Above: Tadpoles lose their tails when they become frogs. That’s an example of apoptosis. Image © NicolasMcComber,iStockPhoto.com

Cells are like Lego blocks. You can use them to build different tissue structures, such as hearts or brains.

Sometimes, these structures can change. For example, when a tadpole metamorphoses (changes) into a frog, it loses its tail. The tail doesn’t magically disappear. It’s composed of many different cell types. Each of these cells remove themselves by initiating their own death until there is no more tail. It’s like popping bubbles!

This self-initiated cell death process is called apoptosis. Apoptosis, also sometimes called “programmed cell death” or “cell suicide,” plays many important roles in our bodies. Let’s explore three reasons why cells might remove themselves: metamorphosis, sickness, and restoring balance.

Did you know? Ten billion cells die every day in human adults through apoptosis!

Metamorphosis

Remember how I said that apoptosis plays an important role in the metamorphosis (transformation) of a tadpole to a frog? Specifically, the cells of the tadpole’s tail go through apoptosis. The result? No more tail!

Did you know? The metamorphosis of tadpoles into frogs is initiated by a single important molecule: the thyroid hormone. It signals to cells, like those in the tadpole’s tail, to undergo apoptosis.

Transformations like these don’t just happen in frogs. When you were an embryo, your hands and feet transformed. At first, they were basically blocks of tissues. But these blocks eventually separated into fingers and toes. What caused this transformation? You guessed it - apoptosis! Cells within these blocks of tissues died, creating gaps that helped distinguish each of your digits.

Sickness

Cells can get sick. Sometimes, these sick cells can repair themselves and function normally again. Other times, cells are too sick to repair themselves. In other words, they are irreversibly damaged. These cells will then undergo apoptosis.

Here’s an example: when you get a cold, cells infected by the cold virus will undergo apoptosis. This prevents the virus from spreading to the rest of your body. Otherwise, the virus would be able to reproduce within the living cell and then move on to infect your other cells.

Cells with DNA damage might also undergo apoptosis. For example, your cells might have DNA damage after a long day at the beach. That’s because your skin cells have been exposed to very strong, harmful UV rays from the sun. If the cell continues to live despite carrying damaged DNA, it might become a cancer cell and cause more problems! Sometimes, the DNA can be repaired. But if it can’t, these cells avoid the risk of cancer by going through apoptosis.

Before it initiates apoptosis, a cell will try its best to fight off damage. For example, if a specific organelle is damaged, then the cell might instead initiate autophagy, the breakdown of certain parts of a cell. The damaged portion is sent off to be broken down by lysosomes, and the rest of the cell continues to live.

Restoring Balance

Cells are constantly dying. At the same time, other cells are constantly being born. The balance between the number of cells that are born and the number of cells that die is important for maintaining the shape and size of your tissues.

Scientists showed this balance through experiments on adult rats. When scientists removed a piece of the rat’s liver, more liver cells were born to replace the removed cells. In contrast, when scientists injected the rats with a chemical that caused the birth of new liver cells, the liver grew. But when the science removed the chemical, a bunch of the rat’s liver cells died. The number of cells that died was the same as the number of cells born while the chemical was in the body. This caused the liver to return to its original size.

The Apoptotic Process

Apoptosis is a very tidy cell death process. A cell neatly begins to break itself down into smaller packages. These smaller packages then protrude out of the cells. (You can view these protrusions, or blebs, using a super-powerful electron microscope.) The packages are then eaten up by immune cells, a process called phagocytosis. These immune cells make sure that the dead cells are cleared up, leaving room for new cells.

Apoptotic Process
Above: Image © ttsz, iStockPhoto.com.

Did you know? There’s another form of cell death that can happen in your body: necrosis. That’s cell death due to injury - and it isn’t neat at all! For example, if you cut yourself, the cells will burst open, and their contents will spill out, leaving the body with a huge mess to deal with.

Apoptosis and Necrosis - Illustration
Above: Image © ttsz, iStockPhoto.com.

A cell’s decision to undergo apoptosis is very important. Once it commits to its decision, there’s no point of return. Because of this, there are many different molecules in the cell that help regulate a cell’s decision to die or not to die. Specifically, there are pro-apoptotic (pro-death) molecules and antiapoptotic (no-death) molecules. Signals from inside or outside the cell will then determine whether the pro- or anti-apoptotic molecules win.

For example, if there is extensive DNA damage, the pro-apoptotic molecules will outweigh the anti-apoptotic molecules. The cell is now committed to apoptosis. It will then activate a special set of proteins called caspases, which get the process started.

Summing up

Apoptosis gives your body ways to sculpt new organs, get rid of unhealthy cells, and balance dying and newly-born cells. Many molecules regulate the critical decision to undergo apoptosis. Once the decision has been made, the cell will then neatly begin breaking itself down into small packages. The immune system cleans these packages up.

And so the cycle continues, keeping you healthy!

Did you know? The word apoptosis comes from a Greek word meaning “falling off,” such as leaves falling off of a tree.

Learn more!

Apoptotic Pathways (2012) Genentech

Apoptosis (2016) Khan Academy

What is apoptosis? (2010) How Stuff Works

References

Programmed Cell Death (2002) Molecular Biology of the Cell

What is Apoptosis, and Why is it Important? (2001) British Medical Journal

Apoptosis (2017) Khan Academy

Apoptosis in Amphibian Organs during Metamorphosis (2010) Apoptosis: an international journal on programmed cell death

Autophagy: When your body eats itself (2016) CurioCity

Moushumi Nath

 I am a graduate student at the University of Toronto in the Department of Physiology. I study how the brain functions in learning and memory! This means I get to play with mice, visualize the brain, and listen in on how neurons communicate with each other. My interests in learning and memory began during my undergraduate degree at McGill University, where I completed a BSc in Honours Neuroscience. I look forward to continuing to engage in the fields of science communication and science policy. Sidenote: I am an avid free-food scavenger.






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