In the brain, nervous structures that come from different progenitors coexist and have evolved independently: the discovery will be useful in research.
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The human brain is made up of two structures distinct in terms of evolutionary history and embryonic development: scientists from the Faculty of Medicine at Stanford University have discovered this. In an article published on Nature Neurosciencedemonstrate that two nervous systems that derive from different progenitor cells coexist in the brain, integrated: a more primitive one, which regulates automatic functions essential for survival and which controls the muscles of the face, tongue and throat, and a more recent and advanced one which presides over “higher” functions such as language, consciousness and abstract reasoning.
The conclusion is important especially for those who deal with the brain in basic research. In fact, for decades it was believed that the entire brain originated from a single progenitor cell in the early stages of embryonic development.
The idea that all brain cells had a common evolutionary imprint clashed, however, with the difficulty of cultivating the neurons of the human hindbrain in the laboratory, a more primitive nervous structure than the one in which the cerebral cortex resides, the most studied part of the human brain. Some rare and progressive diseases (such as Amyotrophic Lateral Sclerosis or ALS) affect the neurons of the hindbrain, and this gap in basic research hindered studies on possible new therapies.
Different stories and functions
The adult brain is organized into three structures, hindbrain, midbrain and forebrain, which divide during the development of the embryo. The forebrain, in the front part of the brain, is the most complex and evolved part: it presides over complex sensory functions (hearing, sight, touch), the regulation of emotions and vital stimuli such as hunger and sleep, and higher cognitive functions.
The hindbrain, located at the back of the skull, is the oldest and least evolved structure, responsible for directing automatic functions such as blood pressure, digestion, heartbeat, alertness, as well as basic movements such as those necessary for swallowing and breathing. The midbrain is a connecting structure between the two others described.
Parallel tracks
By studying one of the very first stages of embryonic development in mice – gastrulation, which occurs during the third week of pregnancy – scientists observed that the hindbrain follows a separate developmental path compared to the forebrain and midbrain. In fact, it derives from a progenitor cell that expresses a gene called Gbx2; forebrain and midbrain, however, from another cell that expresses a gene called Otx2.
The two cell populations develop independently and in parallel from the beginning, without ever interbreeding and without deriving from each other.
The organization of DNA within these cells is also very different.
A recurring pattern throughout evolution
Further research back into 550 million years of evolutionary history of other animals suggests that these two separate systems ended up in a single structure for reasons of economy and efficiency: forming, in fact, what we now simply call the brain.
A pattern similar to ours is found in chickens, zebrafish, and worms that live on the bottom of the sea, while jellyfish, separated from our evolutionary line 600-700 million years ago, actually have two nervous systems, located at opposite ends of the body.
The reason for so many failed attempts
Some rare and progressive genetic diseases, such as ALS, but also spinal muscular atrophy (SMA), also affect the neurons of the hindbrain, gradually depriving patients of the ability to swallow and breathe.
In the past, scientists have long tried to obtain hindbrain cells in the laboratory, but “previous attempts to create hindbrain neurons probably aimed to induce the progenitors of the forebrain and midbrain to transform into hindbrain cells, which our study shows is not possible” explains Rayyan Jokhai, one of the authors of the research.
Thanks to the new discovery, the team managed to induce human pluripotent stem cells (capable of transforming into any type of tissue) to become motor neurons in the hindbrain.
