What is Apoptosis? - Definition, Pathway & Examples

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  • 0:05 Definition of Apoptosis
  • 1:54 Pathway of Apoptosis
  • 2:46 Activating Caspases
  • 4:15 Examples of Apoptosis
  • 5:50 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

This lesson will discuss apoptosis, giving a definition and overview of importance. It will also delve into a few of the pathways by which apoptosis occurs and give some examples of when apoptosis is induced.

Definition of Apoptosis

Have you ever seen a duck's feet? They are webbed, that is, there is skin between the toes. Did you know that your fingers and toes were also webbed before you were born? Yes, your digits were webbed rather like a duck's feet. So why aren't they webbed now?

Apoptosis is the reason you can wiggle your toes separately and wear rings on your fingers. Apoptosis is programmed cell death or cell suicide. Sounds rather morose, but it's an important aspect of survival. Why? Apoptosis is partly responsible for keeping many things (like your organs) the right size, the right shape, in the right place, and functioning correctly.

You may hear this word pronounced different ways. Some say 'a-pop-toe-sis,' while others insist it's 'ape-o-toe-sis,' with a silent second p. Either way, now you know that apoptosis is programmed cell death.

You should know that there are two major forms of cell death. Cells that are wounded don't undergo apoptosis; instead they undergo necrosis, a different type of cell death that involves inflammation, releases toxins to neighboring cells, and often causes further complications.

However, a cell undergoing apoptosis does not involve these effects. Instead, everything is kept inside the plasma membrane, the cell's border to the outside. An apoptotic cell can be characterized by:

  • Condensation of chromatin
  • Membrane blebbing (or formation of irregular bulges)
  • Cell shrinkage
  • Disintegration of the nuclear envelope

A neighboring cell or a macrophage will dispose of the cell by eating it. Yes, the apoptotic cell is cannibalized and its parts reused; it may be a little disturbing but remember that its part of what keeps us going.

Pathway of Apoptosis

How does the cell go about killing itself? The main answer is caspases. Caspases are enzymes (or proteins that speed up chemical reactions) that start degrading parts of the cell. Once caspases are loose, they go on a controlled rampage, destroying the cell. They:

  • Destroy the nuclear membrane
  • Cause DNA destruction by cleaving (chopping it up) or preventing its repair
  • Degrade proteins that keep the cytoskeleton (the cell's skeleton) intact
  • Inactivate or destroy anything that can inhibit (or stop) them.

Caspases exist in an inactive form called procaspase and sit inside the cell waiting to be activated. When a portion of the procaspase is removed, the caspase becomes active and is free to destroy cells or activate other caspases.

Activating Caspases

So how are caspases activated? The whole process of the apoptotic pathway is complex, but we'll review a few major mechanisms that activate caspases.

  1. One reason that caspases become activated is a lack of survival signals. Cells live in communities and support each other by sending out survival signals. If a cell doesn't get its dose of survival signal, it goes into apoptosis mode, activating caspases.
  2. Death receptors also activate caspases. Sounds ominous, doesn't it? Cells have receptors that can bind to proteins. Inside the cell, receptor/protein conglomerates known as a DISC (Death Inducing Signaling Complex) participate in binding procaspases. The binding initiates cutting of the procaspase and activation of the caspase.
  3. A third mechanism involves a protein called cytochrome c, found inside the mitochondria. Cytochrome c can bind to an adaptor protein and form a protein group called an apoptosome, which can free caspase from its procaspase form.
    One way that cytochrome c may be activated is through the transcription factor p53. If a cell incurs DNA damage, p53 can activate genes that induce cytochrome c release from the mitochondria.

Examples of Apoptosis

Now we know how, it's time to talk about why:

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