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Question 9 of 20 Fundamentals Concept

What is apoptosis, and how does it differ from necrosis at the bench?

Short answer

Apoptosis is a regulated death program in which the cell shrinks, keeps its membrane intact and is packaged for clearance, driven by caspases. Necrosis is death with early loss of membrane integrity and leakage of contents. At the bench the practical difference is whether the membrane is intact when the marker appears.

Why

The Nomenclature Committee on Cell Death defines intrinsic apoptosis as death initiated by internal stresses, marked by mitochondrial outer membrane permeabilization and carried out mainly by caspase-3, with the membrane staying intact until late. Extrinsic apoptosis starts at death receptors on the surface and runs through caspase-8. Necrosis is not one process.

The committee's 2018 recommendations describe several regulated forms, including necroptosis, pyroptosis and ferroptosis, each with its own machinery. Assays follow the same logic. Annexin V binds phosphatidylserine exposed on the outer leaflet early in apoptosis, and a dye excluded by intact membranes separates early apoptotic cells from cells that have already lost integrity.

Caspase activity assays and cleaved caspase-3 staining confirm the apoptotic route. Lactate dehydrogenase release into the medium reports membrane rupture. Timing is the most common trap.

In culture, apoptotic cells eventually break down and become secondary necrotic, so a late time point shows a necrotic picture whichever route the cell took. Morphology alone is not a reliable label, and the committee recommends naming the mechanism, supported by more than one assay.

A strong answer covers

  • Apoptosis is regulated, caspase-driven, membrane intact until late
  • Necrosis covers several regulated forms with early membrane loss
  • Annexin V with a membrane-impermeant dye separates stages
  • Caspase activity confirms the route
  • Secondary necrosis makes late time points ambiguous

Common mistakes

  • Calling any annexin V positive cell apoptotic without a viability dye
  • Treating all necrosis as accidental and unregulated
  • Picking one late time point and labelling the death mode from morphology