For half a century, the human brain was believed to contain about 100 billion neurons and one trillion glial cells, with a glia:neuron ratio of 10:1. A new counting method, the isotropic fractionator, has challenged the notion that glia outnumber neurons and revived a question that was widely thought to have been resolved. The recently validated isotropic fractionator demonstrates a glia:neuron ratio of less than 1:1 and a total number of less than 100 billion glial cells in the human brain. A survey of original evidence shows that histological data always supported a 1:1 ratio of glia to neurons in the entire human brain, and a range of 40–130 billion glial cells. We review how the claim of one trillion glial cells originated, was perpetuated, and eventually refuted. We compile how numbers of neurons and glial cells in the adult human brain were reported and we examine the reasons for an erroneous consensus about the relative abundance of glial cells in human brains that persisted for half a century. Our review includes a brief history of cell counting in human brains, types of counting methods that were and are employed, ranges of previous estimates, and the current status of knowledge about the number of cells. We also discuss implications and consequences of the new insights into true numbers of glial cells in the human brain, and the promise and potential impact of the newly validated isotropic fractionator for reliable quantification of glia and neurons in neurological and psychiatric diseases.

The cellular composition of brains shows largely conserved, gradual evolutionary trends between species. In the primate spinal cord, however, the glia-neuron ratio was reported to be greatly increased over that in the rodent spinal cord. Here, we re-examined the cellular composition of the spinal cord of one human and one nonhuman primate species by employing two different counting methods, the isotropic fractionator and stereology. We also determined whether segmental differences in cellular composition, possibly reflecting increased fine motor control of the upper extremities, may explain a sharply increased glia-neuron ratio in primates. In the cynomolgus monkey spinal cord, the isotropic fractionator and stereology yielded 206-275 million cells, of which 13.3-25.1% were neurons (28-69 million). Stereological estimates yielded 21.1% endothelial cells and 65.5% glial cells (glia-neuron ratio of 4.9-5.6). In human spinal cords, the isotropic fractionator and stereology generated estimates of 1.5-1.7 billion cells and 197-222 million neurons (13.4% neurons, 12.2% endothelial cells, 74.8% glial cells), and a glia-neuron ratio of 5.6-7.1, with estimates of neuron numbers in the human spinal cord based on morphological criteria. The non-neuronal to neuron ratios in human and cynomolgus monkey spinal cords were 6.5 and 3.2, respectively, suggesting that previous reports overestimated this ratio. We did not find significant segmental differences in the cellular composition between cervical, thoracic and lumbar levels. When compared with brain regions, the spinal cord showed gradual increases of the glia-neuron ratio with increasing brain mass, similar to the cerebral cortex and the brainstem. Anat Rec, 301:697-710, 2018. © 2017 Wiley Periodicals, Inc.

Background The “isotropic fractionator” (IF) is a novel cell counting technique that homogenizes fixed tissue, recovers cell nuclei in solution, and samples and quantifies nuclei by extrapolation. Studies using this technique indicate that the ratio of glia to neurons in the human brain is approximately 1:1 rather than the 10:1 or 50:1 ratio previously assumed. Although some results obtained with the IF have been similar to those obtained by stereology, the IF has never been calibrated or validated. It is conceivable that only a fraction of glial cell nuclei are recovered intact or recognized after the homogenization step. New Method To rule out this simple explanation for the claim of a 1:1 glia-neuron ratio, we compared cell numbers obtained from adjacent, weight-normalized samples of human and macaque monkey white matter using three techniques: the IF, unbiased stereology of histological sections in exhaustively sectioned samples, and cell numbers calculated from DNA extraction. Results and comparison of methods In primate forebrains, the IF yielded 73,000–90,000 nuclei/mg white matter, unbiased stereology yielded 75,000–92,000 nuclei/mg, with coefficients of error ranging from 0.013–0.063, while DNA extraction yielded only 4,000–23,000 nuclei/mg in fixed white matter tissues. Conclusions Since the IF revealed about 100% of the numbers produced by unbiased stereology, there is no significant underestimate of glial cells. This confirms the notion that the human brain overall contains glial cells and neurons with a ratio of about 1:1 far from the originally assumed ratio of 10:1 in favor of glial cells.

Background Inflammatory responses can include recruitment of cells of hematopoietic origin to the tunica muscularis. These cells can secrete a variety of factors which can reset the gain of smooth muscle cells (SMC) and influence motor patterns. Histamine, a major mediator in inflammation, is released by mast cells and exerts diverse effects in SMC by binding to histamine (H) receptors. The profiles of H receptor expression in animal models used to study inflammatory diseases are unknown. Methods H receptor expression and electro-mechanical responses to histamine were tested in simian and murine colonic smooth muscle using qualitative and quantitative PCR, isometric force measurements, microelectrode recordings and patch clamp techniques. Key results H1, H2 and H4 receptor transcripts were expressed at similar levels in simian colonic tissue whereas only the H2 receptor transcript was detected in murine colonic tissue. Stimulation of simian colonic muscles with histamine caused depolarization and contraction in the presence of TTX. Histamine activated non-selective cation channels in simian SMC. In contrast, histamine caused hyperpolarization and inhibited contractions of murine colon. The hyperpolarization was inhibited by the KATP channel blocker, glibenclamide. Histamine-activated K+ currents were inhibited by glibenclamide in murine colonic SMC. Conclusions & Inferences H receptor expression in simian SMC was similar to that reported in humans. However, H receptor profile and responses to histamine were considerably different in mice. Thus, monkey colon may be a more suitable model to study how inflammatory mediators affect the gain of smooth muscle excitability.