Researchers have produced a detailed map of brain cells and the genes that are active within them. Their work, published in Science this week, even revealed a handful of previously unknown types of cells.

In mammals, the cerebral cortex plays a role in cognitive functions like memory and social behaviors, and these all rely on a variety of cell types: from neurons and their protective glial cells to blood vessel cells. There are about 100 million cells in a mouse brain, and 65 billion cells in a human brain. A Swedish team led by Jens Hjerling-Leffler and Sten Linnarsson of Karolinska Institutet wanted to create a more detailed brain map that could help us better understand how brain cells respond to diseases and injuries. Using a technique called single cell sequencing, they were able to classify the cells in the mouse somatosensory cortex and hippocampal CA1 region.

"If you compare the brain to a fruit salad, you could say that previous methods were like running the fruit through a blender and seeing what color juice you got from different parts of the brain," Linnarsson says in a news release. "But in recent years we've developed much more sensitive methods of analysis that allow us to see which genes are active in individual cells. This is like taking pieces of the fruit salad, examining them one by one and then sorting them into piles to see how many different kinds of fruit it contains, what they're made up of and how they interrelate."

The team studied 3,005 cells from the cerebral cortex one at a time, and they compared which of the 20,000 genes were active in each one. After sorting the cells into neat piles, they identified 47 molecularly distinct subclasses of cells, comprising all major cell types of the cortex.

Furthermore, with their detailed brain map, they found brand new cell types: a nerve cell in the most superficial layer of the cortex and six different types of oligodendrocytes—cells that form the myelin sheath around the nerve cells. Knowing more about the electrically-insulating myelin sheath could help us understand diseases like multiple sclerosis.