Carl Zimmer has an interesting article about attempts to estimate how many cells the average human body contains. It depends on how you do it and previous estimates over the past few centuries have ranged widely from two billion to 200 million trillion cells.
Now that we have more sophisticated tools of measurement, one way to do this is by using the average mass of a cell (which turns out to be about one nanogram) and this gives a figure of 70 trillion cells. Another is to use the average volume of a cell (about 4 billionth of a cubic centimeter) and this gives 15 trillion cells. But there are other things to consider as well.
So if you pick volume or weight, you get drastically different numbers. Making matters worse, our bodies are not packed with cells in a uniform way, like a jar full of jellybeans. Cells come in different sizes, and they grow in different densities. Look at a beaker of blood, for example, and you’ll find that the red blood cells are packed tight. If you used their density to estimate the cells in a human body, you’d come to a staggering 724 trillion cells. Skin cells, on the other hand, are so sparse that they’d give you a paltry estimate of 35 billion cells.
That’s pretty huge range. To narrow it down, they took into account the fact that cells in different parts of the body and different organs have different numbers, masses, volumes, and pack in different densities.
After trying to take all these things into account, they arrive at a figure of 37.2 trillion cells.
Does anything actually depend on knowing this number precisely?
Curiosity is justification enough to ponder how many cells the human body contains, but there can also be scientific benefits to pinning down the number too. Scientists are learning about the human body by building sophisticated computer models of lungs and hearts and other organs. If these models have ten times too many cells as real organs do, their results may veer wildly off the mark.
The number of cells in an organ also has bearing on some medical conditions. The authors of the new study find that a healthy liver has 240 billion cells in it, for example, but some studies on cirrhosis have found the disease organ have as few as 172 billion.
It is nice to be able to figure this number out even if it did not have any usefulness.
That’s pretty interesting. So it sounds like they scaled back a bit so to speak, starting with the relatively easy to quantify parts, like organs, blood, etc. Then trying to average out the number of cells in each?
Are they counting the non-human cells as well, that just happen to be inside of us? By some definitions, our gut biome is certainly “in the body”, even if it is not “of the body”… well, I guess by some views it is an active contributor to our life, so maybe it really is part of us.
Anyway, are they counting that?
Wait, this was the previous range of estimates, seriously? 11 fuggin orders of magnitude? That’s the difference between a tremor that rattles your china and 2012. Imagine the news reports: “Details on the damage caused by the quake are sketchy, but it appears that either a few dishes were smashed, or the North American continent sank into the mantle.”
At any rate, it’s nice to have a more accurate answer. And by accurate I mean “not orders of magnitude less accurate than getting wasted and making up a really big number.”
Well, I coulda told you it was greater than 2 billion cells. There are about 100 million cells in a mouse’s spleen, which is about the size of a kidney bean or a bit smaller. You can fit more than 20 kidney beans in the volume of a human.
Tens of trillions sounds like the right range. But you can’t figure it out by weight or volume alone, since some of those quantities is taken up by extracellular matrix and fluids.
We also carry about 2-4 pounds of bacteria in/on/around our bodies, which is said to number ten times the number of our own cells.
Apparently not. I have seen estimates that only about 10% of the cells by number in a human body are human cells (have human DNA). Bacteria are pretty small though, so the percentage by weight is much higher. Are gut bacteria “in the body”? Is a doughnut hole “in the doughnut”? We need a philosopher or a topographer for advice.
No, they are not counting the microbes, just the human cells.