Tuesday 10th November 9:28 am
How many cells in a person?
Tuesday 10th November 2015 9:28 am
It’s a surprisingly hard question to answer, but Dr Karl has tracked down a reasonable estimate of the number of cells in the human body.
It seems such a basic and deeply fundamental question, that we should definitely know the answer – “how many cells are in the human body?” And yet surprisingly, it was only in the last few years that we got even close to a reasonable estimate.
Let me give you an idea of what I mean by reasonable.
Going back over the last two centuries, the estimates for the number of cells in the human body have been in the trillions.
How many trillions? Anywhere between five trillion, and 200 million trillion! An estimate that has a range between 200 million down to just five is perhaps a little too loose to be of any use.
But first, some background to “cells”.
Way back in 1665, about three-and-a-half centuries ago, the British scientist, Robert Hooke, peered down an early compound microscope. He sketched the amazing detail he saw in some 60 objects.
One object was a very thin slice of cork – the rubbery stuff that is still sometimes used to seal bottles of wine. He saw thousands of tiny pores that he called “cells”, from the Latin word “cella”, which meant a small room, like the ones that monks lived in. Robert Hooke didn’t realize (at the time) that these empty “cells” had previously been filled with living tissue.
It took nearly two centuries before Theodor Schwann and Matthias Schleiden formulated what we now call “Cell Theory” – around 1838.
Today, modern Cell Theory is part of the bedrock of Biology. Cell Theory includes the following statements:
1. All living creatures are made of one or more cells. That covers the range from bacteria to the blue whale, the biggest creature we know of;
2. The cell is the fundamental unit of structure and function in all living creatures;
3. All living cells come from previous cells, which grew and then divided;
4. The overall activity of a creature depends on the total activity of all the individual cells;
5. Energy flows into cells and within cells, and can flow out of cells;
6. Cells contain hereditary or genetic information called DNA. This DNA is passed from one cell to its daughter cells; and finally
7. All cells in similar species have the same basic chemical composition.
That’s all very good, but how many cells are in your body?
So back in 2013, Dr Eva Bianconi from the University of Bologna in Italy, and colleagues, tried to answer this question.
They first searched for estimates in the medical and scientific literature. These estimates came from the last two centuries, back to the year 1809. The estimates were all in the trillions (and remember, a trillion is a million million).
While there was an outlier estimate of a colossal 200 million trillion cells in the human body, most of these estimates lay between 70,000 trillion and five trillion. That’s still a range of 14,000 to one.
So what the team did was to go back to basics. First, they defined the average human to be a 30 year-old male, 1.72 metres tall, weighing 70 kg, and with a surface area of 1.85 square metres. They then looked at individual organs or cell types in the body, and worked out how many cells were present – and then, just added up the numbers.
Of course, it was complicated
For example, for bone, it’s fairly easy to start with the total weight of the skeleton. But there are two different types of bone. So they came up with 700 million cells of trabecular bone tissue, but 1.1 billion cells of cortical bone tissue.
They started with adipose (or fat) tissue, and painstakingly worked their way through articular cartilages, the bilary system, blood, bone and so on until they eventually ended up at vessels (such as veins or arteries).
So in the blood vessels, there are about 80,000 kilometres of capillaries, with an average diameter of three-quarters of a millimetre. Blood vessels are lined by endothelial cells that are only about 60 microns (that’s a millionth of a metre) long, and 20 microns wide. That gives you about 2.5 trillion endothelial cells in the capillaries.
Adding all these numbers together gives a total of (drum roll, please) 37 trillion cells. Mind you, with further work, this will almost certainly be adjusted a bit – but hopefully, not by a factor of 14,000!
I do hope that Dr. Bianconi and her team didn’t need a stay in a padded cell, after being driven around the twist with all that endless cell counting
This blog first appeared on Dr Karl's Great Moments in Science
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