How many human cells are there in our body, on average?

Question

How many human cells are there in our body, on average?

Wikipedia says 10¹³:

Bacterial cells are much smaller than human cells, and there are at least ten times as many bacteria as human cells in the body (approximately 10¹⁴ versus 10¹³).

The Nobel site says 10¹⁴:

An adult human being has approximately 100 000 billion cells, all originating from a single cell, the fertilized egg cell.

The Physics fact book insists it's 10¹³:

(length of 1 bp)(number of bp per cell)(number of cells in the body)
(0.34 × 10^-9 m)(6 × 10⁹)(10¹³)

Finally, Wolfram Alpha gives 1.0 × 10¹⁴ as fact:

Estimated number of eukaryotic (human) cells in the human body: 1.0×10¹⁴

I am quite confused: who is right here?

Answer 1

Bianconi et al. 2013 give an estimated lower bound of 3.72 × 10^13 (which, by the way, is approximately the geometric mean of 10^13 and 10^14).

However, from the table in their Supplemental Information (where estimates for about fifty different types of cells are added up), it is clear that the vast majority of these are the erythrocytes, also known as red blood cells: estimated at 2.63 × 10^13. While the second largest class are the glial cells (from the nervous system), estimated at 0.30 × 10^13. Three other classes (endothelial cells {vessels}, dermal fibroblasts {skin}, and platelets {blood}) add up to another 0.58 × 10^13. These, in total, is about 3.5 × 10^13, and we can stop there, because the uncertainty in all these estimates swamps all the other cell type populations, which are much smaller.

This means that if we only consider the 5 most abundant cell types in the body, we already get a very good estimate. Is it likely that we have missed a cell type of which there are a trillion cells in the body? It's not impossible, but unlikely, because it would be hard to miss all these cells.

Maybe there is a long tail (i.e. a large number of minor cell types, each with a small, but significant population) that wasn't considered? Together, these could add up to a significant adjustment to the above figure. I don't know.

This estimate is for a typical human person: 30 year old young adult, 70 kg, 1.72 m tall and with a surface area of 1.85 m^2. For this person, the figure is given with roughly a +-20% uncertainty.

Given all the above, it's probably safe to say "30 to 50 trillion".

(This obviously doesn't take into account any of the bacteria mentioned above.)

Answer 2

In Mike Lynch's "The Origins of Genome Architecture", he claims 10^13. I forget the primary source he cited, and since the book is in my office I can't check it now.

** Found the snippet that contained the reference. I think it's really worth including in its entirety.

A crude estimate of the amount of DNA within currently living organisms can be made by noting that the length spanned by one base of DNA is ~0.3x10^-12 km (Cook 2001).

The number of viral particles in the open oceans is ~10^30 (Suttle 2005). Assuming that there are twice as many viruses on land and in fresh water does not change the global estimate very much at the order-of-magnitude level. Thus, assuming an average viral genome size of 10^4 bp, the total length of viral DNA if all chromosomes were linearized and placed end to end is ~10^22 km.

The estimated global number of prokaryotic cells is ~10^30 (Whitman et al. 1998), and assuming an average prokaryotic genome size of 3x10^6 bp yields an estimated total DNA length of 10^24 km.

With a total population size of 6x10^9 individuals, 10^13 cells per individual (Baserga 1985), and a diploid genome size of 6x10^9 bp, the amount of DNA occupied by the human population is ~10^20 km. Assuming there are ~10^7 species of eukaryotes on Earth (~6 times the number that have actually been identified), that the average eukaryotic genome size is ~1% of humans, and that all species occupy approximately the same amount of total biomass, total eukaryotic DNA is ~10^5 times that for humans, or ~10^25 km.

Given the very approximate nature of these calculations, any one of these estimates could be off by one or two orders of magnitude, but it is difficult to escape the conclusion that the total amount of DNA in living organisms is on the order of 10^25 km, which is equivalent to a distance of 10^12 light years, or 10 times the diameter of the known universe.