The paramecium can swim fast, overcome obstacles with ease, and turn in any direction – despite consisting only of a single cell. In contrast to "Paramecium aurelia", the human body’s cell count is not exactly known. Among scientists, it was long a subject of debate. According to a recent estimate published in 2013 in the "Annals of Human Biology" by an international team of researchers, it is 3.72 × 1013. In other words: The human body consists of some 37.2 trillion cells.
For a long time, scientific estimates of the number of cells in the human body ranged between 1012 and 1016. These calculations were not based on reliable empirical data. Even the title of the 2013 study "An estimation of the number of cells in the human body" points out that the authors' results are merely an approximation.
Researchers doing detective work
The researchers from Italy, Spain, and Greece first browsed several thousand scientific papers and books published from 1809 to 2012 that dealt with the quantity and size of the cells of various organs. The large range of figures found in the literature reflects the uncertainty in the scientific community: It was between 1012 and 1020.
But how were scientists to systematically determine the number of cells in the human body? It is impossible to count trillions of cells under the microscope. Some older calculations were based on the estimated average cell weight. Using the weight of one nanogram (1 billionth of a gram) as the basis of calculations, an adult weighing 70 kg should consist of 7x1013 (some 70 trillion) cells. If using the average cell size of 1/40th millimeter as a basis, an average human body should be made of only 1.63x1013 (some 16 trillion) cells.
Each cell type is different
The two approaches not only arrive at very different results. They also neglect the fact that the complex structure of the human body is composed of some 210 different cell types. Each of these cell types is present in the body in different quantities. In addition, they differ in size and weight.
At about 0.12 to 0.14 millimeters in diameter, the largest cell is the female egg cell. It is big enough to be just barely visible with the naked eye. The smallest human cells, on the other hand, are sperm – at least if you count the cell body without the flagellum. Sperm measure about 60 μm (0.06 millimeters) and are about 5 μm long and 3 μm wide including the flagellum. The reason for this significant difference is that, unlike sperm, an egg cell holds large amounts of cytoplasm and yolk-containing nutrients. However, sperm are ahead if you count the number of mature cells per month: on average there is one mature egg cell compared to 3,000,000,000 mature sperm cells each month. The longest cells in the human body are nerve cells. Their projection, the axon, can be up to one meter long.
The average man as the starting point
In its study, the international team of researchers considered the differences between cell types in terms of size, volume, and quantity. As the basis of their calculations, they used a 30-year-old average man, who is 1.72 m tall and weighs 70 kg. For this man, the researchers determined the number of blood cells, liver cells, bone cells, and skin cells as well as the cells of other organs. Finally, they added the numbers.
It is questionable whether this sum precisely reflects reality. The team therefore considers its contribution merely a basis for a new, systematic inventory of all human cells. Those who wonder about the benefits of knowing how many cells are in the human body can find the answer in the study's conclusion. This knowledge is not only of cultural importance but also benefits medical diagnostics. In addition, it could help improve our understanding of the development of cancer and other diseases.
A common effort
Now the researchers want to get down to details – and they are calling for a common effort to complete the total calculation of all cell types. Ideally, the authors are hoping the last step could be preparation of a forum paper and an online database resource summarizing the complete picture by integrating data from different expert contributors. This survey should remain open for corrections and the participation of other scientists.