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Chapter 9 · Part 9 of 10
  1. Chapter overview
  2. 1Why cells communicate
  3. 2The three stages of cell signaling
  4. 3How receptors recognize signals
  5. 4Four kinds of receptors
  6. 5How a growth factor tells a cell to divide
  7. 6Second messengers and the fight-or-flight response
  8. 7Amplifying, ending, and combining signals
  9. 8One hormone, many responses
  10. 9Programmed cell death
  11. 10How apoptosis is triggered

Programmed cell death

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Over the next hour, more than 2 billion cells in your body will deliberately destroy themselves. That’s normal, and it keeps you healthy.

A cell that takes itself apart

A cell can dismantle itself in an orderly way, following an internal program. This is programmed cell death, or apoptosis. Signals from other cells, the loss of signals a cell needs, or damage to the cell can set the program in motion. An adult human loses roughly 50–70 billion cells this way every day.

In an animal cell, the process runs in a set order:

  1. The cell destroys its own nucleus and cytoskeleton (the protein fibers that give it shape) from within, so it shrinks and becomes rounder.
  2. The plasma membrane bulges out into irregular extensions.
  3. The extensions break off as small fragments of the cell, called blebs, until the whole cell has come apart.

Apoptosis in health and disease

Apoptosis is part of normal life, not a malfunction. It has three main jobs:

The amount has to be right, though. Many diseases involve apoptosis happening too little or too much.

Apoptosis level Disease What goes wrong
Too little Cancer, including some prostate and ovarian cancers Cells that should die survive, so the cancer cells multiply faster
Too much Some viral diseases, such as HIV infection HIV raises the rate of apoptosis in helper T cells, a type of immune cell
Too much Some neurodegenerative diseases, such as Parkinson’s disease Nerve cells that release dopamine die at an unusually high rate

Hormones can order apoptosis

Early evidence that cell death follows a program came from studies of rat adrenal glands, the small glands that sit on top of the kidneys. The researchers, Kerr, Wyllie, and Currie, also coined the word apoptosis.

Two facts were already known. The hormone ACTH increases the number of cells in the adrenal cortex, the gland’s outer layer. The drug prednisolone lowers ACTH production and reduces the number of cells in the cortex.

The researchers injected groups of rats with saline (salt water, the control), prednisolone, prednisolone plus ACTH, or ACTH alone. Two days later, they examined the adrenal cortex under a microscope.

Injection Extra apoptosis? Why
Saline (control) No, only an occasional dying cell ACTH levels are normal
Prednisolone Yes, in 9 of 10 glands Prednisolone lowered ACTH
Prednisolone + ACTH No Injected ACTH replaced what was lost
ACTH alone No; apoptosis expected to be lowest of all The rats’ own ACTH plus the injected ACTH

The conclusion: ACTH keeps adrenal cortex cells alive. When ACTH falls, the cells undergo apoptosis, shrinking and forming blebs as they die. So hormones can control apoptosis, and a cell’s death can follow a program triggered by a change in its signals.

Check yourselfIn the rat experiment, why did adding ACTH to the prednisolone injections prevent the extra apoptosis?

Prednisolone triggers apoptosis by lowering the rats’ own ACTH. The injected ACTH replaced what was missing, so the adrenal cortex cells still received the hormone that keeps them alive.

Practice questions

  1. 1 of 2

    A human embryo’s fingers start out joined by webbing. What role does apoptosis play in shaping the hand?

  2. 2 of 2

    In rat experiments, prednisolone alone caused extensive apoptosis in the adrenal cortex, while prednisolone plus ACTH left apoptosis at normal levels. What does this suggest?