Apoptosis: How we die 300 million times every minute – And still survive

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When human babies are born, they weigh on average a mere 3.3kg, yet over the course of a lifetime, grow and develop into 60kg heavy adults. To the naked eye, it seems as though all we do as we age is grow, yet cell death plays just as vital a role in our development as does cell growth.

Programmed cell death, or apoptosis, is a very common occurrence within the body, and is a vital process which we need to survive. Apoptosis is a carefully controlled process of the cell, where it is broken down mechanically and methodically to ensure that it’s components can be easily used by other cells in a safe way. In essence, apoptosis is a process used to kill cells, which can be triggered due to many reasons.

Around 300 million cells die each minute from apoptosis, yet this is necessary to: allow for certain structures to develop (e.g. our hands from a block of tissue), destroy cells with damaged DNA (to prevent the onset of cancer), and to destroy infected cells and prevent the spread of infection. Without it, your hands would be webbed, frogs would have tails, and life would be unrecognisable as we know it. [1]

But how does apoptosis occur in cells? To answer this question, we could dive into the many little nuances and complexities of the human body, but for the purpose of keeping this as an article and not a research paper, I’ll try my best to explain this concept in a way that’s easy to understand.

Within our cells, there are enzymes present called caspases, which we can think of as really lazy people. They are usually asleep most of the time within our cells, yet when they are put out of their sleep, they are taken over by a fit of rage and devour everything around them. Once a single caspase is woken up, it wakes up all of it’s caspase friends and together, they go on a mission to destroy the cell – yet where this analogy breaks down is that they work in a very methodical and organised way to make sure that the components of the cell are able to be reused by other cells in the neighbourhood. This chain reaction is known as the ‘caspase cascade’, where each caspase is able to activate another, and so the process of apoptosis begins. [2]

The end result of apoptosis is a cell which has been split into many smaller pieces, much like a puzzle which has been disassembled, and each of these pieces, scientifically known as ‘blebs,’ is surrounded by a small membrane, which cells of the immune system are able to recognise and to put it simply, eat them. [3]

For those of you that are really keen on science, below is a more scientific outlook on the process of apoptosis.

Apoptosis can be triggered in one of two different ways: an extrinsic and intrinsic pathway. As the name suggests, an extrinsic pathway consists of an extracellular trigger, namely cells of the immune system, whereas an intrinsic pathway has an intracellular trigger, which triggers the process from the inside. [2]

In extrinsic pathways, T lymphocytes of the immune system bind to receptors of the cell on the surface of it’s plasma membrane, which triggers the caspase cascade and ultimately apoptosis.

In intrinsic pathways, the process is a little more complicated. In the membranes of our mitochondria (which are the organelles in our cells responsible for aerobic respiration), there exist 2 proteins – anti-apoptotic and pro-apoptotic proteins.

In a healthy cell, anti-apoptotic proteins bind to pro-apoptotic ones, inhibiting their action and the process of apoptosis. Yet when a cell stops receiving survival signals, or if irreparable damage is detected, the anti-apoptotic protein is blocked. It is no longer able to inhibit the pro-apoptotic proteins, which then proceed to drill holes through the membrane of the mitochondria, allowing certain chemicals to pass out into which trigger the caspase cascade and in turn the process of apoptosis.

Apoptosis plays a key role in our lives, and serves to be one of nature’s many ironies, as in order to live, we must first die. This death protects us and keeps us healthy, and when apoptosis is stopped, we see the emergence of nasty diseases such as cancer, which is the cause of death for 1 in 6 deaths across the globe. [4]


References

  1. ScienceLine (2012) “How many cells do we have in our body?” https://scienceline.ucsb.edu/getkey.php?key=3926
  2. Elvire Thouvenot-Nitzan (2016) “What is Apoptosis? The Apoptotic Pathways and the Caspase Cascade” Last accessed: 18 October 2016 https://www.youtube.com/watch?v=-vmtK-bAC5E
  3. Khan Academy (2016) “Apoptosis” https://www.khanacademy.org/science/biology/developmental-biology/apoptosis-in-development/a/apoptosis
  4. Hanna Ritchie (2018) “How many people in the world die from cancer” Last accessed: April 2019 https://ourworldindata.org/how-many-people-in-the-world-die-from-cancer

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