Determines the number of human cells

Cell number and cells

Different human cell types (right) and relative proportion of the total cell number. © Ian A. Hatton

Cells form the basic unit of all life – including in our bodies. The range extends from the smallest, highly mobile blood cells to the muscle fibers in our thighs, which are millions of times larger. But how many cells do we have? And how are cell size and number related to the different cell types? A research team has now discovered this. Accordingly, an average man weighing around 70 kilograms consists of around 36 trillion cells, a woman weighing 60 kilograms consists of around 28 trillion cells and a ten-year-old child consists of around 17 trillion cells. Blood cells make up the largest proportion of the cell number, and skeletal muscle cells and fat cells make up the largest proportion by mass. The larger the cell size, the lower their number, as the team determined.

Whether it's a single-celled bacterium, a jellyfish or an elephant: they are all made up of cells, the basic units of life. Every cell contains the genetic material DNA and the cell machinery to carry out its cell metabolism and reproduce through cell division. In multicellular organisms like humans, the cells are specialized in different ways depending on the tissue and function - the range extends from the highly networked nerve cells of our brain to the highly mobile cells of our blood and immune system to the "hard workers" of our muscle fibers. Each cell type has its specific shape and fixed size range. “But despite this uniformity within a cell type, cell sizes in the human body vary over a massive seven orders of magnitude. “That is comparable to the mass range of a shrew to a blue whale,” explain Ian Hatton from the Max Planck Institute for Mathematics in the Natural Sciences in Leipzig and his colleagues. However, how many cells there are in the human body and how cell size, number and mass proportion behave has so far only been partially understood.

Blood cells have the largest number and muscle cells have the most mass

Hatton and his team have therefore compiled and evaluated data on more than 400 cell types in around 60 different human tissues and organs. Based on microscopy and tissue studies, they determined the size of these 1,264 cell groups, how many of them there are in the human body and what proportion of our total mass they make up. They carried out these analyzes using examples for a man weighing around 70 kilograms, a woman weighing around 60 kilograms and a ten-year-old child weighing around 32 kilograms. “For several types of tissue, our new analysis provides far greater resolution than was previously available, including for the muscle fiber size of all striated muscle groups, the neurons and glial cells of the central and peripheral nervous systems, and the blood cells found in various tissues,” explain the researchers.

The evaluations showed a clear connection between body size and weight and the number of cells for the total number of cells: a man weighing around 70 kilograms consists of around 36 trillion cells, while a woman who is ten kilograms lighter consists of 28 trillion individual cells. For the ten-year-old child, Hatton and his team determined an average cell count of around 17 trillion. This does not include the bacteria and other “roommates” that occur in our bodies. According to the analyses, by far the most common cells are red blood cells and platelets with around 29 trillion in adult men, followed by white blood cells with 3.4 trillion. On the other hand, if you look at the proportion of the different cell types in the total biomass of our body, the skeletal muscle cells are far ahead: for a man weighing 70 kilograms, they alone account for 21.5 kilograms. Next come the fat cells with around 13 kilograms. In a woman weighing 60 kilograms, the muscle cells make up around 14 kilograms and the fat cells make up around 18 kilograms, as the scientists determined.

Logarithmic relationship between size and number

“The data reveal a surprising, inverse relationship between cell number and cell size,” report Statton and his colleagues. The larger a cell is, the less common it is in the human body. Specifically, the researchers determined that the total number of cells in each of the 26 logarithmic size classes decreases by a factor of x-0.97 compared to the next. The number and quantity of cells in the human body are balanced in such a way that each size class contributes approximately the same amount to the total mass. “This size distribution appears to be common in nature,” explains the team. For example, the distribution of organisms in the oceans shows a very similar relationship between numbers and sizes: although marine bacteria and blue whales are worlds apart in terms of size, their respective size classes contribute approximately the same amount of biomass in the oceans. The evaluations by Hatton and his colleagues also showed that the range of variation in cell sizes is approximately the same within each size class.

According to the research team, these results suggest that there is a finely regulated balance in terms of cell numbers and sizes in the human body. Knowledge of this balance and the typical values ​​can in turn help to identify anomalies and pathological changes. “Our data serves to establish a holistic frame of reference for the cells in the human body, while also underscoring overarching patterns of cell biology,” said Hatton and his team.

Source: Ian Hatton (Max Planck Institute for Mathematics in the Natural Sciences, Leipzig) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2303077120

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