Introduction This presentation will

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  • This presentation will……

  • be a guide to cell injury and cell death

  • outline causes and pathogenesis of cell injury/death

  • describe the morphological changes of cell injury/death

  • Describe the process of apoptosis


  • General considerations……

  • Adapt or die!

  • Reaction patterns in a given cell/tissue is often limited

  • Degree of injury is a function of type, duration and severity of insult

Types of insult - hypoxia

  • Ischaemia

    • Local e.g. embolus
    • Systemic e.g. cardiac failure
  • Hypoxaemia

    • Oxygen problems e.g. altitude
    • Haemoglobin problems e.g. anaemia
  • Oxidative phosphorylation

    • E.g. cyanide poisoning

Types of insult - chemical

  • Many of the common poisons (arsenic, cyanide, mercury) interfere with cellular metabolism. If ATP levels drop below critical levels, affected cells will die.

  • The list of pharmaceuticals that may have toxic effects on cells is enormous. Some act directly, but most have their effect through breakdown metabolites. Metabolism of alcohol (a type of drug) to acetaldehyde is one example.

Types of insult - infections

  • Fungi, Rickettsiae, Bacteria and Viruses

    • E.g. viruses can take over protein translation machinery and subvert it entirely to the production of new virions.

Types of insult - Physical

  • Direct Physical Effects

    • Exposure of tissue to extreme heat or cold results in direct injury that is often irreversible, resulting in a pattern of coagulative necrosis (see later).
    • Sudden changes in pressure can cause cellular disruption (e.g. a hammer blow to the thumb).
    • Electrical currents can cause direct breakdown of cellular membranes that may be irreversible.

Types of insult –immune

  • Inflammatory mediators such as interferons and interleukins

    • can alter both gene expression and cellular metabolism. The effects are designed to help cells combat an infectious process, but the resulting stress to the cells can be highly injurious and sometimes deadly.
  • Activation of complement

    • can result in direct attack on a cell's surface membrane.
  • Cytotoxic T-cells and NK cells

    • can mediate a direct attack on a target cell's and initiate the self-destruct cascade within a target cell.

Types of insult - nutrition

  • Dietary insufficiency

    • of protein, vitamins and/or minerals can lead to injury at the cellular level due to interference in normal metabolic pathways.
  • Dietary excess

    • can likewise lead to cellular and tissue alterations that are detrimental e.g. fat is the biggest offender, or excess ingestion of "health supplements"

Causes of cell injury - summary

  • Hypoxia

  • Chemical

  • Physical

  • Infection

  • Immune

  • Nutritional deficiency (or excess!)

Principle structural targets for cell damage

  • Cell membranes

    • Plasma membrane
    • Organelle membranes
  • DNA

  • Proteins

    • Structural
    • Enzymes
  • Mitochondria

    • oxidative phosphorylation

Pathogenesis of cell injury - hypoxia

  • Reversible

    • Loss of ATP
      • Failure of Na/K pump
    • Anaerobic metabolism
      • Increased lactic acid and phosphate
    • Reduced protein synthesis

Pathogenesis of cell injury - hypoxia

  • Irreversible

    • Massive intra-cytoplasmic calcium accumulation
    • Enzyme activation

Pathogenesis of cell injury - general

  • Reduced ATP synthesis/mitochondrial damage

  • Loss of calcium homeostasis

  • Disrupted membrane permeability

  • Free radicals

Free radicals

  • Highly reactive, unstable chemicals

  • Associated with cell injury

    • Chemicals/drugs, reperfusion injury, inflammation, irradiation, oxygen toxicity, carcinogenesis

Free radicals

  • Free radical generation occurs by….

    • Absorption of irradiation
      • E.g. OH•, and H•
    • Endogenous normal metabolic reactions
      • E.g. O2-•, and H2O2
    • Transition metals
      • E.g. Fe+++
    • nitrous oxide
      • an important paracrine-type mediator that helps regulate vascular pressure
    • Toxins
      • e.g. carbon tetrachloride

Free radicals

  • Free radicals are removed by….

    • Spontaneous decay
    • Anti-oxidants
      • E.g. Vitamin E, vitamin A, ascorbic acid, glutathione
    • Storage proteins
      • E.g. transferrin, ferritin, ceruloplasmin
    • Enzymes
      • Catalase, SOD, glutathione peroxidase

Free radicals

  • Injure cells by…..

