This photomicrograph is of a cerebellar Purkinje cell in the mouse which has been  revealed by the Golgi stain.
Image kindly provided by:
Giorgio Grasselli, Vivian Wan

Francisca Martínez Traub, a student in the 2014 session of the NeuroMOOC that I teach, emailed me to ask if I had heard of the work of Brazilian neuroscientist, Suzana Herculano-Houzel. I had not. The student wrote that Dr Herculano-Houzel was interested in collecting data on the total number of cells in the brain and what proportion of those cells are neurons. I estimated these two values at 200-500 billion and ~10%, respectively, in my textbook and MOOC. My estimate for the number of cells was a bit higher than most others’ but my estimate that the glia:neuron ratio is 10:1 is the same as most commonly quoted. I forwarded the email I received to a colleague who wrote back:

“yeah, this glial claim has as far as I can tell never had any support. Careful counts in the mouse had completely refuted it a decade or two ago…. when S H-H published her paper on this point (Azevedo FA, Carvalho LR, Grinberg LT, Farfel JM, Ferretti RE, Leite RE, Jacob Filho W, Lent R,
and Herculano-Houzel S, Equal numbers of neuronal and nonneuronal cells make the
human brain an isometrically scaled-up primate brain. J Comp Neurol. 2009 Apr 10; 513(5):53241), I [wrote this  on] F1000 (a subscription service that highlights important papers):

Not everything you learned in grade school is true as this paper shows, there are not ten times as many glia as neurons in the human brain.

My colleague added:

“But more importantly- Wow! there really are a lot of engaged people out there, must be very satisfying!”

Let me address this latter point first. Right-o! Very satisfying indeed. I have been as surprised as my colleague about the incredible thoughtfulness, intelligence and knowledge of NeuroMOOCers. Students in Understanding the Brain continue to amaze and impress. I now am sure that those who underestimate the public’s ability to understand and parse science, do so at their own risk.

Now on to the topic at hand. How many cells are there in the brain and what proportion of the cells are glia? Herculano-Houzel and colleagues figured the answers out using a novel method that her laboratory devised. This method uses flow cytometry, a process that employs an instrument called a cell sorter. Essentially, the process involves mashing up the brain and then labeling the cells of the brain with dyes that recognize either all cells (the dye is DAPI which labels DNA) or recognizes only neurons (NeuN which labels only neurons but not all neurons; a small number of neurons are not labeled with antibodies to NeuN). To cut to the chase, they found that the human brain has about 85 billion neurons and about 85 billion glia for a glia: neuron ratio of 1:1 rather than 10:1.

But there is so much more to this story. Before they homogenized the brain, Herculano-Houzel and colleagues dissected out particular regions. In this way they were able to estimate the number of neurons and glia in various brain regions as well as in the brain as a whole. Moreover, they analyzed brains from a number of other animals besides humans. A review of their findings is available to all. What follows are some particularly eye-catching findings from this work:

The density of cells varies greatly across human brain regions: Herculano-Houzel and colleagues divided the brain into three regions: telencephalon, cerebellum, and rest of the brain. In the human, the number of cells in each region is as follows:

• Telencephalon (82% of brain’s mass): 77 billion (45% of all cells in the brain)
• Cerebellum (10% of brain’s mass): 85 billion (50% of all cells in the brain)
• Rest of the brain (8% of brain’s mass): 8 billion (5% of all cells in the brain)

Right off the bat, we learn that cells (remember this is cells inclusive of both neurons and glia) are not evenly packed through the brain. We have long known that the cerebellum contains far more than its fair share of cells. The commonly repeated adage that cerebellar granule cells constitute roughly half of all the brain’s neurons may be on target as the cerebellum houses half of the brain’s cells (even despite its proportionately small size, 10% of the whole brain).

The proportion of glial cells varies greatly across brain regions: In the human brain, glial cells make up nearly half of the cells in the brain. Herculano-Houzel and colleagues estimate that there are 86 billion neurons in the brain and close to 85 billion glial cells. Thus, for the entire brain, the proportion of glia to neurons is about 1:1. Yet the proportional representation of glial cells varies dramatically across regions:

• Telencephalon (19% of brain’s neurons): 3.8 glia : 1 neuron
• Cerebellum (80% of brain’s neurons): 0.2 :1
• Rest of the brain (<1% of brain’s neurons): 11.4 :1

The following illustrates these data in graphical form:

The glia to neuron ratio varies widely across areas of the brain, with the highest value in the diencephalon + brainstem and the lowest value in the telencephalon.

These numbers tell us that the glia to neuron ratio varies by more than one order of magnitude, from 0.2 in the cerebellum to 11.4 in the diencephalon plus brainstem. It is interesting to note that the glia to neuron ratio appears to be greatest in the region with the greatest neuronal diversity. Of these regions, the cerebellum contains the fewest neuronal types and the brainstem contains the greatest number of neuronal types.

In addition, by picking out the neurons specifically, we see that the cerebellum, while containing about half of all the brain’s cells, contains a whopping 80% of the brain’s neurons. Granule cells make up virtually the entire complement of cerebellar neurons.

Human brain size is in line with  primate brain-to-body scaling: If you are interested in comparing human brains to the brains of other animals (comparative neuroanatomy), a common approach is to express brain size in relation to body size across mammals. Larger animals require larger brains. Essentially the brain:body ratio allows us to compare brains (apples) to brains (apples). Consistently, “smart” species such as humans deviate from the overall mammalian brain:body line, meaning that their brain is bigger than a typical mammalian brain of our body size. However, Herculano-Houzel and colleagues show that humans fall exactly on the primate brain:body line. In fact rather than us having a disproportionately large brain for our body size, it turns out that gorillas and orangutans have much larger bodies than would be expected for their brain size.

Because brain size is not highly correlated to body size, Herculano-Houzel and colleagues speculate that body size may not be predictive or mechanistically related to brain size. Instead, cellular scaling rules that relate the number of neurons to brain size appear highly predictive within (but not between) mammalian orders (see next section).

The scaling rules for brains varies across mammalian orders: Herculano-Houzel and colleagues studied brains from several species in three different orders: primate, rodent, and insectivore. Across rodents, the size of the brain increases faster than does the number of neurons, suggesting that among rodent species, larger brains contain much larger neurons. In contrast, primate neurons are roughly the same size in brains of different sizes. Another way to imagine this is to consider the effect of increasing the number of neurons by 10 times in a typical rodent and a typical primate. A rodent brain would be 35 times as large whereas a primate brain would increase in size proportionately (increasing volume by 11 times) for a 1o-fold increase in neurons.

Herculano-Houzel and colleagues advance the idea that primate scaling is perfect for packing in lots more neurons without making the brain humongous. The rodent brain is far less economical. To illustrate this, a rodent with a 1.5 kg (about 3 pounds) brain would have 1/7th the number of neurons that humans have. Conversely, a rodent with 86 billion neurons (as is found in human brains) would  need a 77 pound (35 kg) brain to house all those neurons at rodent packing densities. In essence we primates have brains that hold many neurons in a relatively small cranium.

Ultimately, this exciting work coming from the Brazilian neuroscientist, Suzana Herculano-Houzel, provides much needed data regarding how anatomical features of brain relate, or don’t relate, to intelligence. Lucky me to be teaching such a stupendous group of students!!