Specialized cells known as microglia—spidery and many-fingered—had been seen crawling around the brain, scrounging for debris, and their role in eliminating pathogens and cellular waste had been known for decades. But Stevens also found them coiled around synapses that had been marked for elimination. Microglia nibble at the synaptic connections between neurons and pare them away. They are the brain’s “constant gardeners,” as one report put it.29
