They trace a fragment of the human brain in astonishing detail – DW – 05/21/2024

57,000 cells, almost 150 million synapses and 230 millimeters of blood vessels. All of this fits into just one cubic millimeter of the human brain, which is equivalent to 1,400 terabytes of data and represents an unprecedented look at the structural organization of brain tissue.

Harvard University (USA) and Google Research have collaborated for nearly a decade to create the largest synaptic-resolution 3D reconstruction of an area of ​​human temporal cortex about half the size of a grain of rice.

Nanoscale reconstruction reported in published study The science, It shows each cell and its network of neural connections in detail, providing unprecedented insight into the structural organization of brain tissue at the supracellular, cellular and subcellular levels.

The human brain is an extremely complex organ and little is currently known about its cellular microstructure, but alterations in synaptic and neuronal circuits are known to influence numerous brain diseases.

A map of excitatory neurons extracted from a new study. The team cataloged about 57,000 cells and 150 million connections between neurons.Image: Google Research, Lichtman Lab and D. Berger/Harvard University.

Reconstruct the wiring of part of the human brain

Researchers led by Jeff Lichtman from Harvard University, and Viren Jain of Google Research’s Connectomics team combined artificial intelligence and electron microscopy to color code and reconstruct the wiring of this small piece of brain.

Such detailed knowledge of human neural circuits entails a number of challenges, such as technological limitations or difficulties in accessing high-quality tissues. Biopsy is rarely performed and is used to examine or remove tumor growths, which prevents the examination of the normal structure of the human brain.

As the basis for the study, the team used a sample taken during a surgical procedure to access the underlying lesion in the hippocampus of a patient with epilepsy.

The researchers reconstructed “thousands of neurons, more than one hundred million synaptic connections, and remaining tissues, including brain matter, glial cells, blood vessels, and myelin,” they explain in the study.

Unprecedented Vision

This unprecedented look at part of the human brain allowed us to discover previously unknown aspects of the temporal cortex, such as the existence of a rare but powerful set of axons (the part of a neuron that transmits brain signals) connected by up to 50 synapses.

One neuron (white) with 5600 axons connected to it (blue). The synapses that make these connections are shown in green. The cell body (central nucleus) of a neuron is about 14 micrometers in size. Image: Google Research, Lichtman Lab and D. Berger/Harvard University.

Other details are the large number of glial cells (responsible for ensuring the survival of neurons) compared to neurons in a two-to-one ratio, or that oligodendrocytes (responsible for the formation of myelin) are the most abundant.

The team has also developed a set of open-access tools that other researchers can use, and hope that “other studies using this resource can provide valuable insights into the mysteries of the human brain,” the study said.

According to the authors, this study demonstrates “the feasibility of using human connectomics approaches to visualize and ultimately understand the physical underpinnings of normal and altered human brain function.”

Connectome, diagram of neural connections

Connectomics, similar to genomics, aims to create complete catalogs of brain structure, down to individual cells and connections. Obtaining complete maps will open the way to new understanding of brain function and diseases about which scientists still know very little.

This version shows excitatory neurons colored according to their depth from the surface of the brain. Blue neurons are closest to the surface, while fuchsia colors mark the innermost layer. The sample width is approximately 3 mm.Image: Google Research, Lichtman Lab and D. Berger/Harvard University.

The connectome is a diagram of neural connections, without it it is impossible to understand how the brain works. The first attempt to create a map of this type was made in the seventies with the help of roundworms, the result was partial.

A big step was taken last year when a complete connectome of the larval brain was obtained Drosophila melanogaster (vinegar or fruit fly), after twelve years of work.

Now, Harvard University and Google are also collaborating on the BRAIN Initiative, a project to map high-resolution neural connections across the entire mouse brain, starting with the formation of the hippocampus, important for neurobiology because of its role in memory and neurology. diseases.

The result will be about 1,000 times larger than the data they just obtained from a one-cubic-millimeter fragment of human cortex.

FEW (EFE, Harvard Newspaper, Science)

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