals (single unpaired e- in outer orbit) are:

- extremely unstable

- readily react with organic or inorganic chemicals

- attack & degrade lipid membranes, proteins and nucleic acids

• free radical-induced injury is an important mechanism of cell damage in many

disease processes

• cell injury occurs when the free radical generation overwhelms antioxidant

defense mechanisms

Generation of Free Radicals

2) Free radical induced injury (Oxidative Stress)

Cellular Metabolism

• produced from cellular Redox Rx’s, eg mitochondria / peroxisomes leakage,

inflammation, altered oxidases (eg reperfusion), O2 therapy

Enzymatic metabolism of exogenous chemicals

• some intermediary metabolites of chemical / drugs are highly reactive free radicals

Ionizing Radiation

• hydrolyzes H2O into hydroxyl (·OH) and hydrogen (H·) free radicals

Divalent Metals

• transition metals (eg Cu & Fe), accept or donate free e-’s

• catalyze free radical formation

• ROS superoxide anions, hydroxyl radical and hydrogen peroxide

Important Reactants

2) Free radical induced injury (Oxidative Stress)

RNS = reactive nitrogen species

Peroxynitrite

• H2O2 generates ·OH radicals from reactions with Cu or Fe ions

Fe2+ + H2O2 ·OH + OH- + Fe3+

(Fenton reaction)

• Fe3+ often reduced by superoxide anions [Fe3+ + O2-. Fe2+]

ROS = ROS =

2) Free radical induced injury (Oxidative Stress)

Arachidonic acid is a polyunsaturated fatty acid present in the phospholipids of cell membranes

Lipid Peroxidation of Cell Membranes

• free radicals steal e- near double bonds in unsaturated fatty acids of membranes

lipid peroxides (unstable and reactive)

autocatalytic reaction (self-propagation)

rapid widespread membrane / organelle damage

Main Sites of Damage

2) Free radical induced injury (Oxidative Stress)

Fig 2-20 (Robbin’s) Excessive

production or inadequate

removal leads to accumulation

of free radicals in cells, which

may damage lipids (by

peroxidation), proteins, &

deoxyribonucleic acid (DNA),

resulting in cell injury.

Damage to Proteins

• free radicals cause fragmentation & cross-linkage between proteins

damage to structural proteins & enzymes degraded by proteosomes

Damage to DNA

• free radicals react with DNA strand breaks & DNA-protein adducts

• cell aging & neoplastic transformation

Main Sites of Damage

a) Storage and transport proteins

• Fe & Cu (which catalyze ROS) are minimized by being bound to storage and

transport proteins.

(eg ceruloplasmin, transferrin, lactoferrin, apoferritin / ferritin)

2) Free radical induced injury (Oxidative Stress)

b) Antioxidants (block formation of free radicals or inactivate/scavenge)

Vitamin A & E - lipid soluble antioxidants (in cell mbr’s)

Vitamin C - aqueous-phase antioxidant

Glutathione - in reduced form (GSH) reacts with H2O2 or ·OH to

form oxidized glutathione (GSSG) + H2O

- also neutralize lipid peroxides

Protective Mechanisms

Superoxide dismutase (SOD)

• in cytosol / mitochondria coverts superoxide anion to hydrogen peroxide

[2O2-. + 2H H2O2 + O2]

Catalase

• in peroxisomes, breaks down H2O2 [2H2O2 O2 + 2H2O]

2) Free radical induced injury (Oxidative Stress)

c) Enzymes which neutralize free radicals:

Glutathione peroxidase

• a selenium-containing enzyme which catalyzes GSH to GSSG

Protective Mechanisms

2) Free radical induced injury (Oxidative Stress)

3) Intracellular calcium and loss of calcium homeostasis

Fig. 1-15 (Zachary) Sources and consequences of

increased cytosolic calcium in cell injury. ER,

Endoplasmic reticulum; ATP, Adenosine triphosphate

4) Mitochondrial damage

• all cells are depend on

oxidative metabolism for long

term survival, regardless of

glycolytic ability

Fig 2–18 (Robbins) Role of mitochondria in

cell injury and death. Mitochondria are

affected by a variety of injurious stimuli and

their abnormalities lead to necrosis or

apoptosis. This pathway of apoptosis is

described in more detail later. ATP, adenosine

triphosphate; ROS, reactive oxygen species.

Severe injury Mild injury

5) Defects in membrane permeability

Figure 1-21 (Robbins) Mechanisms of membrane damage in cell injury. Decreased O2 and increased cytosolic Ca2+ are

typically seen in ischemia but may accompany other forms of cell injury. Reactive oxygen species, which can be produced on

reperfusion of ischemic tissues (and several other causes), also cause membrane damage.

OR direct damage by physical & chemical agents, bacterial toxins, viral proteins, complement / perforins