Necrosis: When a Cell Dies from Injury
Necrosis is a passive form of cell death that does not require energy (ATP) expenditure. This process is always associated with extreme injury or sudden environmental shifts, such as hypoxia (as seen in myocardial infarction).
Pathogenesis of Necrosis:
- Membrane integrity is compromised (both plasma membrane and organelle membranes).
- Water rushes inward, leading to swelling of the cell and nucleus, as well as organelle edema.
- Autolysis occurs: lysosomal enzymes spill into the cytoplasm and haphazardly digest the cellular contents.
- The nucleus condenses and completely dissolves under the action of nucleases (karyolysis).
- The plasma membrane ruptures, spilling the cell contents outward.
The main consequence of necrosis is the release of breakdown products into the extracellular environment, which damages neighboring cells and triggers an inflammatory response.
Apoptosis: Programmed Self-Destruction
Apoptosis is an evolutionarily conserved, genetically controlled cell death process. It requires energy expenditure and serves to eliminate dangerous or unnecessary cells. The main difference from necrosis is that the cell breaks down into neat fragments (apoptotic bodies) enclosed by a membrane. Contents do not spill outward, no inflammation occurs, and the fragments are rapidly engulfed (phagocytosed) by neighboring cells.
Triggers of Apoptosis:
- Intrinsic signals ("internal cues"): Aging of long-lived (neurons) or rapidly renewing (epithelial) cells, as well as the accumulation of critical DNA mutations during division.
- Extrinsic signals ("by command"): Removal of temporary embryonic structures, elimination of autoreactive immune cells, follicular atresia, and cell death due to growth factor withdrawal. The command may arrive via receptor-ligand binding or the loss of survival factor signaling.
Morphology and Biochemical Tools of Apoptosis
Morphologically, apoptosis begins with chromatin condensation (aggregating into dense clumps at the nuclear periphery). The cell then shrinks (rather than swelling as in necrosis), and the nucleus breaks apart. The plasma membrane forms invaginations, and the cell divides into apoptotic bodies.
This process is executed through specific biochemical mechanisms:
- Caspases: A family of enzymes (proteases) that cleave nuclear proteins. They do not completely lyse proteins, but rather cut them into large peptides.
- Nucleases: Enzymes that cleave DNA precisely between nucleosomes, fragmenting the genetic material.
- Membrane proteins: Translocate specific phospholipids to the outer leaflet of the apoptotic body membrane. This acts as an "eat-me signal" for macrophages.
Regulation of Apoptosis: The Role of p53 and Mitochondria
The central regulator of apoptosis is the p53 protein. Its activity increases in response to chromosomal damage or growth factor deprivation. It can arrest the cell cycle, induce oxidative stress, and influence the cellular microenvironment.
The primary pathway for executing p53 signals is the mitochondrial pathway. Mitochondrial membranes contain channels whose activity is controlled by regulatory proteins. Inhibitors (such as Bcl-2) close these channels, while activators (such as Bax) open them.
Under the influence of p53, the channels open, and mitochondrial proteins—cytochrome c and AIF (apoptosis-inducing factor)—leak into the hyaloplasm. These factors trigger a caspase cascade, leading to chromosomal degradation, DNA fragmentation, and the formation of apoptotic bodies.