Researchers have mapped a tiny slice of the human brain on an unprecedented scale, vividly visualizing every brain cell, or neuron, and the intricate networks they form with other cells.
The groundbreaking brain map, created by researchers at Harvard and Google, reveals about 57,000 neurons, 230 millimeters of blood vessels and 150 million synapses, or the connection points between neurons.
Dr. Jeff Lichtman, a professor of molecular and cellular biology at Harvard University who co-led the decade-long project, said he couldn’t believe the detailed map when he first saw it. “I had never seen anything like it,” he told LiveScience.
The human brain is an extremely complex organ approximately 170 billion cells, including 86 billion neurons. Researchers have previously looked inside the brain on a millimeter scale using magnetic resonance imaging (MRI). And more recently, advanced microscopy techniques have revealed details on much smaller scales, improving our understanding of the brain’s inner workings.
Related: The most detailed human brain map ever contains 3,300 cell types
Now, using these microscopy methods and a artificial intelligence (AI) system called machine learning, Lichtman and his colleagues have created a 3D map of a piece of the brain on the scale of a nanometer, or 1 millionth of a millimeter. This provides an image of the organ at the highest resolution scientists have ever achieved.
The resulting cell atlas, described in the journal Science This is also the case on May 9 available for scientists to view online.
This map maps a small part of the brain with a volume of about 1 cubic millimeter – smaller than a grain of rice. A very adult brain is a million times larger.
The brain fragment was sampled from a 45-year-old woman who had undergone brain surgery to treat epilepsy. Doctors removed the piece from the cerebral cortex, the outer part of her brain. After fixing the sample in preservatives, the researchers stained it with heavy metals to better see the cells. They then embedded the tissue in resin and cut it into more than 5,000 slices, each with a thickness of about 30 nanometers.
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“That’s about one-thousandth the thickness of a hair strand,” Lichtman said. The team scanned each of the slices with a high-speed electron microscope, which uses multiple electron beams to illuminate the cells in the sample. They then sent the microscopy data to Google for further analysis using AI.
The Google researchers used machine learning models to identify the same object in different microscopic images and then create a 3D representation of each object in all the images. They then electronically stitched the views together to reconstruct the entire sample in three dimensions. The final 3D map contains a whopping 1.4 petabytes, or 1 million gigabytes, of data.
“The amount and complexity of the data generated in this project required Google’s ability to develop advanced machine learning and AI algorithms to reconstruct the 3D connectome,” Viren Jaina senior staff scientist at Google who co-led the project, told LiveScience in an email.
The scientists’ detailed map contains several surprises. For example, they found that some of the neurons’ output wires, or axons, wound themselves into knots, forming whorls that Jain described as “mysterious but beautiful.” The team also found rare connections between neurons, with single axons connecting to as many as 50 synapses.
“We’re still investigating the function of these connections, but they could explain how very fast responses or very important memories are encoded,” Jain told LiveScience.
It remains to be seen whether the whorls and super-strong synapses have anything to do with the tissue donor’s epilepsy, or whether they would be seen in the brains of people without the condition, Lichtman noted. He added that the team is now examining brain tissue from a person with Parkinson’s, so that could answer the question.
He added that it is unlikely that brain tissue samples from two people will look exactly the same, partly because the way the brain connects depends on an individual’s experiences.
The team next wants to map the entire brain of a mouse, which would be 500 times the size of this human brain sample. They start with the hippocampus, a key area for learning and memory.
“We have already started the ambitious task,” Lichtman said.
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