Unit 1: Cell Degeneration and Death

We introduced the terms cell degeneration and necrosis, and discussed the four mechanisms underlying them: impaired energy production, impaired cell membrane function, metabolic derangements, and genetic abnormalities. You should now be able to define and describe the following terms: fatty degeneration fatty liver degeneration local hemosiderosis jaundice and its mechanisms microscopic evidence of necrosis – cloudy swelling, pyknosis, karyorrhexis, karyolysis gross evidence of necrosis – coagulative, caseous, liquefactive, and fat necrosis problems/clinical evidence of necrosis post-mortem changes apoptosis I’ve included a short self-assessment quiz, to help you assess how well you have learned the material in Part 1. Your final examination will consist of multiple-choice questions similar to these.

What are the causes of depleted impaired cell energy (ATP)?

• Hypoxia

• Hypoglycemia

• Enzyme inhibition

• Uncoupling of oxidative phosphorylation

What is hypoxia?

Insufficient oxygen in cells caused by:

• pulmonary emphysema

• pneumonia

• COPD

• anemia

→ deplete ATP

What is hypoglycemia?

Low blood glucose levels - glucose is the main substrate for energy production and the only source for neurons

→ deplete ATP

What is enzyme inhibition?

Anything that inhibits key enzymes in the respiratory chain

• e.g. Cyanide interfering with cytochrome oxidase

→ deplete ATP

What is uncoupling of oxidative phosphorylation?

Anything that alters how enzymes and chemical rxns are organized on the mitochondrial membrane

→ deplete ATP

What does defective ATP production effect first?

Effects are first seen in cells with high basal metabolic rate (oxygen demand)

• e.g. brain cells

What are the effects of depleted impaired cell energy (ATP)?

• Intracellular accumulation of water

• Swelling of cytoplasmic organelles

• Switch to anaerobic glycolysis

What is intracellular accumulation of water?

Low ATP → dysfunction of sodium pump → water and sodium enter the cell

What is swelling of cytoplasmic organelles?

Seen in the mitochondria which causes physical uncoupling of oxidative phosphorylation

What is the switch to anaerobic glycolysis?

Aerobic → anaerobic glycolysis → lactic acid production → ↓intracellular pH → damage to lysosomal membranes → releases of lysosomal enzymes into cytoplasm

What can cause an impaired cell mambrane?

• Free radicals (reactive particles that cause degradation of nucleic acids)

• Activation of the complement system (final compounds cause degradation of cell membranes)

• Membrane lysis (caused by enzymes, viruses, and physical/chemical agents)

What are the effects of impaired cell membrane function?

• loss of structural integrity

• loss of function (abnormal entry of water into cell = cloudy appearance)

• deposition of lipofuscin/damaged cell membranes are deposited into the cell membrane

What is lipofuscin?

They’re damaged cell membranes that are deposited in cell membranes

• Appearance: granular, golden brown pigment

• Has no effect on cell function

• Considered normal wear and tear

• Can also be seen in cases of starvation and chronic disease

What are intracellular accumulations that impact cell degeneration and necrosis?

• Fatty degeneration

• Iron deposition

• Bilirubin accumulation

What is fatty degeneration/hepatic lipidosis?

It’s triglyceride accumulation in cytoplasm of parenchymal cells

• Nonspecific response to many types of injury

• Normal cells: free fatty acids carried in portal blood from intestine → liver → processed then complex with apoproteins → secreted into plasma

→ Intracellular accumulations

What are the causes of fatty degeneration/hepatic lipidosis?

• Increased mobilization of adipose tissue

  • Starvation, diabetes mellitus

• Overactivity of certain enzymes increase conversion of FA into triglycerides

• Oxidation of triglycerides to other forms is decreased

  • Anemia, hypoxia

• Apoprotein synthesis is decreased

  • Protein malnutrition, specific hepatotoxins

→ Intracellular accumulations

What are the characteristics of fatty degeneration/hepatic lipidosis?

• Pale/beige colouring

• Enlarged

• Friable tissue

→ Intracellular accumulations

What is iron deposition?

Excess iron can accumulate in tissues as hemosiderin

• Visible as golden-brown granules within cytoplasm

  • Does not generally harm the cell

Local accumulation of iron

• Occurs when hemoglobin is broken down at sites of hemorrhage

  • Causes colour changes in bruises

• Iron deposited in macrophages or in connective tissues as hemosiderin

→ Intracellular accumulations

What is the functional significance

When deposited in parenchymal cells (functional cells of an organ), can cause cellular injury

• Ex. bilirubin accumulation in hepatocytes after cholestasis → toxic cellular injury → cell death

• Ex. bilirubin accumulation in brain cells → neuronal dysfunction (kernicterus) and cell death

→ Intracellular accumulations

What causes genetic abnormalities?

