Cell Injury and Cell Death Notes

Introduction to Pathology

Pathology is the study of structural, biochemical, and functional changes in cells, tissues, and organs that underlie disease.
It involves understanding the causes (aetiology) and the underlying mechanisms (pathogenesis) of disease.
Pathology uses molecular, microbiological, immunological, and morphologic techniques.
It provides a rational basis for clinical care, treatment, and medicine development.

Major Aspects of Disease Processes

Aetiology: Causes of the disease, which can be genetic or environmental (e.g., toxins, hormonal changes, mutations, polymorphisms, infections).
Pathogenesis: Sequence of cellular, biochemical, and molecular events that occur after exposure to an injurious agent.
Morphological Changes: Structural alterations in cells or tissues characteristic of a disease or diagnostic of an etiologic process.
Functional Abnormalities: The end result of genetic, biochemical, and structural changes in cells and tissues, leading to clinical manifestations.

Steps in the Development of Disease

Etiology: Causes of disease (e.g., hypoxia, ischemia, toxins, infections, abnormal immune reactions, genetic abnormalities, nutritional imbalances, physical agents).
Pathogenesis: Mechanisms of disease involving biochemical and structural changes at the molecular and cellular levels.
Abnormalities in Cells and Tissues: Molecular, functional, and morphologic changes.
Clinical Manifestations: Signs and symptoms of the disease.

Cellular Responses to Stress and Noxious Stimuli

Cells maintain physiological parameters within a narrow range (homeostasis).
When cells encounter stress or pathological stimuli, they undergo adaptations, which are reversible functional and structural responses to changes to reach a new steady state while preserving viability and function.
If adaptive capability is exceeded or stress is inherently harmful, cell injury develops.
Mild or transient stress results in reversible injury where cells return to their original state.
Severe or persistent stress leads to irreversible injury and cell death (e.g., changes in ion levels like $K^+$ ).

Adaptation

Response to increased load (e.g., hypertrophy in myocytes due to gym or hypertension; decreased oxygen supply due to hypoxia or ischemia).

Cellular Adaptation to Stress

Reversible changes in number, size, phenotype, metabolic activity, or functions of cells in response to environmental changes.
* Physiological adaptations: Responses of cells to normal stimulation by hormones or endogenous chemical mediators.
* Pathological adaptations: Responses to stress that allow cells to modulate their structure and function to escape injury.

Hypertrophy

Increase in size of cells resulting in an increased size of the organ.
Can be physiological (e.g., hormone-induced enlargement of breast and uterus during pregnancy) or pathological and is caused by increased functional demand or hormonal stimulation.
Occurs when cells are incapable of dividing.
Results from increased production of cellular proteins.

Hyperplasia

Increase in the number of cells in a tissue or organ.
Adaptive response in cells capable of replication.
Often occurs with hypertrophy.
Can be physiological or pathological.
- Physiological hyperplasia: Hormonal (e.g. breast during puberty) or compensatory (e.g., after tissue removal).
- Pathologic hyperplasia: Caused by excessive hormonal or growth factor stimulation (e.g., benign prostate hyperplasia).

Atrophy

Reduction in the size of an organ or tissue due to a decrease in cell size and number.
Shrinkage in size of cells due to loss of cell substance.
Cells may have diminished function but are not dead.
Can be physiological or pathological.
Causes include decreased workload, loss of innervation, decreased blood supply, inadequate nutrition, loss of endocrine stimulation, and aging.
Represents a retreat by the cell to a smaller size at which survival is still possible, indicating a new equilibrium between cell size and diminished blood supply, nutrition, and stimulation.
Results from decreased protein synthesis or increased protein degradation due to decreased metabolic activity.

Metaplasia

Reversible change in which one adult cell type (epithelial or mesenchymal) is replaced by another adult cell type.
Cells sensitive to a particular stress are replaced by other cell types better able to withstand the adverse environment.
Results from reprogramming of stem cells in normal tissues or undifferentiated mesenchymal cells.
Often results in some loss of function.
Persistent metaplasia can initiate malignant transformation.
Example: Smoking-induced metaplasia in the trachea where squamous cells replace the normal epithelium, providing toughness but reducing cilia and mucus secretion.
Another example: Barrett's esophagus, where esophageal squamous cells are replaced with intestinal-like columnar cells due to acid reflux.

Cell Injury and Cell Death

Reversible Cell Injury: In early stages or mild forms, functional and morphologic changes are reversible if the damaging stimulus is removed.
Cell Death: With continuing damage, the injury becomes irreversible, and the cell cannot recover and dies.

Reversible Cell Injury Hallmarks

Characterized by reduced oxidative phosphorylation with resultant depletion of ATP.
Cellular swelling results from changes in ion concentrations and water influx.
Intracellular organelles (e.g., mitochondria and the cytoskeleton) may show alterations.

Cell Death

Cell death is a key event in the evolution of disease in any tissue or organ.
Results from diverse causes (e.g., ischemia, infection, and toxins).
Cell death is also a normal and essential process in the development of organs (embryogenesis) and the maintenance of homeostasis.
Two main pathways of cell death: necrosis and apoptosis.

Necrosis

Pathological process of cell death.
Main pathway of cell death from common insults (e.g., ischemia, toxins, infectious agents, trauma).
Involves unregulated enzymatic digestion of cell components.
Occurs when damage to membranes is severe.
Enzymes leak out of lysosomes into the cytoplasm and digest the cell.
Cell contents also leak out through damaged plasma membranes and initiate inflammation.

