As kids, we all take falls and get tiny scrapes growing up. Try to look for these scrapes and they would have all recovered by now. Ever exercised too much and felt super sore the next day? That is the feeling of damaged muscles but even that fully heals given time. Our body can even regenerate our liver if we donate half of it. How does our body manage to recover from such grave wounds but is unable to recover from injuries to the spinal cord or eyeball? Let’s look at what is happening in our body at every moment of our lives.

Our body consists of many different types of cells, all with different roles. Our largest and most visible organ is the skin. The skin functions as the body’s first line of defence against the environment, protecting us from bacteria, the Sun’s radiation and other physical threats. As we go through our daily life, it will inevitably be damaged. Thus, our body will have to constantly repair and replace it to ensure we stay safe. It takes just about 4 to 8 weeks for the bottom layer of our skin, the basal cell layer, to push its way to the top layer of our skin, the epidermis, fully replacing itself. So where does the old skin go? Well, when you take a shower, you scrub it off. It also sheds off our body and joins the dust all around us!

What decides the longevity of our cells? Each cell in our body consists of a few basic components, the cell membrane, and numerous other organelles such as the nucleus and mitochondria. Depending on the makeup of the cell, it can self-repair and function for a longer time. For example, the heart muscle is constantly getting damaged from the strain of beating and pumping blood around our body. The self-repair mechanism allows our heart to function well despite the constant stress placed on it.

What happens when cells start getting old and need to be replaced? You may have seen pictures of the human chromosome before. At each end of the chromosome is a region known as the telomere. As the cell replicates itself, the telomere shortens with each division. When the telomere gets too short, the cell goes through a process known as senescence where the cell no longer replicates albeit still being metabolically active and functional. Telomere shortening thus limits the number of replications a cell can perform, which can contribute to aging and the limitation of our lifespans.

The skin is but one example of the many body parts that is constantly replacing itself. Our fastest replacing cells are generally the ones that need to face the harshest conditions. This includes our digestive system that uses acid to break down food, lungs that take in smoky air, and tongues that get scalded by hot food. They have a replacement rate starting from as little as 3 days!

Other body parts and cells can last quite a bit longer. Our fat cells have an average turnover of about 10 years while some others will last our lifetime. The female oocyte is an immature egg cell, as females reach puberty, a few slowly mature to become egg cells that are involved in reproduction. These immature eggs last about 50 years and are made during a foetus’ gestation in the womb, the human body does not make new ones as we age. The eye lens cells that refracts and allows light into our eyes last our entire lifetime, our body cannot replace or repair it, we should be very careful to make sure not to damage them!

As mentioned, the shortening of the telomere prevents us from living forever. This function is a fact of our body but what if there an exception to this rule, a line of cell that lives today as healthily as it did 70 years ago?

A specific line of cervical cancer cells, known as the HeLa cell line, does not appear to undergo telomere shortening; special due to its ability to endlessly replicate. The HeLa cell line comes from cervical cancer cells taken from a lady known as Henrietta Lacks in the 1950s. This is because the telomeres are active, whereby an enzyme copies the telomeres over and over again, leading to unlimited cell division and immortality. The cell line is commonly used by researchers and scientist in their studies to this day. This is very unlike other cell lines that eventually dies off.

With the help of her cells, over 50,000 scientific articles have been written, ranging from research on virus and cancer to genealogy and radiation. Her cells continually reproduce which provides an avenue to test on human cells before moving on to human testing. As the same cell line can be continually used for a given research, it provides researchers a baseline to work off, standardising an important component. Although she has passed away, her cells and legacy still live on to this day.

Now we know that as we age, our cells replace themselves and we become a whole new person. This begs the question, are you still you? The ship of Theseus is a thought experiment about identity and perfect for us to ponder upon this point.

Let’s say we bought a brand-new car. As it ages, we slowly replace the old components. Changing out the worn-out tires, broken windows, and faulty engine. Eventually, all parts of our car are replaced, and we have a perfectly new one. Would we consider it the same car as the one bought all those years ago? Apply this same concept to ourselves, if our bodies are constantly being replaced throughout life, does our identity change along with it? This philosophical question has no correct answer, it is up to us to individually come up with a conclusion.

Having learnt all these, do you think it is possible to live forever? What if our cells are modified to be like the HeLa cell line which has an overactive telomerase. Doesn’t that mean we would never grow old. Are there potential consequences for our bodies with such a big modification? Things are not as simple as they might look!

References
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002533
https://www.scientificamerican.com/article/our-bodies-replace-billions-of-cells-every-day/

Written by Oh Wei Sian
Illustrated by Lim Daphne