• Interference with mitosis in actively dividing cells

  • Ex. damage to RBC precursors can lead to anemia

• Failure of synthesis of structural proteins

• Failure of growth-regulating protein

  • Can lead to cancer

• Failure of enzyme synthesis

  • In embryo: Can lead to congenital diseases

  • In later life: Can lead to degenerative changes or cell necrosis

Necrosis - basics

• Cell death

• Can be caused by persistent cell degeneration or sufficient injury

• Cell may look normal until ~6 hours later

What are examples of gross (big), morphological evidence of necrosis?

1. Coagulation

2. Liquefaction

3. Caseation

4. Fat

5. Fibrinoid

6. Gangrenous (combo)

Coagulation/Coagulative necrosis

• Cause: ischemia (lack of blood supply)

• Appearance: 

  • Proteins are denatured (‘coagulated’)

    • Proteolysis is blocked

  • Microscopic: basic cell outline preserved, nuclei lost

  • Gross: pale, dry demarcated areas

  • Tissues become firm

• Examples of occurrence: 

  • Injection sites

  • Infarcts (necrosis due to lack of blood supply)

Liquefaction/Liquefaction necrosis

• Cause: Bacterial or fungal infections (due to leukocyte enzymes), Hypoxic cell death (within central nervous system) 

• Appearance: 

  • Cells completely digested → viscous liquid

  • Caused by acute inflammation → creamy yellow pus

• Examples of occurrence:

  • Abscess from bacterial infections

  • Cerebral infarcts

Caseation/Caseous necrosis

• Cause: Tuberculous infection, some fungal infections

• Appearance:

  • Gross: Cheeselike, friable, yellow-white appearance

  • Microscopic:

    • Fragmented cells

    • No distinguishable cellular outline

    • Amorphous granular pink appearance, surrounded by inflammatory border → granuloma

• Examples of occurrence:

  • Tuberculosis lung lesions

  • Granulomatous inflammation

Fat necrosis

• Cause: Release of pancreatic lipases, trauma to adipose tissue

• Enzymatic:

  • Causes release of pancreatic enzymes (mainly lipase) into adjacent fat tissue

  • Breaks down triglycerides in fat cells into glycerol and fatty acids → complex with plasma Ca+ ions to form Ca2+ soaps

  • Appearance:

    • Gross: chalky white areas (fat saponification = fatty acids + calcium)

    • Microscopic: shadowy outlines of necrotic fat cells with calcium deposits and inflammation

• Nonenzymatic:

  • Occurs in other fat deposits, usually following trauma

  • Appearance: 

    • Induces an inflammatory response which is typically granulomatous

• Examples of occurrence:

  • Acute pancreatitis

  • Traumatic fat necrosis in breast tissue

Fibrinoid necrosis

• Cause:

  • Immune-mediated vascular injury (immune complex deposition in vessel walls)

  • Severe hypertension damaging vessel walls

• Appearance:

  • Bright pink, amorphous, fibrin-like deposits on H&E

  • Seen in vessel walls with protein leakage

• Examples of occurrence:

  • Polyarteritis nodosa (immune vasculitis)

  • Malignant hypertension.

Gangerous necrosis (note: it’s a combination of other necrosis types)

• More of a clinical descriptor, not histologic

• Cause: severe ischemia of a limb

• Appearance:

  • Dry: coagulative, involving multiple layers

    • Black, mummified tissue

  • Wet: superimposed infection leads to liquefaction

    • Swollen, foul-smelling

• Examples of occurrence: peripheral vascular disease→ ischemic limb necrosis

Cytoplasmic evidence of necrosis

• Requires light or electron microscope

• Cytoplasm becomes more homogenous and deeply staining

  • Cause: denaturation of cytoplasmic proteins, loss of ribosomes

  • Stains more pink

• Cytoplasm may have a vacuolated (bubbly) appearance

  • Cause: impaired ATP production → failure of cell membrane Na+ pump → water and Na+ move into cell, mitochondria swell

• Dystrophic calcification

  • Abnormal deposition of calcium salts in dead/dying tissues 

    • Even with normal serum calcium levels

  • Can be associated with necrosis of any type

  • Cause: cells die → lysosomal membranes rupture, releasing enzymes → break down cellular components, freeing up phosphate groups → bind with calcium in extracellular fluid → form deposits

  • Ex. fatty plaques of atherosclerosis

Nuclear evidence of necrosis

• More definitive indicator of necrosis than cytoplasmic changes

• Nuclear chromatin clumps

• Nucleus shrinks and becomes more densely staining (darker blue)

  • Undergone pyknosis 

• Breaks up into fragments due to lysosomal enzymes

  • Process of karrhyorhexis or complete lysis (karrhyolysis)

What are the clinical problems associated with tissue necrosis?

