Herculano-Houzel has proposed over the last 10 or so years that there are only about as many glia as neurons in the nervous system of mammals. In reaching this estimate she excluded Schwann cells from the calculations - she only included oligodendrocytes, astrocytes and microglia among the glia as far as I can see. There was a previous estimate I have seen attributed to Eric Kandel, of a 10:1 ratio when Schwann cells are included. I understand this estimate has appeared in the Principles of Neural Science textbook by Kandel and Schwartz. Does anyone have a peer-reviewed publication in mind of the glia:neuron ratio with the Schwann cells included?
Dear Nickolaos,
I have only investigated the role of Schwann cells in promoting axon regeneration, so I am unaware of anyone who has actually counted Schwann cells. That would be a huge task, given the distribution of the PNS.
In regards to counting CNS neural cells, I agree with Hussein that the method used for counting matters, as well as which material you label to count and what you use to label the types of cells. To add to the articles suggested by him, you may want to look at the 2016 review in Journal of Comparative Neurology:
https://www.ncbi.nlm.nih.gov/pubmed/27187682
There is a nice historical summary of neuronal numbers in Table 6, as well as in the text, including the numbers quoted in Principles of Neural Science.
As for the omission of Schwann cells, the numbers quoted in Principles of Neural Science are for the CNS, at least in my 1991 copy (the ratio is described as "between 10 and 50 times more glial cells than neurons in the central nervous system of vertebrates."). This ratio, whether correct or incorrect, would exclude Schwann cells since it is CNS.
Cheers!
Jill
Thank you both for your interesting contributions. The current edition of the Principles of Neural Science has 2-10 times as many glia as neurons in the CNS, so this excluded the Schwann cells then but is still more than Herculano-Houzel and Helen Barbas and their colleagues have claimed. There is still no reference about where that figure came from. If we are only talking of the CNS then all the evidence we have favours the Herculano-Houzel claim, except in that I think these researchers only used cortex, i.e. I believe they excluded white matter. If glia cells in the white matter were added then maybe the ratio would shift away from the approximate 1:1 ratio that seems to be the new standard.
Dear Nicholaos,
The glial cells in white matter tracts in the CNS are oligodendrocytes, astrocytes, and oligodendrocyte precursor cells (or NG2 positive cells). There are no Schwann cells, which is in part responsible for poor axon regeneration following CNS lesions.
It is interesting that the current edition of Principles of Neural Science still quotes this number, but as the JCN review details (and Hussein also pointed out) there is still a controversy over numbers depending on the method used to quantify neurons. The review also reviews the historical literature and how the different numbers were obtained.
Cheers!
Jill
Indeed there are no Schwann cells but the glial cells in white matter are not being included in any case as far as I can tell. Only cortex is being used for the assays of Herculano-Houzel and Barbas and colleagues.
I saw somewhere a response by one of the Principles of Neural Science co-editors of the more recent editions of the Principles of Neural Science who insisted on an estimate of 5-10:1 on the basis of genetic cell markers he had investigated. Still, I find it hard to believe that estimate was drawn out of a hat. Perhaps it is from Cajal or someone a long time ago but it must be somewhere.
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