Two main types of cell death
In any multicellular organism, two fundamental variants of cell death are generally distinguished, which differ fundamentally in their nature and consequences:
- Necrosis. This is an exclusively pathological process. The cell dies in response to the severe action of unfavorable damaging factors. At the same time, destruction is chaotic in nature.
- Apoptosis. This is physiological (or programmed) death. The process is strictly controlled by the genetic apparatus and occurs independently of sudden traumatic agents. The cell neatly "dismantles" itself into fragments that are safe for surrounding tissues.
Physiological role of apoptosis
Normal embryonic development and the maintenance of homeostasis in the adult organism are impossible without programmed cell death. This process is often under strict hormonal regulation.
The most important examples of normal apoptosis:
- Metamorphosis in animals. A classic example is the disappearance of structures that are no longer needed: the involution of a tadpole's tail during transformation into a frog, or the reduction of gills in newts.
- Formation of the urogenital system. During mammalian embryogenesis, targeted death of the cells of the so-called Wolffian or Müllerian ducts occurs, which is necessary for correct sex determination.
- Renewal of the reproductive system. In adults, apoptosis ensures the resorption of ovarian follicular cells after ovulation has occurred. Also, it is via apoptosis that the lysis of mammary gland cells occurs after the lactation period ends.
How cellular "suicide" is regulated
The process is not triggered spontaneously. Activation of the program requires the action of certain triggers or, conversely, the disappearance of supporting signals. Three key factors triggering apoptosis are distinguished:
- Deficiency of regulatory molecules. If a cell is deprived of necessary trophic support, it dies. A striking example: if in vitro nerve growth factor (Nerve Growth Factor, NGF) is removed, nerve cells will inevitably undergo apoptosis.
- Action of specific signaling molecules. There are so-called "death receptors" and their ligands. For example, tumor necrosis factor (Tumor Necrosis Factor, TNF) is able to send a direct self-destruction signal to a cell.
- Disruption of cellular metabolism. The program can be activated due to the action of exogenous toxins that irreversibly break internal metabolic processes.
Clinical significance of apoptosis dysregulation
Any deviation in the genetically programmed death system leads to severe diseases. The problem can develop according to two scenarios: either cells refuse to die, or they die too actively.
Inability to undergo apoptosis (pathological accumulation of cells):
- Oncological diseases: follicular lymphomas, breast tumors, ovarian and prostate cancer.
- Viral infections: some viruses (e.g., herpes simplex viruses or adenoviruses) block host cell apoptosis to use it for their own replication.
- Autoimmune pathologies: systemic lupus erythematosus (SLE), glomerulonephritis (autoaggressive lymphocytes survive).
Abnormally enhanced apoptosis (pathological cell loss):
- Immunodeficiencies: acquired immunodeficiency syndrome (AIDS), in which immune system cells die en masse.
- Neurodegenerative diseases: Alzheimer's disease, Huntington's disease (there is a steady loss of functional neurons).
Comparative characteristics: Apoptosis and Necrosis
To better understand the differences, let us examine their main characteristics:
| Sign | Apoptosis | Necrosis |
|---|---|---|
| Nature of the process | Genetically controlled, physiological | Pathological, response to external damage |
| Scale of damage | Autonomous (single cells), neighbors are not involved | Massive (involves bulk areas of tissues) |
| Inflammatory reaction | Completely absent | Always develops |
Briefly: answers to topical questions
What morphological changes and stages does a cell undergo during apoptosis?
The duration of the apoptotic process is 1–3 hours. The cell undergoes the following morphological changes:
- Nuclear changes — chromatin condensation with the formation of dense masses (hemispheres and clumps) at the periphery of the nucleus (margination), appearance of osmiophilic bodies, lobulation of the nucleus and its fragmentation (karyorrhexis).
- Changes in the cell as a whole — reduction in size and cell shrinkage, increase in cytoplasmic eosinophilia.
- State of organelles — relative preservation of organelles, especially mitochondria; aggregation of ribosomes into semi-crystalline structures; appearance of cytoskeleton microfilament bundles; dilation of the smooth ER with vesicle formation.
- Formation of apoptotic bodies — appearance of deep plasmalemma invaginations (blebbing), division of the cell into fragments — apoptotic bodies, which may contain nuclear fragments or cytoplasm only.
What enzymes (caspases) are involved in executing apoptosis?
Apoptosis is executed by a cascade of proteolytic enzymes — caspases (cysteine proteases) that cleave the polypeptide chain after aspartic acid residues. They are subdivided into:
- Initiator caspases — caspase 8 (activated in the extrinsic/receptor pathway) and caspase 9 (activated in the intrinsic/mitochondrial pathway).
- Executioner (effector) caspases — caspase 3 (main), less commonly caspases 6 and 7. They are activated by initiator caspases and destroy cellular structures by cleaving cytoskeleton and nuclear proteins.
What is the difference between the intrinsic (mitochondrial) and extrinsic (receptor) pathways of apoptosis?
| Sign | Mitochondrial (intrinsic) pathway | Receptor (extrinsic) pathway |
|---|---|---|
| Triggering conditions | Increased mitochondrial membrane permeability, intracellular damage (DNA) | External signals, ligand-receptor interaction |
| Trigger factors | Bcl-2 family proteins (Bax, Bak, Bid, etc.), p53 | Death receptors (Fas/CD95, TNFRI) and their ligands (FasL, TNF, TRAIL) |
| Signal transduction factors | Formation of the apoptosome (Apaf-1, cytochrome c, ATP, procaspase 9) | Assembly of the DISC complex (FADD, TRADD adapter proteins, procaspase 8) |
| Initiator caspase | Caspase 9 | Caspase 8 |
How do phagocytes recognize apoptotic bodies for engulfment?
During apoptosis, phosphatidylserine and other markers are expressed/extracellularized on the cell surface. Macrophage membrane receptors recognize these molecules, after which phagocytosis of the apoptotic cells/apoptotic bodies takes place. This type of phagocytosis is associated with the removal of apoptotic structures without the development of an inflammatory response.
It is easy to remember the difference from necrosis by association: Apoptosis is "Accurate" (according to a genetic plan, without inflammation, cell by cell). Necrosis is "Messy" (massive destruction, chaos, and obligatory inflammation).
Class and exam questions
Does inflammation develop during apoptosis?
No, never. The cell neatly breaks down into fragments that are engulfed by phagocytes without the release of inflammatory mediators. Inflammation is characteristic only of necrosis.
What role does nerve growth factor (NGF) play in apoptosis?
Its absence serves as a death trigger for nerve cells. A deficiency of such regulatory molecules signals to the cell that it is no longer needed by the organism.
What else to study on this topic
- Molecular mechanisms of death receptor action (interaction with TNF)
- The role of caspases in the process of cell disassembly during apoptosis
- Mechanisms of apoptosis inhibition by herpesviruses and adenoviruses
- Features of apoptosis in autoimmune diseases (using SLE as an example)
- Effect of exogenous toxins on metabolism and the initiation of programmed cell death
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