<center>![Picture]( https://cdn.pixabay.com/photo/2017/10/15/00/02/eye-color-change-2852261_960_720.jpg)</center>

>***”Don’t you know? The cells in your body replace themselves all the time! In 7 years, you’ll be a whole different person than you are right now.”***

I’ve heard and read this more than once, I even believed it for a while. And why not? It seems kind of logical. We know, that our cells replace themselves on a regular basis. Seems only logical that at one point, they’ll all be replaced at least once. So we’re a new person at 7 years old, 14, 21, …

But wait, do we start counting at birth? Or at conception? From the moment a specific organ is formed? All organs are formed? When do the seven years begin? 

This alone already makes the statement a bit weird. But let’s look at some examples for cell lifespans in your body, shall we? I dug out some for you.

Blood, let’s start with blood. According to what I found (source linked, as always, at the end), the mean lifespan of a red blood cell is about 115 days. But a “mean value” doesn’t give you the specific age for each cell, can live longer or shorter. And in fact, they do. The survival rates for red blood cells range from 70 to about 140 days. <sub><sub><sub><sub><sub><sub><sub><sub><sub><sub>@suesa</sub></sub></sub></sub></sub></sub></sub></sub></sub></sub>
Let’s assume all red blood cells survive 140 days. That would mean in seven years, they’re completely switched out roughly 18 times. Not too shabby, certainly doesn’t disprove the statement.

<center>![](https://cdn.pixabay.com/photo/2013/01/18/06/56/blood-75302_960_720.jpg)</center>

How about skin? What do you think, how long does your skin take to replace itself? A week? Two weeks? A month?

Epidermal cells, the ones that make up the outer layer of your skin, are replaced roughly every 40 to 56 days. That means that, in 7 years, you switch them out about 45 times. That’s even more frequent than the blood cells! Should we correct the 7 years “new body” time period? Make it shorter?

Your [immune system]( https://steemit.com/science/@suesa/innate-vs-adaptive-immune-system-why-there-is-no-story-today) kind of disagrees.

T cells, which are important for immune response, mostly live for about 2 weeks after they’ve been activated (= reacted to a pathogen). I’m saying *mostly* as that’s not the fate of every T cell. The memory T cells, those who remember what the pathogen looked like and what’s required to fight it again, can live several years.

How many years *exactly* depends. Is it 7? Maybe in some cases, but it can be a lot longer.

And did you ever wonder how your *bones* fit into all of this? Sure, their matrix (the part that isn’t cells) is made from mineral crystals and other things, but there *are* cells that synthesize that matrix, it doesn’t appear out of nothing.

Synthesizing it is the job of *osteoblasts*, which regulate the growth and destruction of bone. After 200 days, most of them (60-80%) die. Some of the surviving cells are then embedded into the minerals of the bone (then called osteocytes) and survive for … 1 to 50 **years**.

<center>![](https://cdn.pixabay.com/photo/2017/04/23/05/43/knee-2253047_960_720.jpg)</center>

Okay, you won’t have a completely new body after 7 years. But surely, you’re a completely new person? Your personality has changed, *you’ve* changed!

Except … not really. It depends on how you define “you”, because the neurons (the cells that make up the brain and are not [glial cells]( https://steemit.com/science/@suesa/the-10-myth-and-what-it-implies). They’re what makes us *think*. And while the connections between change over time and as we learn, the cells itself don’t - as long as there isn’t some kind of injury or sickness.

Neurons don’t age through replication. Normally, cells replicate and changes happen in their replicated genomes (mutations, shortening of telomeres, those things). This causes them to “age” and at one point, stop replicating. One reason for this halt in replication is to avoid cancer. Cancer cells have accumulated a lot of mutations over time and at one point, start proliferating in a way they’re not supposed to.

A neuron cell can, in theory, live as long as the rest of the organism. Interestingly, it even seems like neurons can live *longer* than the organism they belong to!

Researchers have implanted neurons from one mouse into a younger mouse, that outlived the first. And the neurons just adapted and kept on living their life.

Now that we’ve cleared up the myth, one question remains, at least for me: Where did it come from? And why, out of all time spans, 7 years?

The first part of the question I kind of answered in the beginning. People know their cells are switched out regularly, which probably lead, at one point, to the conclusion, that *all* your cells will have been replaced.

<center>![](https://cdn.pixabay.com/photo/2018/01/14/23/25/wood-mouse-3082871_960_720.jpg)</center>

But why 7 years?

Have you ever noticed, that the number seven seems to pop up everywhere, especially in religion? The seven days of creation in the Bible, the seven hells and heavens in Islam, the seven days of mourning in Judaism and the seven Chakras in Hinduism. Seven seems to have always been associated with spirituality of some sort.

Not surprising then that it has been chosen for a myth that promises you a new start every seven years.

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##### Sources:

[Measurement of Red Cell Lifespan and Aging]( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3678251/)

[Cell Division and Cancer]( https://www.nature.com/scitable/topicpage/cell-division-and-cancer-14046590)

[Aging and Bone]( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2991386/)

[Lifespan of neurons is uncoupled from organismal lifespan]( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3600460/)

[Extending the lifespan and efficacies of immune cells used in adoptive transfer for cancer immunotherapies–A review]( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4489929/)

[Making an epidermis]( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2861991/)

[7]( https://en.wikipedia.org/wiki/7#Religion_and_mythology)

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*Pictures taken from pixabay.com*

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