We’ve discussed how cells can grow and divide through the cell cycle and a process called mitosis. Equally as important to cells growing and dividing is the ability of cells to die. Why do cells have to die? During human development cell death is necessary. For example, in the womb fingers and toes are attached to one another by a webbing made of cells. During development, these cells die so that your fingers and toes are separate. A great non-human example of cells dying during development is the metamorphosis of a tadpole into a frog. The cells of the tadpole tail die to make a mature frog that does not have a tail. As an adult, your hair, skin, gut and other cells constantly divide. In fact ~ 60 billion cells are made each day. Imagine if cells didn’t also die each day – you’d be ENORMOUS!
This cell suicide is called apoptosis (pronounced a-pah-toe-sis) after the Greek meaning “dropping off ” or ” falling off ” of petals from flowers, or leaves from tree. Apoptosis is a mechanism that every single cell in your body has to commit suicide. Why in the world would this mechanism exist? In part, it exists for the reasons mentioned above – to remove unneeded tissue during development or to balance out cell growth. But apoptosis also provides a fail safe for cells to remove themselves if they become damaged so that they don’t damage the rest of the organism.
So how does apoptosis work? Obviously your cells don’t just kill themselves willy nilly. The cell must receive a trigger the initiates the process. These triggers can either come from inside or outside the cell. For example, UV light from outside of the cell can trigger damage to the DNA. If this damage isn’t repaired, it will start the process of apoptosis. As another example, when cancer cells are treated with chemotherapy, this often damages the DNA or messes with the cell cycle so much that it triggers apoptosis. Once triggered, proteins are activated that act like protein scissors, cutting up proteins and DNA inside the cell. This does a few things: it shuts down activity within a cell and makes the pieces of the cell smaller so that they can be packaged up and thrown away. It’s like a kitchen demo (or any kind of demolition) – you knock down the cabinets with a sledgehammer so that they don’t work to hold your dishes anymore and then break them into small enough pieces that they can easily be thrown in the dumpster. Once cell pieces are broken down, the cell packages up the contents (called blebs – see picture at right) and these blebs are eaten (actually, they are absorbed…but “eating blebs” is more fun to say) by neighboring cells. What’s so awesome about this process is that no trace of the cell is left. It’s a clean suicide that leaves no trace of the body behind. Why is this important? We can compare apoptosis to another type of cell death called necrosis. If you cut your arm, cells die by necrosis and they spill their contents everywhere. When this happens, your arm can get inflamed and this inflammatory reaction can be bad for you. During apoptosis, since everything is cleaned up nice and neat, there is no inflammation and the body can just move along as if nothing happened.
Now what if the trigger is defective or the machinery is broken and cells don’t die when they are supposed to? This is one of the causes of cancer. Of course cancer is a result of too many cells, but this can either be from cells growing too fast OR from cells not dying when they are supposed to OR a combination of both. On the other hand, what if too many cells die when they aren’t supposed to? This can cause the neurodegeneration found in Alzheimer’s disease or the loss of immune cells in HIV/AIDS infection. Therefore, understanding apoptosis and the exact way that cells die can help scientists to induce cell suicide (e.g., to kill cancer cells) or prevent it when needed.