Billions of cells in your body die every day. Some go out with a bang, others with a whimper.
They can die accidentally if injured or infected. Alternatively, if they outlive their natural lifespan or begin to fail, they can carefully arrange a desirable demise, with their remains neatly disposed of.
Originally, scientists thought these were the only two ways an animal cell could die: accidentally or by that neat and tidy version. But in recent decades, researchers have collected many more new cell death scenarios, some of which are specific to certain cell types or situations. Understanding this panoply of death modes could help scientists save good cells and kill bad ones, which could lead to treatments for infections, autoimmune diseases and cancer.
“There are a lot of different flavors here,” says Michael Overholtzer, a cell biologist at Memorial Sloan Kettering Cancer Center in New York. He estimates that there are now more than 20 different names to describe cell death varieties.
Here, Knowable Magazine profiles a handful of classic and new modes that break the camel's back for cells.
Unplanned cell death: necrosis
Many bad things can happen to cells: they become injured or burned, poisoned or deprived of oxygen, infected by microbes, or otherwise sickened. When a cell accidentally dies, it is called necrosis.
There are several types of necrosis, but none are pretty: In the case of gangrene, when cells run out of blood, the cells rot. In other cases, dying cells liquefy and sometimes turn into yellow goop. Lung cells damaged by tuberculosis become mushy and white – the technical name for this type, 'cheesy' necrosis, literally means 'cheesy'.
Any form of death, other than necrosis, is considered 'programmed', meaning it is deliberately carried out by the cell because it has been damaged or has outlived its usefulness.
A good, clean death: Apoptosis
The two main categories of programmed cell death are “silent and violent,” says Thirumala-Devi Kanneganti, an immunologist at St. Jude Children's Research Hospital in Memphis, Tennessee. Apoptosis, first mentioned in 1972, is the original silent type: it's a neat, clean form of cell death that doesn't wake up the immune system.
This is useful when cells are damaged or have served their purpose. Apoptosis allows tadpoles to discard tail cells when they become frogs, for example, or human embryos can discard the tissue between developing fingers.
The cell shrinks and separates from its neighbors. Genetic material in the nucleus breaks down into pieces that scramble together, and the nucleus itself fragments. The membrane bubbles and puffs, and the cell disintegrates. Other cells gobble up the pieces, keeping the tissue tidy.
