Our cells die all the time, in vast numbers.  Cells are programmed to die when all kinds of things happen: They may have reached the end of their productive life (as with cells of the gut or skin); they may detect damage to their DNA (as in cancer); or they may have detected viral infection. (See here, where I posted a movie of cells undergoing programmed death.)

In principle, dying is fine,  because most cells can be easily replaced. 1 Unexpected cell death isn’t fine, mind you — it means there’s something abnormal going on, and the immune system detects uncoordinated cell death as danger and responds with inflammation — but the usual form of cell death is highly coordinated, with the cell carefully tidying up before blowing out the candles.

This video2 helps show just how organized programmed cell death is.  Here we’re seeing a cell whose mitochondria are double-stained.  Cytochrome c is green (it’s part of the mitochondrial energy-generating system); red is a dye that indicates functioning, respiring mitochondria. The cell is forced into programmed cell death.3 At the start the mitochondria are both green and red, making yellow.  Watch the dual waves sweep over the cell: First the cytochrome leaks out, leaving only the red behind, and then — this 15-second movie shows about 10 minutes of activity — the mitochondria wink out altogether as they stop breathing, leaving behind a peaceful corpse.

A time-lapse movie of a HeLa cell expressing cytochrome c-GFP (green), stained with TMRE (red), and treated with TRAIL to induce programmed cell death

  1. Obviously there are exceptions, like neurons, that aren’t as easily replaced.[]
  2. This is one of the supplementary videos from
    Bhola, P., Mattheyses, A., & Simon, S. (2009). Spatial and Temporal Dynamics of Mitochondrial Membrane Permeability Waves during Apoptosis Biophysical Journal, 97 (8), 2222-2231 DOI: 10.1016/j.bpj.2009.07.056[]
  3. By treatment with TRAIL[]