Where do senescent cells come from, and why does our body create them?
As over time and as you get older, you accumulate DNA damage in your cells.
DNA damage can happen in a number of different ways, such as radiation and environmental factors, but can also happen naturally through normal cell growth and division.
This DNA damage accumulates and causes what’s known as senescent cells. Senescence means that the cells are old and have undergone aging at the cellular level.
But some cells don’t experience senescence and seem to be able to undergo healthy division almost indefinitely.
Let’s examine the differences between cells that experience senescence and those that don’t.
Somatic Cells: telomere shortening and cell division
In human bodies, our cells divide 2 trillion times per day.. that’s 2,000,000,000,000. 
As somatic cells divide, the caps at the end of our cells (called telomeres) slowly lose information after each subsequent cell division.
When telomeres become too short, the cells stops dividing.
Telomere shortening and other types of DNA damage results in these cells becoming senescent – which means they autonomously stop dividing.
Doing so helps prevent DNA damage from continuing to happen, and being carried over into future child cells. 
Our somatic cells can divide between 60-70 times before the telomeres get too short (called the Hayflick limit).
Stem cells: telomerase and DNA repair
Although somatic cells undergo this natural progression towards cellular senescence after a certain number of divisions, not all cells in the body go through these stages.
Stem cells are special – they don’t experience cellular senscence.
Stem cells are able to maintain their mitotic capacity (ability to divide) and avoid senescence because they have a special enzyme called telomerase.
Telomerase helps cells “stay young” and avoid the Hayflick limit because it is able to rejuvenate the lost ends of telomeres after dividing. 
Humans have stem cells in all different types of tissue and organs throughout our bodies.
Why don’t somatic cells produce telomerase?
In regular cell function, telomerase is specific to stem cells, and somatic cells are not supposed to produce telomerase.
However, because our body creates 100 billion new cells every single day, mutations can happen where a somatic cell will produce telomerase. This is actually a BAD mutation!
And while telomerase might sound like the key to keep our cells young, unfortunately, its more complicated than that.
Telomerase production in somatic cells results in uncontrollable cell division and growth, which causes cancer.
Even at an extremely low rate of mutation, the high number of cells means more opportunities for cancer to form.
Problems caused by senescent cells
Similar to brain neurons and heart muscle cells, senescent cells don’t divide. They shut down and stop copying DNA and dividing.
Somatic cells undergo senescence to stop dividing and to avoid becoming cancerous. And as humans age, we have more senescent cells in our bodies.
Although good for the prevention of cancer, too many senescent cells is a detriment to human health.
Senescent cells secrete proteins and molecules that cause inflammation. (senescence associated secretory phenotype, or SASP).
Remember, inflammation is BAD. Due to the SASP factors from senescent cells increasing inflammation in our bodies, we experience debilitation and disease at increased rates.
So although we want our somatic cells to avoid growing uncontrollably and causing cancer, we also want to avoid the presence of senescent cells if at all possible.
The future of cellular aging
Scientists are looking into ways to remove senescent cells to improve health. Senolytic drugs, for example, are built to kill these cells.
How do you think the future of cellular aging will look?
Tweet @espressoinsight and let us know what ideas you have.
- The Hallmarks of Aging https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3836174/
- SENS Overview of Cell Senescence https://www.sens.org/overview-of-cell-senescence/
- Telomeres and cellular senescence https://www.youtube.com/watch?v=R5YiO6rKr-w
- Cell division https://askabiologist.asu.edu/cell-division#:~:text=Organisms%20grow%20because%20cells%20are,trillion%20cells%20divide%20every%20day.