The prefrontal cortex, the essential component of human identity
The prefrontal cortex is a part of the brain found in humans, but also in primates. It is located in the front part of the brain along with the motor cortex and the premotor cortex. It is a structure that has a fundamental role in memorization, in the management of emotions as well as in attention. This region of the brain is connected to many other areas of the brain, including those that control the secretion of certain neurotransmitters such as dopamine, norepinephrine, and serotonin. These molecules are involved in mood management. For this reason, depression is often accompanied by a decrease in the activity of the prefrontal cortex.
In this prefrontal cortex is the dorsolateral prefrontal cortex designated dlPFC essential for human survival. This area is involved in the development of cognitive processes, attention and time planning.
This prefrontal cortex, which is one of the most significant brain structures of human identity, also exists in primates such as chimpanzees, macaques and marmosets. In humans it is different from that of primates and it is this difference that makes us unique in the great history of life on Earth. However, researchers are still wondering what makes it unique to humans. It is to try to find an answer to this question that a research team from Yale University embarked on this study.
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Specific brain immune cells in humans and chimpanzees
The researchers first wondered if there were cell types in the dorsolateral prefrontal cortex that only exist in humans and/or in one or another primate species. Using complex RNA sequencing techniques, they analyzed several thousand cells from this area of the brain in humans, but also in three other primate species.
After pooling cells with similar profiles, they identified 109 identical cell types in humans and the other three primate species, but also five cell types not shared by these four species. Among these 5 cell types, a cell type present in humans and chimpanzees and a second cell type only present in humans stood out. This human-specific cell type is a type of microglia.
Microglia cells are cells belonging to the glial cells that cohabit with neurons and represent up to 50% of brain cells. These cells are actually macrophages present in the brain and spinal cord. These cells, belonging to the macrophages, have the capacity for phagocytosis and also proliferate in case of injury at the level of the central nervous system where they act in the immune response.
When there is no infection in the central nervous system, microglia cells monitor the cellular environment and clean up debris. On the other hand, in the case of an infection, they switch to pro-inflammatory or anti-inflammatory mode depending on the nature of the inflammation.
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A gene that makes us human and predisposes us to neuropsychiatric diseases
This type of human-specific glial cell is present throughout life, suggesting that these cells are involved in maintaining brain maintenance rather than fighting infection.
By analyzing these microglia cells more specifically using molecular biology techniques, the researchers revealed the presence of a gene that only exists in humans, the FOXP2 gene. This discovery is surprising, because variants of this gene in humans are linked to verbal dyspraxia, which is a condition in which people have difficulty speaking and expressing themselves. This gene is also prominent in neurological diseases such as schizophrenia, epilepsy, and autism.
The study of the expression of the FOXP2 gene in microglia cells in humans paves the way for new research to better understand certain diseases and in the study of language.
However, it should be noted that this genetic difference is only one more index to try to understand the difference between the brain of Man and that of primates. Let us not forget that the chimpanzee genome differs from ours by only 1.2% and that of the gorilla differs from ours by 1.6%. We are also genetically different from the orangutan by only 3.1%. Therefore, genetics alone cannot explain the difference between the human brain and that of our closest cousins.
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Shaojie Ma, Mario Skarica, Qian LiChuan XuRyan D. Risgaard, Andrew T. N. Tebbenkamp, Xoel Mato-Blanco, Rothem Kovner, Željka Krsnik, Nenad Sestan, “Molecular and Cellular Evolution of the Primate Dorsolateral Prefrontal Cortex,” SciencesAugust 25, 2022, https://www.science.org/doi/10.1126/science.abo7257