What Is Apoptosis?
Apoptosis is a controlled, energy-dependent process of programmed cell death that removes damaged, infected, or unneeded cells without harming their neighbors. It is essential for normal development and tissue maintenance, in contrast to necrosis, which is uncontrolled and triggers inflammation.
Apoptosis is programmed cell death: a tightly regulated process in which a cell activates enzymes called caspases to dismantle itself in an orderly way, without triggering inflammation.
- 1↓SignalAn intrinsic (DNA damage, stress) or extrinsic (death receptor) trigger initiates the pathway
- 2↓Initiator caspase activationInitiator caspases (e.g. caspase-8, -9) are activated by the signal
- 3↓Executioner caspase activationInitiator caspases activate executioner caspases (e.g. caspase-3)
- 4↓Cell shrinkage & DNA fragmentationThe cell shrinks, blebs, and its DNA is cleaved into fragments
- 5Phagocytic clearanceApoptotic bodies are engulfed by phagocytes without spilling cell contents
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Step-by-step worked examples
A tumor biopsy of 1,000 cells shows 25 cells with condensed, fragmented nuclei (apoptotic). Calculate the apoptotic index.
Apoptotic index = (apoptotic cells ÷ total cells) × 100 = (25 ÷ 1000) × 100 = 2.5%
During limb development, webbing between fingers is removed by apoptosis. A paddle-shaped hand bud has 4 interdigital regions, each losing about 2,000 cells. How many cells die in total to form separate fingers?
Total apoptotic cells = regions × cells per region = 4 × 2,000 = 8,000 cells This sculpting explains how separate fingers form from a webbed limb bud
A drug inhibits caspase-3, an executioner caspase. Predict the effect on a cell that has received a strong apoptotic signal.
Caspase-3 normally cleaves structural proteins and DNA-repair enzymes to execute cell death Blocking it stops the execution phase even though initiator signals fired The damaged cell may survive despite being marked for death This is one mechanism by which cancer cells evade apoptosis
Flashcards
Quick quiz
Q1.Which enzymes execute apoptosis?
Q2.How does apoptosis differ from necrosis?
Q3.What happens to a dying cell's remains during apoptosis?
Q4.Why is apoptosis important during development?
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Common mistakes
Apoptosis is a random, accidental process. — Correct: Apoptosis is a tightly regulated, genetically programmed process.
Apoptosis and necrosis are the same thing. — Correct: Apoptosis is controlled and doesn't cause inflammation; necrosis is uncontrolled and does.
Apoptosis only happens when cells are diseased. — Correct: Apoptosis also occurs normally in healthy development and routine tissue turnover.
Caspases are always active in a cell. — Correct: Caspases exist as inactive precursors (procaspases) and are only activated by specific signals.
FAQ
What is apoptosis?
It's programmed cell death — an orderly process that eliminates damaged or unneeded cells.
What is the apoptosis formula for measuring cell death?
Apoptotic index = (apoptotic cells ÷ total cells) × 100, used to quantify tissue samples.
What are examples of apoptosis in the body?
Removal of webbing between fingers, shedding of the uterine lining, and elimination of virus-infected cells.
How is apoptosis triggered?
Through intrinsic signals (DNA damage, stress) or extrinsic signals (death receptors like Fas), both activating caspase cascades.