Why Do Some Cells Die?

Shaping up

Complex multicellular colonies of protoctists form. In many successful communities cells become specialists. A different kind of dedication to one's community prevails as the "superorganism" (the new larger individual) evolves. Colonies elaborate on techniques developed for seasonal relief of doubling and invent organized "celli-cide" as the superorganism grows and increases in complexity. Eventually, exquisitely organized individuals, hundreds of millions of cells working together, emerge.

The hundred trillion cells in the human body shape themselves through differentiation and selective death. Programmed death of certain cells is required for differentiation. Think of a block of marble. Without Michelangelo chipping away just the right bits, no figure of David ever appears. Without death on cue, no embryo, brain, or immune system develops. Scientists call such programmed death "apoptosis," a Greek word meaning "the falling away of petals from a flower."