What human and mouse brains do and don’t have in common

The brains of mice and people are mostly similar, except when they’re not.

That finding, from a detailed comparison of thousands of individual brain cells from both species, reveals new ways in which human brains are distinct from those of mice (SN: 8/17/19, p. 22). The results, published August 21 in Nature, emphasize that the brains of laboratory mice are not always good proxies for human brains.

Human brains and mice brains do have roughly the same number of cell types in the cortex, the outer layer of the brain that handles many sophisticated jobs like planning, decision making and even consciousness, neuroscientist Rebecca Hodge at the Allen Institute for Brain Science in Seattle and colleagues found. That number — 75 — comes from comparing the behavior of genes in single mouse cells with the behavior of genes in single nuclei from human cells.

But amid that overall similarity lie big differences, some of which may be behind people’s high-powered mental skills and susceptibility to human diseases, the researchers suspect. Some of the most striking differences were in the behavior of genes that hold instructions for how cells sense the chemical messenger serotonin. The type of cell that senses serotonin in mice is different from the type of cell that senses serotonin in people, the results imply. That distinction may mean laboratory mice aren’t good models for disorders that may involve serotonin, such as depression.

And microglia, the brain’s resident immune cells, differed in their gene behavior between humans and mice. Microglia are under scrutiny for their role in human diseases such as Alzheimer’s disease and Parkinson’s.