A question I get asked almost every year in one class or another is "How many cells are in me?"
At first I answered with "a lot", but then did a quick google search, found the answer "10 trillion" somewhere and quickly that became my stock answer.
10 trillion cells, 10^13 cells, 10 000 000 000 000 cells
It was a nice big number, it answered the question, it wowed my students. All good, oh and I'd read it somewhere, so it must be right.
Right?
Then a couple of weeks ago a friend emailed me a biological curiosity which, as part of its preamble, mentioned that humans had
100 trillion, 10^14, 100 000 000 000 000 cells.
Well that was clearly wrong. Right? I had read somewhere that it was 10 trillion, and this jumped up "somewhere else" was now telling me it was 100 trillion. How audacious. My hackles were raised. For the love of decent, multicellular Homo sapiens everywhere, how could they make such an outrageous claim! I mean if there is one thing I should be able to count on is an unconfirmed, unchecked, human cell count? Right?
Hrumph....
So in high dudgeon I decided to re-find my original reliable 10 trillion count, and find a good old fashioned sense of smug conceit as I laugh at the foolish inaccuracies of yet another website claim.
Aaah,
there was my problem.
I did find my 10 trillion statement eventually, and a whole bunch of other numbers too.....
None gave references, none explained where they had got there numbers from.
Oh dear.
But then I found a blog pointing to some research published by scientists in the Italian city of Bologna
thank you Bologna!
You did not agree with my 10 trillion (which I promise I will use say again), but you did work out an answer.
That's right - work out. Not guess, imagine or make up!
Thank you!
Eva Bianconi and colleagues at the University of Bologna have carefully, meticulously calculated (with error bars), a cell number. Not the exact number - that may never happen, but the best (and it seems, by far the most data driven) estimate so far.
They first did an amazingly detailed search through publications, databases and websites from as long ago as 1809 (publication not website) and found an amazing range of guestimates - from 5 trillion to 10 000 trillion!, with, admittedly, the most around my 10 trillion range.
Clearly some sorting out needed to be done.
They then looked at both taking average cell masses and dividing them into a human's body mass (70 trillion cells) and average cell volume and divide into a human's volumes (around 15 trillion cells) . These are pretty impressively different numbers - suggesting there is a problem with at least one of these methods - and this is probably due to the fact that these "average" cell sizes/masses do not accurately take into account the proportion of each cell size/mass in a body.
So our Italian researchers painstakingly calculated the average cell size for pretty much every organ and tissue they could and then that organ/tissue's volume. And carefully added up their separate cell numbers to get their new (and greatly improved) estimate.
So here it is 37.2 trillion cells!!!!!
And this is probably lower than the actual number - as the researchers point out that not every tissue has been counted yet!
So I wasn't to far off - only 27 trillion-ish - hardly anything really....
Disclaimer Part 2: Were there were no pictures in this fun fact. If this upsets you look below for a picture of the beautiful city of Bologna