1. Altered function

2. Loss of tissue

3. Secondary infection

4. Systemic effects

5. Local effects

6. Release of enzymes from necrotic cells

Explain altered function

• Can result if sufficient number of cells become necrotic

  • How much depends on the type of tissue affected

• Ex. myocardial infarction

  • Blood supply to heart muscle is occluded, necrosis of affected area occurs

  • Heart may fail to contract normally, electrical info may not flow normally

→ clinical problem associated with tissue necrosis

Explain loss of tissue

• Loss of affected tissue or organ

  • Sometimes referred to as gangrene 

• Ex. frostbite, burns

• Typically due to loss of blood supply

  • Loss of tissue tends to follow patterns of blood flow

  • This is why frostbite typically affects fingers, toes, nose, etc

    • Furthest ends of circulatory system, easiest to impair blood flow to

• Appearance:

  • Darkly coloured

  • Clearly demarcated from normal adjacent tissues

  • Bacterial infection: wet, swollen, foul-smelling, liquefied

    • May make lesion less demarcated from healthy tissue

  • Uninfected: dry, brittle, shriveled tissue

→ clinical problem associated with tissue necrosis

Explain secondary infection

• Necrotic tissue often has little to no inflammation

  • Blood flow is necessary for inflammatory cells to enter

  • Ideal for infections, especially if moist

    • Because immune system can't reach it

• Necrotic cells release chemical signals that can encourage inflammation around edges of dead/dying cells where blood supply is intact

→ clinical problem associated with tissue necrosis

Explain systemic effects

• Fever 

  • Due to release of pyrogens (fever-inducing agents) from necrotic cells and WBCs

• Increased WBC count

  • Due to inflammatory response

→ clinical problem associated with tissue necrosis

Explain local effects

• Depends on tissue affected and extent of necrosis

• Ex. gastric ulceration

  • Part of gastric mucosa has become necrotic

  • Sloughs off forming an ulcer

→ clinical problem associated with tissue necrosis

Explain release of enzymes from necrotic cells

• Cytoplasmic enzymes from necrotic cells released into blood

• Useful diagnostically

  • Ex. elevation of certain enzymes could mean necrosis of liver cells

→ clinical problem associated with tissue necrosis

What are the types of programmed cell death?

1. Apoptosis

2. Autophagy

3. Necroptosis

4. Pyroptosis

What are post mortem changes?

• Rigor mortis: stiffening of dead body

  • Due to reduction in ATP in muscles

• Post mortem lividity: settling of blood in lower parts

  • Breakdown of hemoglobin = green discolouration

• Post mortem blood clotting: formation of large clots like in chambers of the heart

• Putrefaction: process of decay or rotting

  • Due to fermentation caused by saprophytic bacteria

  • Gas accumulation → rupture of stomach or a foamy liver (full of gas bubbles)

• Autolysis

  • Gross: disintegration of tissues or organs as a whole

    • Occurs after death due to action of their own enzymes

  • Inflammatory cells generally not seen

Explain apoptosis

• Occurs as part of normal development 

• Physiological apoptosis:

  • During embryogenesis

  • Hormonally-driven regression of tissues

  • Tissues with normal rapid turnover (skin, intestines)

  • Elimination of potentially harmful self-reactive lymphocytes during maturation

• Pathologic apoptosis (cells are damaged beyond repair):

  • Damage to DNA

  • Accumulation of misfolded proteins

  • Viral infections

  • Atrophy of organs

• Cytotoxic T-cells: capable of inducing apoptosis in neighbouring cells

• Basic process

  • Signaling mediated by mix of internal and external signals

  • Enzymes (caspases) degrade cells nuclear DNA and nuclear + cytoplasmic proteins

  • Produces neatly packaged cell fragments

  • Cleared by macrophages → Recycles cell components

  • Little leakage of cellular contents

    • Does not elicit inflammatory reaction

• Diseases driven by apoptosis can exhibit altered function

  • Tend not to induce loss of tissue, predisposition to bacterial infection, local and systemic effects, and release of cellular enzymes

Explain autophagy

• Cause: starvation of cell

• Process where cell will eat and recycle cytoplasmic organelles that are not essential for cell survival to provide energy

  • Cytoplasmic lysosome fuses with autophagic vacuole containing organelle → digest contents, providing nutrient source

• → apoptosis if cell can't cope with energy depletion

• → Recovery is nutrients are restored

Explain necroptosis

• Receptor-interacting protein (RIP) kinases are activated

• Results in dissolution of cell, like necrosis

• Has features of both necrosis and apoptosis

Explain pyroptosis

• Associated with activation of inflammasome (cytosolic danger-sensing protein complex)

• Results in activation of caspases

  • Induce production of cytokines (induce inflammation)