Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain.
Level 4 - case-series / case-control
Descriptive post-mortem human tissue quantification study (case series)
PubMed 19226510 · doi:10.1002/cne.21974
What was done
Researchers quantified the absolute numbers of neuronal (NeuN-positive) and nonneuronal (NeuN-negative) cells in adult male post-mortem human brains using the isotropic fractionator method. They also compared the observed cell counts and regional distributions (such as in the cerebral cortex) against allometric expectations for a generalized primate scaled to human proportions.
What was found
The adult male human brain contained on average 86.1 ± 8.1 billion neurons and 84.6 ± 9.8 billion nonneuronal cells, indicating an approximately 1:1 ratio rather than the traditionally cited 10:1 glia-to-neuron ratio. The cerebral cortex comprised 82% of total brain mass but contained only 19% of all neurons. Glia-to-neuron ratios across human brain structures matched values expected for an isometrically scaled primate brain.
Why it matters
This study overturns widespread textbook assumptions that the human brain contains 100 billion neurons with a 10-fold excess of glia, demonstrating that human brain cellular composition matches standard primate allometric scaling.
Limits
The abstract does not report the sample size (n), donor age range, cause of death, or clinical history. Analysis was restricted to adult male brains, leaving female brain cellular composition unaddressed in the abstract. Findings rely on post-mortem tissue preservation and the accuracy of NeuN as a definitive neuronal marker.