Cell Injury
This article is part of a Pathology Series for healthcare professionals
Cells are constantly exposed to changes in their environment. Most of the time, they maintain homeostasis by adjusting their metabolism, membrane transport, energy production, and other cellular processes. But when a stress becomes too severe, lasts too long, or overwhelms the cell’s ability to adapt, the cell becomes injured. Cell injury exists on a spectrum. Some injuries are temporary and can be reversed once the harmful stimulus is removed. Other injuries cause permanent damage and eventually lead to cell death. Understanding cell injury provides the foundation for understanding many diseases. Whether tissue is damaged by lack of oxygen, infection, toxins, physical trauma, or immune reactions, the underlying process often begins with injury to individual cells.
Causes of Cell Injury
Cell injury can result from many different types of stress. Although the causes are diverse, they ultimately interfere with the cell's ability to maintain homeostasis.
Hypoxia and Ischemia: One of the most important causes of cell injury is inadequate oxygen or blood flow. Hypoxia means that cells do not have enough oxygen, while ischemia means that blood flow to a tissue is reduced or interrupted. Because cells rely on oxygen for aerobic cellular respiration and ATP production, hypoxia can reduce the energy available for essential cellular processes. Ischemia can be even more damaging because it reduces not only oxygen and nutrient delivery but also the removal of metabolic waste.
Physical Agents: Physical factors such as mechanical trauma, extreme temperatures, radiation, electrical injury, and changes in pressure can directly damage cells. Depending on the type and severity of the stress, these agents can disrupt cell membranes, damage proteins, alter DNA, or interfere with normal cellular structures.
Chemical Agents and Toxins: Chemicals can injure cells by interfering with enzymes, damaging cell membranes, disrupting mitochondrial function, or altering DNA. Some substances are directly toxic, while others are converted inside the body into harmful substances that damage cells.
Infectious Agents: Viruses, bacteria, fungi, and parasites can damage cells directly or indirectly. Infectious organisms may destroy cells, interfere with their normal functions, or produce substances that are harmful to the host. The immune response against an infection can also contribute to cellular and tissue injury.
Immune Reactions: The immune system protects the body from harmful substances and infectious organisms, but an excessive or inappropriate immune response can injure healthy cells. Immune cells and inflammatory mediators can damage surrounding tissue when the response is too strong, prolonged, or directed against the body's own cells.
Genetic Abnormalities: Genetic abnormalities can cause cells to produce abnormal proteins, produce too little of an essential protein, or disrupt important cellular processes. Because cells depend on precisely regulated proteins and genetic instructions, these abnormalities can interfere with normal structure and function and eventually lead to cell injury.
Nutritional Imbalances: Both nutritional deficiencies and excesses can injure cells. Cells require appropriate amounts of nutrients to produce energy, build cellular components, and maintain normal metabolism. Too little of an essential nutrient can impair these processes, while excessive nutrients can also disrupt metabolism and damage cells.
Reversible vs. Irreversible Cell Injury
Not all cell injury leads to cell death. The key question is whether the cell can recover after the harmful stimulus is removed. Cell injury can therefore be reversible, where the cell remains viable and can return toward normal, or irreversible, where the damage has progressed beyond the cell's ability to recover.
Reversible Injury: In reversible cell injury, the cell is damaged but remains alive and functional enough to recover. Early injury commonly involves reduced ATP production, cellular swelling, changes in ion and water balance, reduced protein synthesis, and altered cellular metabolism. For example, when oxygen supply temporarily decreases, ATP production falls and energy-dependent membrane pumps become less effective. Sodium and water then accumulate inside the cell, causing it to swell. If the oxygen supply is restored before severe damage occurs, ATP production can recover and the cell can gradually return toward normal.
Irreversible Injury: In irreversible cell injury, the damage has progressed beyond the point at which the cell can recover. Two critical events are severe mitochondrial dysfunction and severe membrane damage. When mitochondria can no longer produce enough ATP, the cell loses the energy needed to maintain essential processes. At the same time, damage to the plasma membrane and membranes of intracellular organelles disrupts the normal movement of ions and cellular contents. Once these critical systems are permanently damaged, the cell can no longer maintain homeostasis and eventually dies.
