What’s so special about the human brain?

Looking at brain cells closely has shown some interesting patterns. Over the past five years, techniques that enable scientists to catalogue the genes expressed in a single cell have been revealing the many different types of cell that make up a brain — at a level of detail much higher than anything achieved before.

Last year, a team based at the Allen Institute for Brain Science in Seattle, Washington, reported the most-comprehensive atlases yet of cell types in both the mouse and human brain. As part of an international effort called the BRAIN Initiative Cell Census Network (BICCN), researchers catalogued the whole mouse brain, finding 5,300 cell types²; the human atlas is unfinished but so far includes more than 3,300 types from 100 locations³; researchers expect to find many more.

Some regions do have distinct cell types — for instance, the human visual cortex contained several types of neuron that were exclusive to that area⁴. But in general, human-specific cell types are rare.

The overall impression, when comparing the cell types of the human brain with other species, is one of similarity. “I was expecting bigger differences,” says Ed Lein, a neuroscientist at the Allen Institute, who is involved in efforts to catalogue cells in human, mouse and other brains. “The basic cellular architecture is remarkably conserved until you get down to the finer details”, he says.

Most human brain regions differ from primates and mice in the relative proportions of cell types that appear⁵, and in the ways those cells express their genes: it’s not the ingredients that are different, but the recipe.

Take these two comparable regions of the human and mouse cortex, which both process auditory information. The mouse area contains a higher proportion of excitatory neurons, which propagate signals, relative to inhibitory neurons, which dampen activity. The human region had a much greater proportion of non-neuronal cells, such as astrocytes, oligodendrocytes and microglia. These cells support neurons and also help to prune and refine their connections during development. The ratio of these cells to neurons was five times that of mice.