Cell injury and necrosis

When an injury is severe enough that the cell can no longer adapt, the cell develops reversible and/or irreversible changes. These changes may progress to necrosis, apoptosis, and ultimately cell death.

Reversible cell injury

• Reduced oxidative phosphorylation and ATP formation
• Generalized swelling of the cell and organelles
• Formation of plasma membrane blebs
• Detachment of ribosomes from the RER
• Clumping of nuclear chromatin
• Myelin figures appear

Irreversible cell injury

• Mitochondrial damage with swelling and amorphous deposits
• Necrosis
• Apoptosis
• Lysosomal enzymes digest cytosolic structures
• Increased cellular swelling
• Swelling and disruption of the lysosomes
• Disruption of cellular membranes
• Nuclear pyknosis, karyorrhexis and karyolysis
• Prominent myelin figures

Causes of cell injury

Cell injury can result from many different causes, acting at both gross and microscopic levels.

1. Hypoxia: Inadequate oxygenation of tissue. In hypoxia, tissues either can’t utilize oxygen or they don’t receive enough oxygen. Hypoxia may be due to hypoxemia (reduced PaO2 from low inspired oxygen concentration, hypoventilation, V/Q defects, shunts, diffusion defects and circulatory shock), anemia, CO poisoning, methemoglobinemia, cyanide poisoning and shock. It results in reduced ATP formation by the mitochondria.

2. Physical injury: Physical trauma, extremes of temperatures, electric shock, barometric pressure changes, etc. can all cause cellular injury (e.g., frostbite in extreme cold weather).

3. Chemical injury: Adverse and toxic effects of drugs and poisons (e.g., carbon tetrachloride, strong acids and alkalis), and heavy metals (e.g., lead, arsenic, mercury) are part of a long and ever-increasing list of chemical agents that cause cell injury and cell death.

4. Infectious agents: All types of pathogenic microbes, including viruses, bacteria, fungi, rickettsia, parasites and prions, can cause cell injury.

5. Immune reactions: Failure of immune regulation may result in host tissue damage due to immune reactions. Injury may also occur from autoimmunity and complement mediated damage (e.g., Grave’s disease, Goodpasture’s syndrome, paroxysmal nocturnal hemoglobinuria).

6. Nutritional imbalances: Both nutritional deficiencies and nutritional excess can cause cell injury, ranging from kwashiorkor, marasmus, and anemia to obesity and metabolic syndrome.

7. Genetic defects: Genetic mutations lead to various disorders such as cystic fibrosis, sickle cell anemia, and Marfan’s syndrome. Most of them are inherited.

Mechanisms of cell injury

Cell injury produces multiple defects in cellular function.

1. Depletion of ATP: This is seen in hypoxic and chemical cell injuries. Injury may impair oxidative phosphorylation in the mitochondria and glycolysis, leading to reduced ATP production.

   ATP depletion leads to several downstream effects:

   • Na+K+ATPase pump dysfunction: Intracellular Na+ increases, water follows by osmosis, and the cell swells.
   • Increased anaerobic glycolysis: Lactic acid accumulates, intracellular pH decreases, and some enzymes may become inactivated.
   • Ca++ pump dysfunction: Ca++ enters the cell.

   Increased intracellular Ca++ activates enzymes such as ATPases, phospholipases, proteases and endonucleases, and increases mitochondrial membrane permeability, which can lead to apoptosis.

   ATP depletion also interferes with ribosomal protein synthesis, causing detachment of ribosomes from the RER and dissociation of polysomes. Protein misfolding occurs.

2. Mitochondrial damage: Damage to the mitochondrial membrane may be reversible early on, but it becomes irreversible if the injury persists or is severe. Membrane damage causes mitochondrial swelling and loss of the mitochondrial proton motive force, so ATP production by oxidative phosphorylation stops. Cyt c is released from the mitochondria, initiating apoptosis.

3. Influx of Ca++ into the cell: Most intracellular Ca++ is sequestered in the endoplasmic reticulum and mitochondria. Ischemia and toxins can cause release of Ca++ from intracellular stores, leading to activation of lytic enzymes.

4. Free radical injury: Free radicals are reactive oxygen species with a single unpaired electron in their outer orbit. Free radical-mediated injury is seen in radiation and chemical induced injury, ischemia-reperfusion injury and aging.

   Free radicals are generated from several sources:

   • Exposure to ionizing radiation (OH and H free radicals)
   • Normal metabolism in all living cells and immune cells (O2- or superoxide anion, H2O2 and OH)
   • Nitric oxide and nitrites
   • Metals like iron and copper
   • Metabolism or detoxification of drugs and toxins like paracetamol

   Free radicals damage cells through multiple mechanisms:

   • Lipid peroxidation: Peroxidation of lipids in cell and organelle membranes forms peroxides that trigger an autocatalytic reaction, leading to more severe cellular damage.
   • Protein oxidation: Oxidative modification of proteins can lead to protein breakdown.
   • DNA damage: Single stranded breaks occur in both nuclear and mitochondrial DNA.

Stepwise reduction of O2 to H2O

The body uses several mechanisms to counteract and neutralize free radicals:

• Catalase breaks down H2O2 into O2 and H2O.
• Superoxide dismutase converts superoxide to H2O2.
• Glutathione peroxidase uses reduced glutathione to scavenge free radicals like H2O2 and OH-, converting them to H2O. Reduced glutathione is converted to oxidized glutathione in the process.
• Glutathione reductase uses NADPH from the pentose phosphate pathway to keep glutathione in the reduced state.
• The body keeps metals like iron and copper bound to storage and transport proteins (ferritin, transferrin, ceruloplasmin, etc.) to prevent formation of OH free radicals from free iron and copper.
• Vitamins A, E, C and glutathione are naturally occurring antioxidants.

1. Membrane damage: Membrane phospholipids are affected in cell injuries. Phospholipases cleave cell membrane phospholipids. Cytoskeletal filaments are broken down, causing the cell membrane to detach from the cytoskeleton and promoting cellular swelling and rupture. Fluid and ions flow into the cell. Rupture of lysosomes releases enzymes such as proteases, DNSases, RNAases, cathepsins, etc., leading to dissolution of cellular components.