The lab
The Herculano-Houzel lab started in 2005, with the publication of the cell-counting method created by the PI, the isotropic fractionator, which really consists of... turning brains into soup. The motivation was figuring out how many cells composed the brains of different animals, because only one thing was certain then: the human brain was NOT made of "100 billion neurons and 10 times as many glial cells", for the simple reason that nobody had really counted yet.
Take your fixed brain structure of interest, slice and dice it, and dissolve it in a salty detergent solution, using a glass homogenizer. You end up with a suspension of free cell nuclei that can be made homogeneous ("isotropic") by agitation.
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As long as every cell in the tissue has one and only one nucleus, counting DAPI-labeled nuclei - which can be done very quickly under a fluorescence microscope - is equivalent to counting cells. Next, fluorescent markers that make only your cells of interest red will let you work out in a couple of hours what fraction of the cells are of that type.
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The verdict? The human brain has on average 86 billion neurons and just as many non-neuronal cells, which makes it just a scaled-up primate brain, remarkable in that it has the most neurons in the cerebral cortex of any species - but not extraordinary, for it is not an exception to the rules of evolution.
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By turning all sorts of brains into soup, the Herculano-Houzel lab and their collaborators discovered that there is no single way to put neurons in a brain or body. Each group of animals shares their own relationship between brain mass and number of neurons.
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Still, there are only two ways for a mammal to have more than 1 billion neurons in its cerebral cortex: either it is gigantic, or else it is a primate! Or else... it is a bird, as later studies showed. Bottomline: you never know until you look.
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Suzana Herculano-Houzel and Bruno Mota discovered that the extent to which the cerebral cortex is folded is determined by pure physics - the same rules that determine how much you can crumple a sheet of paper of a certain surface area and thickness. How many neurons are in the cortex is... irrelevant.
More recently, the PI discovered that something really important comes with a certain number of neurons in the cerebral cortex: time to live. Formerly believed to be determined by body mass, it turns out that the best predictor of the maximal lifespan of warm-blooded animals is simply how many neurons it has in the cerebral cortex. In the figures below, you see that the human species takes just as long to reach puberty and then lives just as long as one would expect of a generic warm-blooded vertebrate with its number of cortical neurons!
The Herculano-Houzel lab is now focused on understanding what determines how much energy different brains use, and how that relates to how much energy is available per neuron, which has important implications for healthy aging and longevity.
Sources:
Azevedo et al. (2009) J Comp Neurol
Herculano-Houzel (2016) The Human Advantage, Cambridge, MIT Press
Herculano-Houzel (2019) J Comp Neurol
Mota and Herculano-Houzel (2015) Science
Azevedo et al. (2009) J Comp Neurol
Herculano-Houzel (2016) The Human Advantage, Cambridge, MIT Press
Herculano-Houzel (2019) J Comp Neurol
Mota and Herculano-Houzel (2015) Science