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Ten years ago, Dr. Jeff Lichtman — a professor of molecular and cellular biology at Harvard University — received a small brain sample in his lab.

Although tiny, the 1 cubic millimeter of tissue was big enough to contain 57,000 cells, 230 millimeters of blood vessels and 150 million synapses.

“It was less than a grain of rice, but we began to cut it and look at it, and it was really beautiful,” he said. “But as we were accumulating the data, I…