    • Membrane lipid peroxidation
      • Autocatalytic chain reaction
    • Interaction with proteins
      • Protein fragmentation and protein-protein cross-linkage
    • DNA damage
      • Single strand breaks (genomic and mitochondrial)

General protective mechanisms

  • Heat shock response genes

    • comprise a large group of genes
    • expression is up-regulated in the face of cell stressors and
    • serve to protect proteins from stress-related damage
    • "clean up" damaged proteins from the cell.
  • Many tissues and organs can survive significant injury if they are "pre-stressed"

    • Ways to exploit this phenomenon to improve organ transplantation and tissue repairs are being tested in clinical trials.

Cell injury - morphology

  • Reversible

  • Irreversible

Cell injury - morphology

  • Light microscopy

    • Cytoplasmic changes
    • Nuclear changes

Cell injury - morphology

  • Abnormal accumulations

    • Lipid
    • Protein


  • Definition

    • Death of groups of contiguous cells in tissue or organ
  • Patterns

    • Coagulative
    • Liquefactive
    • Caseous
    • Fat necrosis
    • (gangrene)
    • (Infarct)
      • Red/haemorrhagic
      • White

Coagulative necrosis

  • Cells have died but the basic shape and architecture of the tissue endures

  • Most common manifestation of ischaemic necrosis in tissues.

  • Affected tissue maintains solid consistency.

  • In most cases the necrotic cells are ultimately removed by inflammatory cells.

  • The dead cells may be replaced by regeneration from neighboring cells, or by scar (fibrosis).

Coagulative necrosis

Coagulative necrosis

Liquefactive necrosis

  • Complete dissolution of necrotic tissue.

  • Most commonly due to massive infiltration by neutrophils (abscess formation).

    • Release of reactive oxygen species and proteases
  • Liquefaction is also characteristic of ischaemic necrosis in the brain.

Liquefactive necrosis

Caseous necrosis

  • Accumulation of amorphous (no structure) debris within an area of necrosis.

  • Tissue architecture is abolished and viable cells are no longer recognizable.

  • Characteristically associated with the granulomatous inflammation of tuberculosis. Also seen in some fungal infections.

Caseous necrosis

Caseous necrosis

Fat necrosis

  • Results from the action of lipases released into adipose tissue.

    • pancreatitis, trauma.
  • Free fatty acids accumulate and precipitate as calcium soaps (saponification).

    • These precipitates are grossly visible as pale yellow/white nodules
  • Microscopically, the digested fat loses its cellular outlines. There is often local inflammation

Fat necrosis

Gangrene ("gangrenous necrosis")

  • Not a separate kind of necrosis at all, but a term for necrosis that is advanced and visible grossly.

    • If there's mostly coagulation necrosis, (i.e., the typical blackening, desiccating foot which dried up before the bacteria could overgrow), we call it dry gangrene.
    • If there's mostly liquefactive necrosis (i.e., the typical foul-smelling, oozing foot infected with several different kinds of bacteria), or if it's in a wet body cavity, we call it wet gangrene.

Gangrenous necrosis


  • An area of ischaemic necrosis in a tissue or organ

    • White
      • Arterial occlusion in most solid tissues
    • Red/haemorrhagic
      • Venous occlusion
      • Loose tissues
      • Dual blood supply
      • Previously congested

White infarct

Red infarct

Apoptosis - basics

  • is a distinct reaction pattern which represents programmed single-cell suicide.

  • Cells actually expend energy in order to die.

  • Derived from Greek "falling off" (as for autumn leaves)

  • Apoptosis is "the physiological way for a cell to die", seen in a variety of normal situations.

Apoptosis - morphology

  • Necrosis:

    • pathological response to cellular injury.
    • Chromatin clumps, mitochondria swell and rupture, membrane lyses, cell contents spill, inflammatory response triggered
  • Apoptosis

    • DNA cleaved at specific sites - 200 bp fragments.
    • Cytoplasm shrinks without membrane rupture
    • Blebbing of plasma and nuclear membranes
    • Cell contents in membrane bounded bodies, no inflammation

Apoptosis - normal

Apoptosis -pathological

Apoptosis - triggers

  • Withdrawal of growth stimuli

  • Death signals

    • E.g. TNF and Fas
  • DNA damage

    • p53 plays an important role

Apoptosis - mechanisms

  • Extrinsic factors

    • E.g. by members of the TNF family
  • Intrinsic mechanisms

    • E.g. hormone withdrawal


  • This talk has covered….

    • Causes of cell injury
    • Cellular targets
    • Pathogenesis
    • Morphology of cell injury
    • Patterns of necrosis
    • Apoptosis

Final thought…

  • Our lives are filled with joys and strife,

  • And what is death but part of life?

  • Will come the day that we must die,

  • And leave behind those learning why.

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