In an effort that began more than ten years ago, scientists have estimated just how many cells make up the human body: Using data from more than 1,500 published sources, researchers determined that men are composed of about 36 trillion cells, women have some 28 trillion cells, and a 10-year-old child has roughly 17 trillion cells.
What’s more, these cells follow a mathematical pattern based on size and mass that appears elsewhere in nature. The new data revealed that if a human’s cells of similar sizes are grouped together, each group contributes about the same total amount of mass to the body. The international team of researchers published its results Monday in Proceedings of the National Academy of Sciences.
“The key was looking for papers that described the number of cells in different tissues,” co-author Eric Galbraith, an ecologist at McGill University in Canada, tells New Scientist’s Jason Arunn Murugesu. “And then knowing that those kinds of tissue were made up of particular cells and knowing what the size range of those cells were.”
Human cells, rather than all being roughly the same size, vary widely. The researchers estimated cell mass, size range and cell count for 1,200 cell groups, which can further be broken down into 400 cell types across 60 different tissues. They found that smaller cells, such as blood cells, are more common in our bodies, while larger ones, like muscle cells, are less plentiful. But the amount of matter that makes up cells in each size category is largely constant.
Similar size distributions can be seen across scientific fields. In ecology, for example, marine organisms from bacteria to whales display the same pattern. If the creatures are sorted by size along a logarithmic scale—where the values on each axis get exponentially larger—smaller organisms are more common than larger ones, but the total mass of organisms in each category mostly stays consistent for all size classes, writes Science News’ Darren Incorvaia.
The data parallels Zipf’s law, a frequency pattern recorded in city size, asteroids and even linguistics. For instance, a small variety of short words (such as “the”) show up frequently in the average book, while longer words are less common.
This wide-ranging pattern suggests there may be “some deep underlying mechanism that could be common to all these different things,” lead author Ian Hatton, a biologist with the Max Planck Institute for Mathematics in the Sciences in Germany and at McGill University, tells Science News. But the team doesn’t know what that mechanism is: “We’re not there yet.”
Knowing more about cell frequencies in the body could be critical to understanding human health and diseases—especially cancer, which occurs when cell numbers are thrown off balance by rapid division, researchers tell Science News. The team also estimated humans have about four times more total lymphocytes in our bodies than previously thought. Lymphocytes are white blood cells that aid in fighting off infection and disease.
Still, the study based its numbers on previous research that was largely focused on men, so the cell counts come with some uncertainty, especially the estimates for women and children. This new research was based on three distinct anatomical models: a 155-pound man, a 130-pound woman and a 70-pound child.
“It is just fascinating from a pure science perspective to have some sort of quantification of cellular diversity in the human body,” John Runions, at Oxford Brookes University in England, tells New Scientist. “When I lecture students about cell biology and development, I say something like, ‘We all start as a single fertilized cell, the zygote, which undergoes successive rounds of cell division accompanied by differentiation to produce an adult organism with X cells.’”
“The X has always been the tricky part,” he tells the publication. “I am delighted that my statement of cell number can now be at least in the correct order of magnitude.”