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PH-FD02

Apoptosis and necrosis

Two ways a cell dies: apoptosis, a controlled shrinking that removes cells quietly, and necrosis, a swelling and bursting that inflames the tissue, with the patterns necrosis takes in different organs.

By the end, you can

  • Explain why healthy tissues need apoptosis and give examples of it in normal life.
  • Describe the intrinsic and extrinsic pathways that activate caspases.
  • Compare the cellular changes of apoptosis and necrosis, and the tissue response to each.
  • Relate the main patterns of necrosis (coagulative, liquefactive, caseous, fat) to their settings.
  • Explain how too little or too much apoptosis contributes to disease.
  • Recognise the classic appearances of each pattern of necrosis and of apoptotic cells, and explain how restoring flow, removing dead tissue and cancer drugs act on cell death.

What it is

Apoptosis is programmed cell death: an energy-requiring sequence, run by enzymes called caspases, in which a cell shrinks and breaks into membrane-bound pieces that are eaten without inflammation. Necrosis is death after severe injury, in which the cell swells, its membranes rupture and its contents leak, provoking inflammation. Necrosis takes different patterns depending on the tissue and the cause.

3 types to tell apart
Intrinsic (mitochondrial) apoptosis
Triggered from inside, by DNA damage, stress or loss of survival signals; mitochondria release cytochrome c, which activates caspase-9.
Extrinsic (death-receptor) apoptosis
Triggered from outside, when a ligand such as Fas ligand binds a death receptor on the cell; caspase-8 is activated.
Necrosis
Unregulated death after severe injury such as ischaemia, toxins or burns; it affects sheets of neighbouring cells and inflames the tissue.

Normal function

Cells die every day, by design

Healthy tissues balance cell division with cell death. The lining of the small intestine is a clear example: stem cells in the (crypts) divide, their daughters climb the , and old cells are shed at the tips within a few days, largely by apoptosis.

Apoptosis also shapes and protects the body. It removes the webbing between developing fingers, deletes lymphocytes that would attack the body’s own tissues, and eliminates cells with DNA damage that cannot be repaired. Cytotoxic T cells use it to kill virus-infected cells.

Because apoptosis is quiet and quick, it is rarely seen under the microscope: a dying cell is eaten within an hour or so, without inflammation.

Point to it on the model