Physiological and pathological role
Apoptosis is essential for the body both in normal conditions and in the development of pathologies. It is a universal tool for «housekeeping» of unnecessary or dangerous cellular material.
- In normal conditions and development: Ensures cell elimination during embryogenesis (e.g., death of up to 75% of neuroblasts during brain formation). Helps remove immunocytes after the immune response is complete or autoreactive clones in the thymus. Causes tissue aging — endometrial involution, follicular atresia in women, and prostate changes in men.
- In infections: The introduction of viral or bacterial nucleic acid activates the death program. This is characteristic of viral hepatitis, HIV, diphtheria, and tuberculosis.
- In injuries: Sublethal doses of radiation, hypoxia, high temperature, or cytostatics trigger apoptosis without driving the cell to gross destruction.
- In tumor growth: Activation of apoptosis (e.g., by tumor necrosis factor TNF-α) underlies antitumor therapy. However, cancer cells often learn to block this process using anti-apoptotic genes.
Apoptosis and necrosis: key differences
Both processes lead to cell death, but their scenarios differ radically.
| Sign | Apoptosis | Necrosis |
|---|---|---|
| Cause | Information signal | Direct destructive action |
| Character | Programmed, energy-dependent | Violent, passive |
| Finale | Breakdown into membrane-bound bodies | Lysis (rupture) of the cell |
| Tissue response | No inflammation | Inflammation develops |
Four stages of apoptosis
The process of cellular «suicide» is strictly ordered and proceeds through four sequential stages:
- Initiation. Receptors capture the death signal. Signals can be transmembrane (growth factor deficiency, influence of FasL or TNF) or intracellular (acidosis, oxidative stress, viruses, penetration of glucocorticoids). All of them direct information to the genome.
- Programming. The cell decides its fate. The signal can go directly (via adapter proteins and the release of cytochrome c from mitochondria) or indirectly (via activation of pro-death genes and suppression of protective genes).
- Execution (effector stage). Destructive enzymes come into play. Effector caspases cleave cytoskeleton and nuclear proteins, and endonucleases fragment DNA. The cell shrinks and breaks down into fragments — apoptotic bodies.
- Removal. Specific marker ligands appear on the surface of apoptotic bodies. Phagocytes recognize them, engulf and digest the remnants. The contents of the dead cell do not leak out, so the tissue remains intact.
Molecular regulators: promoters and inhibitors
At the programming stage, the balance of activity of specific genes and their protein products plays a key role:
- Promoters (stimulators) of apoptosis: Products of Bad and Bax genes, as well as anti-oncogenes p53 and Rb. They promote the activation of destructive enzymes.
- Inhibitors (protectors) of apoptosis: Products of expression of Bcl-2 and Bcl-XL genes. Their main task is to reduce the permeability of mitochondrial membranes and prevent the triggering factor (cytochrome c) from entering the cytosol.
Briefly: answers to topical questions
Which specific markers (ligands) appear on the surface of apoptotic bodies for their recognition by macrophages?
For rapid recognition and phagocytosis by macrophages, specific molecules are expressed on the surface of apoptotic cells.
The main markers of apoptosis include:
- Phosphatidylserine — normally located on the inner surface, but during apoptosis, membrane asymmetry is disrupted, and it is exposed on the outside.
- Thrombospondin — a specific marker recognized by phagocyte receptors.
- Desialylated residues of membrane glycoconjugates — ensure rapid uptake of apoptotic bodies.
What two main pathways of apoptosis triggering (extrinsic and intrinsic) are distinguished, and what is their essence?
In the mechanism of apoptosis execution, two main pathways are distinguished, which boil down to the activation of caspases.
| Apoptosis pathway | Essence of the process |
|---|---|
| Extrinsic (receptor-mediated) | It is mediated by the interaction of an extracellular ligand with death receptors on the cell membrane surface, leading to complex assembly and caspase 8 activation. |
| Intrinsic (mitochondrial) pathway | It is associated with increased mitochondrial membrane permeability, release of cytochrome c into the cytosol, and formation of the apoptosome, where caspase 9 is activated. |
Which "death receptors" are involved in the transmembrane signaling of apoptosis?
Transmembrane signal transduction is carried out via specific cell surface molecules, most of which contain an intracellular "death domain".
The following "death receptors" are involved in triggering apoptosis:
- Fas receptors (CD95 or Fas/APO) — interact with the FASL ligand.
- Tumor necrosis factor receptor type I (TNFRI) — binds to tumor necrosis factor (TNF).
- DR4 and DR5 receptors — interact with the TRAIL ligand.
- DR3 and DR6 receptors — are activated by the TL1A ligand.
To avoid confusing apoptosis-regulating genes, use this mnemonic: Bax and Bad are the "bad" guys for the cell; they stimulate its death. Bcl-2 acts as a "blocker", an inhibitor that protects the cell from death.
Class and exam questions
Why does apoptosis not cause inflammation?
Because the cell membrane retains its integrity until the very end. The cell breaks down into neat membrane vesicles (apoptotic bodies); their aggressive contents do not spill into the intercellular space and are rapidly cleared by phagocytes.
What role does cytochrome c play in cell death?
Normally, it resides inside the mitochondria. When apoptosis is triggered, mitochondrial membrane permeability increases, cytochrome c leaks into the cytosol, and acts as a potent trigger for the activation of effector enzymes (caspases).
Can apoptosis be induced by viruses?
Yes, infection of a cell with foreign nucleic acid (viruses or bacteria) is a strong intracellular signal for apoptosis. This is a protective mechanism characteristic of hepatitis, HIV infection, and other diseases.
What else to study on this topic
- Classification of transmembrane signals: positive, negative, and mixed
- Effect of physicochemical factors (acidosis, free radicals) on the initiation of apoptosis
- Necroptosis: programmed necrosis involving the necrosome and RIPK kinases
- Pyroptosis: inflammatory cell death mediated by the inflammasome and caspase-1
- Mechanisms of apoptosis suppression in tumor cells (the role of bcl-2 and c-fos genes)
Master the entire topic in the "Pathophysiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Apoptosis" and 4,913 other pages in this subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 135 video lessons on the subject
Other topics in "Chapter 4: Cell Injury as the Basis for the Development of Various Forms of Human Pathology"
Free video lessons: pathophysiologyShort lessons with lecture notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.