Answer:
In many organisms, programmed cell death is a normal part of development. In some cases, apoptosis during development occurs in a very predictable way: in the worm C. elegans, 131131131 cells will die by apoptosis as the worm develops from a single cell to an adult (and we know exactly which ones they are)^1
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Apoptosis also plays a key role in human development. For instance, as we saw in the introduction, your hand started out as a paddle-like block of tissue when you were an embryo. The block was “carved” into fingers by apoptosis of the cells in between the developing fingers.
Microscope images from a scientific paper, showing a developing mouse paw. The cells between the developing digits are stained by a marker that indicates apoptotic cells.
Microscope images from a scientific paper, showing a developing mouse paw. The cells between the developing digits are stained by a marker that indicates apoptotic cells.
Image modified from "Duplication of digit 4 is preceded by reduced apoptosis and expanded chondrogenesis in the posterior limb mesenchyme," by Farin et al. ^2
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This process occurs in all sorts of vertebrate species that have finger- or toe-like digits, and less apoptosis results in more webbing between the digits. Sometimes, if a small mistake happens during finger or toe development, apoptosis may be incomplete (leading, for instance, to fused toes).
Other examples of apoptosis during normal development include the loss of a tadpole’s tail as it turns into a frog, and the removal of unneeded neurons in as neural circuits in the brain are “wired.”
Apoptosis can eliminate infected or cancerous cells
In some cases, a cell can pose a threat to the rest of the body if it survives. For instance, this may be the case for cells with DNA damage, pre-cancerous cells, and cells infected by viruses. If these cells undergo apoptosis, the threat to the rest of the organism (such as cancer or spread of a viral infection) is removed.
When a cell’s DNA is damaged, it will typically detect the damage and try to repair it. If the damage is beyond repair, the cell will normally send itself into apoptosis, ensuring that it will not pass on its damaged DNA. When cells have DNA damage but fail to undergo apoptosis, they may be on the road to cancer.
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