American scientists have announced that they have created the most complete three-dimensional "map" of neuronal connections inside the brain of a mammal, specifically a mouse. It is another significant advance in the long-standing effort to understand how the human brain works and its neurodegenerative diseases. However, to create a similar "map" for the human brain, it will take several more years.
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The researchers, led by Honghui Zheng of the Allen Institute for Brain Science in Seattle, who published the relevant publication in the journal "Nature", according to Reuters and "New Scientist", described the achievement of creating an analytical "map" (Allen Mouse Brain Connectivity Atlas) with all the connections that form the various functional circuits of the brain as a "landmark".
As neuroscientist David van Essen of Washington University in St. Louis said, “this is the most detailed analysis of brain ‘connectivity’ that we have today for any mammalian brain.” The “map” shows how millions of neurons (brain cells) connect and communicate with each other through their extensions (axons) to produce memories, behaviors, and emotions. Anatomical and functional problems in these circuits cause brain diseases.
Before this mouse brain map, scientists had only the complete—but much simpler—neuronal “connectivity” of the worm C. elegans, which has just 302 neurons. By comparison, the mouse brain contains 75 million neurons and the human brain has around 85 billion neurons, each of which makes up to 10,000 connections with other neurons.
The "mapping" of the mouse brain was achieved with the help of viruses, through which a fluorescent protein was introduced into the animal's brain, which "illuminated" the neuronal circuits green under the microscope. Among other things, it was found that neuronal connections limited to only one hemisphere of the brain are always stronger than those that spread across both hemispheres.
To "map" it, the mouse brain was cut into 500,000 sections - cubes, each with a side of 100 micrometers (millionths of a meter). This "map" has further room for great improvement in terms of resolution, as it "traces" the connections between tiny areas of brain tissue (each containing 100 to 500 neurons), but does not include the precise connections (synapses) of all the individual neurons between them.
Such a detailed "map" of the human brain (cell by cell) is the ultimate goal. It is taken for granted, however, that, beyond the many similarities, such a "map" will be unique to each individual person, in the same way that each of us is a unique personality.
The genetic "atlas" of the human fetal brain
In a second study, scientists also from the Allen Institute, who published the relevant article in the same journal, released the first detailed 3D images of the stages of fetal brain development in the womb. The research, part of the “BrainSpan Atlas” project, is hoped to help understand disorders that are “triggered” before birth, such as schizophrenia and autism.
The researchers, led by Ed Line, analyzed the brains of four human fetuses between the 15th and 21st weeks of gestation, thus creating the first "atlas" of sequential genetic changes in the developing brain, that is, which genes are sequentially activated and deactivated as the brain develops.
"This was just the beginning. We want to understand the blueprint by which we build a brain, and this was the first step in that direction, as we began to map out how genes guide the whole process," said Ed Line. "Knowing where and when a gene is expressed in the brain can give us important clues about its role.".
In the future, when neuroscience research has advanced further, scientists hope to be able to compare the human brain with that of other mammals, such as monkeys and mice, to understand where human uniqueness lies. The ultimate goal is to discover the neurobiological basis of that greatest of all mysteries: human consciousness.
Link: For original scientific papers (with subscription) at the addresses:
https://www.nature.com/nature/journal/vaop/ncurrent/full/nature13186.html and
https://www.nature.com/nature/journal/vaop/ncurrent/full/nature13185.html
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