The researchers have only a dim notion of what the newly discovered cells do.
NEW YORK: An international team of scientists has mapped the human brain in much finer resolution than ever before. The brain atlas, a $375 million effort started in 2017, has identified more than 3,300 types of brain cells, an order of magnitude more than was previously reported.
The researchers have only a dim notion of what the newly discovered cells do. The results were described in 21 papers published in Science and several other journals. Ed Lein, a neuroscientist at the Allen Institute for Brain Science in Seattle who led five of the studies, said that the findings were made possible by new technologies that allowed the researchers to probe millions of human brain cells collected from biopsied tissue or cadavers.
“It really shows what can be done now,” Dr. Lein said. “It opens up a whole new era of human neuroscience.” Still, Dr. Lein said that the atlas was just a first draft. He and his colleagues have only sampled a tiny fraction of the 170 billion cells estimated to make up the human brain, and future surveys will certainly uncover more cell types, he said.
Biologists first noticed in the 1800s that the brain was made up of different kinds of cells. In the 1830s, the Czech scientist Jan Purkinje discovered that some brain cells had remarkably dense explosions of branches. Purkinje cells, as they are now known, are essential for fine-tuning our muscle movements.
Later generations developed techniques to make other cell types visible under a microscope. In the retina, for instance, researchers found cylindrical “cone cells” that capture light. By the early 2000s, researchers had found more than 60 types of neurons in the retina alone. They were left to wonder just how many kinds of cells were lurking in the deeper recesses of the brain, which are far harder to study.
With funding from the National Institutes of Health, Dr. Lein and his colleagues set out to map the brain by inspecting how brain cells activated different genes. At least 16,000 genes are active in the brain, and they are turned on in different combinations in different types of cells. The researchers collected brain tissue from several sources, including people who had recently died and those who were undergoing brain surgery.
When studying fresh brain tissue, the scientists attached glass tubes to the surface of individual cells to eavesdrop on their electrical activity, injected dye to make out their structure, and finally sucked out the nuclei from the cells to inspect them more closely.
Rather than carrying out these procedures by hand, the researchers designed robots to work efficiently through the samples. The robots have inspected more than 10 million human brain cells so far, Dr. Lein estimated.
Some of the newly identified cells were found in layers of cerebral cortex on the brain’s outer surface. This region is essential for complex mental tasks such as using language and making plans for the future. But the new studies reveal that much of the brain’s diversity lies outside of the cerebral cortex. A vast number of the cell types uncovered in the project lie in the deeper regions of the brain, such as the brain stem that leads to the spinal cord.