Apoptosis

Tightly regulated type of cell death.
Active, energy-dependent process.
Occurs in specific situations.
Serves many normal functions and is not necessarily associated with pathological injury.
Serves to eliminate cells that are no longer needed and to maintain a steady number of various cell populations in tissues.
Occurs when a cell is deprived of growth factors or DNA or proteins are damaged beyond repair, causing the cell to kill itself.
Mediated by the activation of caspases (protease enzymes).
Characterized by enzymatic degradation of proteins and DNA.

Necrosis vs. Apoptosis

Cell Size: Necrosis involves enlarged cells (swelling), while apoptosis involves reduced cells (shrinkage).
Nucleus: Necrosis shows pyknosis → karyorrhexis → karyolysis, while apoptosis shows fragmentation into nucleosome-sized fragments.
Plasma Membrane: Necrosis involves a disrupted plasma membrane, while apoptosis has an intact but altered structure.
Cellular Contents: Necrosis causes enzymatic digestion and leakage out of the cell, while apoptosis releases contents in apoptotic bodies.
Adjacent Inflammation: Necrosis frequently induces inflammation, while apoptosis does not.
Physiologic or Pathologic Role: Necrosis is invariably pathologic, while apoptosis is often a physiologic means of eliminating unwanted cells but can be pathologic after some forms of cell injury, especially DNA and protein damage.

Morphologic Alterations in Dying/Dead Cells

Necrosis: Increased eosinophilia, nuclear shrinkage, fragmentation and dissolution, breakdown of plasma membrane and organelle membranes, calcification, leakage, and enzymatic digestion of cellular contents.
Apoptosis: Nuclear chromatin condensation and formation of apoptotic bodies.

Irreversible Cell Injury - Necrosis

Swelling of endoplasmic reticulum and mitochondria.
Membrane blebs.
Swelling of endoplasmic reticulum and loss of ribosomes.
Lysosome rupture.
Clumping of chromatin.
Fragmentation of cell membrane and nucleus.
Myelin figures.
Nuclear condensation.
Swollen mitochondria with amorphous densities.

Mechanisms of Cell Injury

ATP depletion.
Reactive oxygen species.
ER stress/misfolded proteins.
DNA damage.
Inflammation.

Causes of Cell Injury

Ischemia/hypoxia.
Chemical agents/toxins.
Infectious agents.
Immune reactions.
Mutations.
Nutritional imbalances.
Physical agents/radiation.
Aging.

Cellular responses to injurious stimuli depend on the nature of the injury, duration, and severity.
Consequences depend on the type, state, and adaptability of the injured cell.
Cell injury results from different biochemical mechanisms acting on essential cellular components.

Endoplasmic Reticulum Stress

ER controls folding of proteins during synthesis.
Proteins may not fold into a normal configuration, leading to altered or dysfunctional proteins or accumulation.
Misfolded proteins are responsible for numerous disorders (proteopathies).
The ER membrane contains sensors that detect misfolded proteins, and accumulation of these can trigger apoptosis.

ATP Depletion

Reduction in ATP levels is a main cause of necrotic cell death.
Associated with both hypoxia and chemical injury (toxins).
Failure of energy-dependent functions leads to reversible injury and then necrosis.
Reversible if oxygenation is restored.
Damage due to decreased activity of the plasma membrane $Na^+ K^+ ATPase$ .
Changes in cellular energy metabolism.
Influx of $Ca^{++}$ due to failure of $Ca^{++}$ pumps.
Reduction in protein synthesis.

Consequences of Decreased ATP

Ischemia leads to decreased oxidative phosphorylation and decreased ATP.
Decreased $Na^+$ pump, increased anaerobic glycolysis, detachment of ribosomes, influx of $Ca^{2+}$ , $H_2O$ , and $Na^+$ , decreased glycogen and pH, decreased protein synthesis, efflux of $K^+$ , ER swelling, cellular swelling, loss of microvilli, and blebs.

Reactive Oxygen Species

Highly unstable “free radicals.”
ROS include superoxide radicals, hydrogen peroxide, and hydroxyl radicals.
Cause covalent modification of cell proteins, lipids, and nucleic acids.
Uncontrolled free radical production causes damage to cell membranes, proteins, inactivation of enzymes, and damage to nucleic acids.
Increased permeability of cellular membranes, typically culminating in necrosis.
Accumulation of damaged DNA and misfolded proteins triggers apoptosis.

Production of ROS: Pathologic Effects

Lipid peroxidation causes membrane damage.
Protein modifications cause breakdown and misfolding.
DNA damage leads to mutations.

Autophagy

Process in which a cell eats its own contents.
Survival mechanism.
Initiated by nutrient-sensing proteins.
Sequestration of cellular organelles into cytoplasmic autophagic vacuoles that fuse with lysosomes and digest the enclosed material.
Plays a role in human diseases including cancer, neurodegenerative disorders, and infections.

Cellular Aging

As cells age, function and viability decrease.
Caused by the accumulation of cellular and molecular damage due to the effects of exposure to exogenous influences.
- Main mechanisms:
  - Accumulation of DNA damage and defective DNA repair mechanisms.
  - Replicative senescence - reduced capacity of cells to divide.
  - Defective protein homeostasis.

Cell Senescence

Healthy human cells are mortal and have a limited capacity for replication.
After a fixed number of divisions, cells become arrested in a terminally nondividing state.
Reduced capacity of cells to divide because of decreasing amounts of telomerase and progressive shortening of chromosomal ends (telomeres) after around 50 divisions (Hayflick limit).