Irreversible cell injury occurs when a cell is damaged beyond its capacity to recover, leading to cell death. This is a critical point where the cell can no longer restore its normal structure and function even if the damaging stimulus is removed. Key Points: Point of No Return: The defining feature is that the cell cannot recover. Mitochondrial Dysfunction: A primary cause of irreversibility. Severe damage to mitochondria leads to: Failure of oxidative phosphorylation* and ATP production. Increased production of reactive oxygen species* (ROS). Formation of mitochondrial permeability transition pores*, leading to loss of membrane potential and release of pro-apoptotic proteins. Membrane Damage: Extensive damage to plasma membranes, lysosomal membranes, and mitochondrial membranes is crucial. Plasma membrane damage* leads to loss of osmotic balance, influx of calcium, and leakage of cellular contents. Lysosomal membrane damage* results in the release of hydrolytic enzymes (e.g., proteases, nucleases) into the cytoplasm, digesting cellular components. Calcium Influx: A sustained increase in intracellular calcium (Ca²⁺) is a major mediator of irreversible injury. High Ca²⁺ activates various enzymes (ATPases, phospholipases, proteases, endonucleases) that degrade cellular components and deplete ATP. Protein Misfolding/DNA Damage: Severe damage to proteins and DNA that cannot be repaired also contributes to irreversible injury. Two Main Forms of Cell Death: Necrosis*: Occurs when external factors like toxins or ischemia cause uncontrolled cell death, leading to inflammation. Characterized by cell swelling, membrane rupture, and leakage of cellular contents. Apoptosis*: Programmed cell death, often triggered by internal signals or severe irreparable damage, leading to controlled removal of cells without inflammation. Characterized by cell shrinkage, DNA fragmentation, and formation of apoptotic bodies. In summary, irreversible cell injury is marked by severe and irreparable damage to mitochondria and cellular membranes, leading to a catastrophic loss of cellular homeostasis and ultimately, cell death, primarily through necrosis or apoptosis.