Final answer:
Apoptosis is a programmed cell death mechanism triggered by cellular damage or abnormalities. It allows cells to die in a controlled manner, preventing the release of harmful molecules. Internal and external signals can initiate apoptosis, such as in amphibian metamorphosis and immune cell development.
Step-by-step explanation:
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 checkpoints monitor a cell's health, and if abnormalities are observed, apoptosis is initiated. External signaling or signals mediated by specific molecules triggers apoptosis.
For example, in amphibian metamorphosis, thyroid hormone signals tadpole metamorphosis, causing tail cell digestion and nutrient reabsorption. Another example is the immune system's screening of immature T-cells to determine self vs. non-self targeting; T-cells that bind to self proteins initiate apoptosis to remove potentially dangerous cells.