A few years ago, a scientist named Nenad Sestan began throwing around an idea for an experiment so obviously insane, so “wild” and “totally out there,” as he put it to me recently, that at first he told almost no one about it: not his wife or kids, not his bosses in Yale’s neuroscience department, not the dean of the university’s medical school.
Like everything Sestan studies, the idea centered on the mammalian brain. More specific, it centered on the tree-shaped neurons that govern speech, motor function and thought — the cells, in short, that make us who we are. In the course of his research, Sestan, an expert in developmental neurobiology, regularly ordered slices of animal and human brain tissue from various brain banks, which shipped the specimens to Yale in coolers full of ice. Sometimes the tissue arrived within three or four hours of the donor’s death. Sometimes it took more than a day. Still, Sestan and his team were able to culture, or grow, active cells from that tissue — tissue that was, for all practical purposes, entirely dead. In the right circumstances, they could actually keep the cells alive for several weeks at a stretch.
When I met with Sestan this spring, at his lab in New Haven, he took great care to stress that he was far from the only scientist to have noticed the phenomenon. “Lots of people knew this,” he said. “Lots and lots.” And yet he seems to have been one of the few to take these findings and push them forward: If you could restore activity to individual post-mortem brain cells, he reasoned to himself, what was to stop you from restoring activity to entire slices of post-mortem brain?
To do so would be to create an entirely novel medium for understanding brain function. “One of the things we studied in our lab was the connectome — a kind of wiring map of the brain,” Sestan told me. Research on the connectome, which comprises the brain’s 90 billion neurons and hundreds of trillions of synapses, is widely viewed among neuroscientists as integral to understanding — and potentially treating — a range of disorders, from autism to schizophrenia. And yet there are few reliable ways of tracing all those connections in the brains of large mammals. “I thought, O.K., let’s see if this” — slices of cellularly revived brain tissue — “is the way to go,” Sestan said. In 2012, Sestan approached two members of his lab, Mihovil Pletikos and Daniel Franjic, and asked them to assist him on the project. Through the spring of 2014, the scientists, often laboring in time they stole from other projects, managed to develop a customized fluid that could preserve centimeter-thick chunks of mouse, pig and human brain for long periods. “Six days was our record,” Sestan recalled. “Six days, and the cells were still culturable.” But there was a hitch: The tissue stayed intact only when the samples were stored in a fridge. Once they were removed and brought to room temperature (any accurate modeling of neuronal function would have to occur at 98.6 degrees Fahrenheit), decomposition rapidly set in.
The primary issue appeared to be one of oxygenation. Mammalian brains are tangled knots of arteries and capillaries, each of which is instrumental in circulating blood (and with it, oxygen and nutrients) throughout the organ. In slicing an entire brain into extremely thin leaves of tissue, the delicate interior architecture was decimated. But Sestan is stubborn, several of his colleagues later told me — in the manner of a dog locking his jaws on a length of knotted rope, he has trouble letting things go. “I get an idea, and I want to finish it,” he admitted. “I have to finish it.” The experiment, he went on, “was constantly on my mind. Like, What is the solution here?”
One afternoon, he dropped by Yale’s pathology department to discuss an unrelated issue with a colleague, Art Belanger, the manager of the university’s morgue at the time. “I look over, and there’s this human brain in a sink, mounted upside-down,” Sestan recalled. As he watched, preservative from a nearby plastic bottle dripped through a few lines of tubing and into the organ’s arteries. The rig, a so-called gravity feed, was being used to “fix” the brain, Belanger explained — to preserve it for further study. Sestan nodded. In his lab, he frequently fixed organs, usually by freezing the specimens or immersing them in formaldehyde. “Trust me,” Belanger told Sestan. “Perfusion is much more effective.”
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