How many stem cells do we have & can you run out?

Sometimes it’s useful to ask questions like, “How many stem cells are in the human body?”

As we try to answer them we end up going through key aspects of cell biology and stem cell identity more generally.

How many stem cells in the human body?

It might seem like possibly an easy question at first, but there’s much more to the answer than you might think.

One of the challenges here is that scientists aren’t even 100% sure how many cells in general of any type are in the human body. Estimates vary from 30-40 trillion, which seems more impressive when written out (like on the lower end): 30,000,000,000,000.

If we took a wild stab at the specific stem cell number and said that about 1 in 1,000 cells are of the stem variety then you might guess that there are 30 billion stem cells in the body. A rate of one in 10,000 would give us 3 billion, while a rate of one in 100,000 would give us 300 million stem cells overall.

So which is it?  I would estimate it’s somewhere between those last two numbers so there are between 300 million and 1 billion stem cells in the body.

But it’s much more complicated than some kind of general ratio and the numbers likely change throughout life.

Why is this such a hard number to pin down?

How many stem cells are in each type of organ?

First of all, every organ has different numbers and types of stem cells.

Bone marrow has a substantial number of blood or hematopoietic stem cells along with a few other cells that might be similar to like MSCs or mesenchymal cells. The stem cell abundance in the marrow is likely in part because the body’s blood cells turnover so fast. Also, we can rapidly need more blood cells if we get an infection, especially one that kills blood cells.

Similarly the intestine is a tissue with rapid cell turnover and so loads of stem cells. Billions of cells die every day in the gut so the abundant stem cells are there to keep up a supply of replacements.

Estimates vary on the number of blood stem cells, with a recent paper reporting a range of 50,000-200,000 in a healthy adult person.

Secondly, in terms of cell identity, there are also gray areas where we aren’t always sure whether a given cell is a stem cell or some other similar cell type like a progenitor cell.

So this gray area complicates the question of stem cell numbers too.

Making things more fun, it is also possible that some cells transition between being a stem cell and being another kind of cell (and maybe back again) over periods of days, weeks, or years. Illnesses or injuries may influence such transitions and stresses like hypoxia may temporarily increase the number of cells in a tissue.

Do some organs have no stem cells?

In adults some organs like the brain appear to have relatively few true stem cells.

The argument has been made that the adult human brain may have no real stem cell-based regenerative capacity, in which case functionally speaking there may be no stem cells at all there. I kind of doubt that though. Neuroscience research seems to ping pong back and forth between papers “proving” a stem cell population does exist in the adult human brain and papers that argue the opposite.

There is also controversy over heart stem cells. The best overall evidence to date suggests there may not be meaningful numbers of these cells in the adult human heart.

Conversely, some organs have great regenerative capacity probably driven via stem cell-like cells. For instance, the liver can readily regrow large portions of itself. This is thought to be mediated by general liver cells called hepatocytes, which under stress may act more like stem cells. However, again, some of this gets tricky since by definition stem cells can make more of themselves (which hepatocytes can do) and of other cell types, which is less clear for these liver cells.

Can we run out of stem cells?

Back around the time when I was writing my book Stem Cells: An Insider’s Guide, I came up with this idea that aging might in many cases be directly related to declining numbers of stem cells.

I’m sure that others have had the same or similar kinds of ideas, but at the time it felt somewhat original.

Conversely, this idea of aging being linked to stem cell numbers also fits with the audacious (and not very well supported by data) hope that stem cells may help fight aging somehow.

References

• Stem Cells: An Insider’s Guide, Paul Knoepfler.
• Lee-Six, H., Øbro, et al. (2018). Population dynamics of normal human blood inferred from somatic mutations. Nature.