5.6B: Cell Signaling and Cell Death

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When a cell is damaged, unnecessary, or dangerous to an organism, a cell can initiate the mechanism for cell death known as apoptosis.

Learning Objectives

Describe how apoptosis is initiated

Key Points

Apoptosis allows a cell to die in a controlled manner by preventing the release of damaging molecules from inside the cell.
Internal checkpoints to monitor a cell’s health exist; if abnormalities are observed, a cell can also spontaneously initiate the process of apoptosis.
In some cases, such as a viral infection or cancer, the cell’s normal checks and balances fail.
External signaling can also initiate apoptosis.
Apoptosis is also essential for normal embryological development; unnecessary cells that appear during the early stages of development will eventually be eliminated through cell signaling.

Key Terms

apoptosis: a process of programmed cell death
glycoprotein: a protein with covalently-bonded carbohydrates

Apoptosis

When a cell is damaged, superfluous, or potentially dangerous to an organism, a cell can initiate a mechanism to trigger programmed cell death, or apoptosis. Apoptosis allows a cell to die in a controlled manner that prevents the release of potentially damaging molecules from inside the cell.

Internal Signaling

There are many internal checkpoints that monitor a cell’s health; if abnormalities are observed, a cell can spontaneously initiate the process of apoptosis. However, in some cases such as a viral infection or uncontrolled cell division due to cancer, the cell’s normal checks and balances fail.

External Signaling

Apoptosis: The histological section of a foot of a 15-day-old mouse embryo, visualized using light microscopy, reveals areas of tissue between the toes which apoptosis will eliminate before the mouse reaches its full gestational age at 27 days.

External signaling can also initiate apoptosis. For example, most normal animal cells have receptors that interact with the extracellular matrix, a network of glycoproteins that provides structural support for cells in an organism. The binding of cellular receptors to the extracellular matrix initiates a signaling cascade within the cell. However, if the cell moves away from the extracellular matrix, the signaling ceases, and the cell undergoes apoptosis. This system keeps cells from traveling through the body and proliferating out of control, as happens with tumor cells that metastasize.

Another example of external signaling that leads to apoptosis occurs in T-cell development. T-cells are immune cells that bind to foreign macromolecules and particles, targeting them for destruction by the immune system. Normally, T-cells do not target “self” proteins (those of their own organism), a process that can lead to autoimmune diseases. In order to develop the ability to discriminate between self and non-self, immature T-cells undergo screening to determine whether they bind to so-called self proteins. If the T-cell receptor binds to self proteins, the cell initiates apoptosis to remove the potentially dangerous cell.

Apoptosis and Embryos

Apoptosis is also essential for normal embryological development. In vertebrates, for example, early stages of development include the formation of web-like tissue between individual fingers and toes. During the course of normal development, these unnecessary cells must be eliminated, enabling fully separated fingers and toes to form. A cell signaling mechanism triggers apoptosis, which destroys the cells between the developing digits.

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