Disease, traced to its root, is cells failing. Cell injury is therefore the starting point of pathology: understand how cells are damaged, where the line between recovery and death lies, and how they die, and you have the mechanism behind an enormous range of clinical problems.
What injures cells
- Oxygen deprivation — hypoxia and ischaemia (loss of blood supply) are the most common and important causes; cells starved of oxygen can't make energy.
- Toxins and drugs — chemical injury, from poisons to therapeutic agents in overdose.
- Infections — direct damage by pathogens, or by the immune response to them.
- Physical agents — trauma, temperature extremes, radiation.
- Immunological reactions — the body's own defences damaging cells.
Reversible vs. irreversible
The pivotal distinction is whether injury is reversible — the cell can recover if the stress is removed — or irreversible, past the point of no return. The transition is marked by severe mitochondrial damage and membrane failure. Recognising that threshold is what determines whether an intervention can still save tissue (the logic behind treating a heart attack fast).
Almost everything that goes wrong in the body starts as a cell under stress. Pathology is, at bottom, the study of that stress and its limits.
The mechanisms
A few interlinked mechanisms do most of the damage: ATP depletion (energy failure), mitochondrial damage, uncontrolled calcium influx, reactive oxygen species (free-radical damage), and loss of membrane integrity. These feed one another in a vicious cycle that tips reversible injury into death.
Two ways cells die
Necrosis is uncontrolled death from injury — the cell swells, ruptures, and spills its contents, provoking inflammation. Apoptosis is programmed, orderly death — the cell dismantles itself cleanly, without inflammation, as a normal part of development and tissue turnover (and a process that goes wrong in cancer). Distinguishing them is central to understanding tissue damage.
Understand disease at the source
My Cell Injury Pathology course covers the causes, mechanisms, and the reversible/irreversible threshold of cell injury, plus necrosis and apoptosis — the cellular basis of disease.
View the course →Questions
What's the most common cause of cell injury?
Oxygen deprivation — hypoxia and ischaemia. Cells starved of oxygen lose the energy needed to maintain themselves, which is why restoring blood flow quickly is so critical clinically.
What's the difference between necrosis and apoptosis?
Necrosis is uncontrolled death from injury, with inflammation; apoptosis is programmed, orderly self-dismantling without inflammation. Both matter in disease.