What is it that separates humans from most other animals? Beyond the obvious like opposable thumbs and language, some big clues are to be found in the uniqueness of our brains. Scientists have made an exciting discovery in this area, identifying an entirely new type of brain cell not seen in animals studied in the laboratory, providing them with a new vehicle to tease out the differences between us and the rest of the animal kingdom.
Discovered by an international team of researchers, the newly identified brain cells were spotted in post mortem samples of brain tissue taken from the top layer of the cortex. This part of the brain is responsible for human consciousness and other higher functions.
Dubbed "rosehip neurons," the cells are named so for the tight web of nerve fibers concentrated around their center, giving them the appearance of a rose after its petals have been shed. According to the team, which includes Rebecca Hodge, a senior scientist at the Allen Institute for Brain Science, this is unlike any cells ever observed in mice, or other animals used for lab studies for that matter.
"These neurons have a distinctive shape characterized by compact, very bushy appearing branches that extend from the cells, and this shape has not been observed in neurons in the rodent cerebral cortex," Hodge explains to New Atlas. "In addition to this distinctive shape, we also found that rosehip cells express a unique set of genes that we don't see turned on in other types of neurons in the brain."
This unique genetic signature sees the rosehip neurons link up with other neurons in another part of the cortex to form synapses, a bridge through which information flows between cells. The special appearance of the rosehip nuerons and the way they trigger this process led the scientists to categorize them as inhibitory neurons, a type of cell that applies the brakes to other, more excitable neurons in the brain.
"Inhibitory neurons can be identified based on the genes that they express and by examining their physiological characteristics," says Hodge. "For example, the rosehip cells express a gene called Glutamic Acid Decarboxylase 1, which is involved in making the neurotransmitter gamma-Aminobutyric acid inside of cells. This neurotransmitter is released from inhibitory neurons and acts on neighboring cells to essentially tamp down their excitability."
What is particularly interesting about the rosehip cells, however, is the way they form a connection with the other cells. They appear to only attach themselves to a specific section, which suggests that they might control the flow of information, or inhibit their excitability, in an entirely unique way. Fleshing out these details, however, will be the subject of further study.
The rosehip cells join a small bunch of neurons that might exist only in humans, primates and other large animals like elephants and whales. Another example is the spindle cell, also know as the von Economo neuron. Scientists have studied the spindle cell as a way of exploring the social awareness of these creatures, with a view to better understanding human evolution.
"Like the rosehip cells, these neurons have a distinctive shape not seen in neurons in the rodent brain," Hodge tells us. "Interestingly, spindle cells are not only found in humans and primates, but also in other large-brained social mammals such as whales. The question of what they tell us about what makes the human brain special is indeed an intriguing question, but one that has proved difficult to answer because of the challenges of working with human brain tissues."
It is important to note that the scientists don't know whether the rosehip cells are unique to humans, only that they don't exist in rodents or other animals used in lab studies. That speaks to one of the inconvenient subplots of discoveries like this one, the doubt they cast on the use of rodents as models for studying brain disease in humans. As more and more differences are uncovered, what does it tell us about the effectiveness of this approach?
"Many of our organs can be reasonably modeled in an animal model," says study co-author Gábor Tamás, neuroscientist at Hungary's University of Szeged in Szeged. "But what sets us apart from the rest of the animal kingdom is the capacity and the output of our brain. That makes us human. So it turns out humanity is very difficult to model in an animal system."
By the same token, cells that are unique to human brains may deliver unparalleled insights into how neuropsychiatric disorders develop. Scientists will now look for rosehip neurons in other parts of the brain and examine them in postmortem brain samples of donors with brain disease. This will allow them to see if they undergo changes as a response to brain disease and their wider role in what makes humans human.
"We really don't understand what makes the human brain special," said Ed Lein, Investigator at the Allen Institute for Brain Science. "Studying the differences at the level of cells and circuits is a good place to start, and now we have new tools to do just that."
The research was published in the journal Nature Neuroscience.
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