Mechanisms of Cell Injury
When cells are injured—whether by hypoxia, toxins, or physical trauma—several interconnected biochemical pathways are triggered.
1. ATP Depletion
One of the most critical consequences of reduced oxygen or mitochondrial damage is the decrease in ATP production. This leads to a cascade of failures:
- Failure of the Na+/K+ pump: This causes an influx of sodium and water, leading to cellular swelling.
- Increased anaerobic glycolysis: As the cell struggles to produce energy without oxygen, it switches to anaerobic pathways, which increases lactic acid production and decreases intracellular pH.
- Ribosomal detachment: The swelling of the endoplasmic reticulum causes ribosomes to detach, which reduces protein synthesis.
2. Mitochondrial Damage and Oxidative Stress
Mitochondria are highly sensitive to hypoxia, toxins, and calcium accumulation. Damage to these organelles results in:
- Further reduction of ATP.
- Increased production of Reactive Oxygen Species (ROS).
- Release of apoptotic proteins, which can trigger programmed cell death.
The resulting oxidative stress causes damage through lipid peroxidation (damaging cell membranes), protein oxidation, and DNA damage.
3. Loss of Calcium Homeostasis
Injury causes an abnormal increase in intracellular calcium (Ca2+). This excess calcium acts as a "death signal" by activating several destructive enzymes:
- Phospholipases: Damage cell membranes.
- Proteases: Break down cellular proteins.
- Endonucleases: Fragment DNA.
- ATPases: Further deplete ATP.
4. Membrane and Lysosomal Damage
If the plasma or lysosomal membranes are breached, the cell faces catastrophic failure. The leakage of lysosomal enzymes into the cytoplasm leads to autodigestion, where the cell's own enzymes digest its proteins, lipids, and nucleic acids.
Intracellular Accumulations
Sometimes, cell injury does not result in immediate death but rather in the abnormal buildup of substances within the cell due to metabolic disturbances. These accumulations can be temporary and reversible, or they may eventually contribute to permanent injury.
| Substance | Description | Common Example |
|---|
| Lipids (Fat) | Excessive accumulation of triglycerides | Fatty change (steatosis) in the liver |
| Proteins | Due to excessive production, defective folding, or impaired transport | Accumulation in kidney tubules |
| Glycogen | Excessive storage due to metabolic abnormalities | Diabetes mellitus |
| Pigments | Colored substances that accumulate within cells | Bilirubin, hemosiderin, lipofuscin |
Detailed Breakdown of Accumulations
- Lipids: The most common form is fatty change (steatosis), which primarily affects the liver. This can be caused by alcohol abuse, obesity, or diabetes. Under a microscope, fat appears as clear vacuoles in the cytoplasm.
- Proteins: These accumulate when the cell cannot properly fold, transport, or degrade them. For example, if there is excessive protein filtration in the kidneys, protein droplets may appear in the renal tubular cells.
- Pigments: These are colored substances. Examples include bilirubin (related to bile), hemosiderin (related to iron), and lipofuscin (often associated with aging).
Cell Death: Necrosis vs. Apoptosis
When cell injury becomes irreversible, the cell must die. There are two primary modes of cell death:
- Necrosis: This is "accidental" or uncontrolled cell death resulting from severe injury, such as prolonged ischemia. It is characterized by membrane rupture, leakage of cellular contents, and often triggers inflammation.
- Apoptosis: This is "programmed" cell death. It is a highly regulated process that can be triggered by specific signals, such as significant DNA damage or the release of apoptotic proteins from damaged mitochondria.
Note on Infarction: An infarction is an area of tissue necrosis caused by a local lack of blood supply (ischemia).
Summary of Other Contributing Factors
While hypoxia and metabolic disturbances are major drivers, cell injury can also be caused by:
- Physical Agents: Trauma, extreme temperatures, radiation, or electric shock.
- Infectious Agents: Bacteria, viruses, fungi, and parasites.
- Chemicals/Drugs: Poisons, heavy metals (lead, mercury), and even therapeutic drugs when used in toxic doses.
- Immunological/Genetic: Autoimmune diseases, allergies, gene mutations, and inherited metabolic disorders.
- Nutritional/Aging: Obesity, vitamin deficiencies, and the natural accumulation of damage over time.
