Break out your botanical dictionary. You're going to need it.
The human brain is one of the most complex structures ever to evolve on this planet, but we're still barely able to understand exactly what sets it apart. We may be a little closer to figuring it out, thanks to a study published Monday in the journal Nature Neuroscience that reports a new type of brain cell—one unique to human beings.
It's called the rosehip neuron, and they comprise about 10 to 15 percent of the upper layer of the human neocortex—the portion of the brain responsible for much of our advanced cognition. The purpose of the study was to understand the diversity of the neocortex and see whether humans possess types of neurons that are absent in other animals.
“We've never seen anything like it before,” says Gábor Tamás, a researcher from the University of Szeged in Hungary and coauthor of the new study. “The results are surprising, but we're not surprised to find elements of this human neurocircuit that are possibly unique to our species. We're not surprised that humans might have more enriched or elaborate neuron networks.” What is surprising, Tamás says, is how unique the neuron's structure is, which seems to allow it to fire off signals in a super-specialized way. Unearthing more about that process and it works might shed more light on what gives the human brain dominion over the rest of the animal kingdom.
The neuronal survey of the upper layer of the human neocortex used two different techniques. At the Allen Institute for Brain Science in Seattle, researchers sequenced the gene profiles of individual cells isolated from people who had donated their brains after death. Meanwhile, Tamás and his team in Hungary conducted a systematic search of neurons with structures and other features that weren't found in rodent brains, by measuring electrical activity in individual cells. Combined, those two approaches allowed researchers to identify the rosehip neuron and understand its shape, how it forwards signals to other neurons, and what genes govern its functions.
The peculiar feature of the rosehip neuron is the shape of its axon terminal, the signaling device of the neuron which releases neurotransmitters to other cells. Neurotransmitters basically act like a sort of language, which neurons use to communicate with one another. “This axon terminal on the rosehip cell,” says Tamás, “looks like a rosehip”—the accessory fruit of the rose plant that's leftover after the petals fall off. This bushy, spindle-shape “is what initially caught our attention.”
The overall size of the cell is extremely compact. Take your smallest known animal neurons from previous research, and the rosehip neuron is virtually half that. The researchers are still not totally certain about their function, but they suspect that rosehip neurons selectively inhibit brain activity by putting the kibosh on activating (“excitatory”) neurons. Their compact, unique shape would make them well suited for this role. “This function is so specific that we haven't seen anything like it,” says Tamás.
Although rodents lack the rosehip neurons, it doesn't eliminate the possibility these cells are found in other animal brains. “There are other examples of cell types that are found in the human but not rodent brains, such as spindle neurons,” says Trygve Bakken, a scientist at the Allen Institute and a coauthor of the study. “These spindle neurons are also found in other highly social, large-brained mammals such as monkeys and dolphins, and it may be that rosehips are also found in these species.” We may be able to find rosehip neurons in other animals through the same techniques the team used, and figure out how they differ from human rosehips.
These cells, Tamás believes, are a good illustration of how evolution works: the process by which nature enriches biology over time and creates variety that will allow a species to develop more precise and more elaborate functions. “Evolution, in this case, provides food for thought,” he says. “The most important outcome of this study is that now we have proof that human matter is enriched, compared to standard animal models.”
In more practical terms, the findings could shed light on how we can predict and treat the onset and progression of a host of different neurological disorders. The complexity of the human brain is tempered by the fact that we're also victim to many kinds of afflictions that don't affect other animals. “If rosehips are implicated in brain disorders, then we can design genetic tools to target this cell type to study and perhaps eventually for treatment,” says Bakken.
That work will be limited by the fact that, if these cells don't exist in other animal models, then it's extremely difficult to ethically study their role in treating disorders like Alzheimer's or schizophrenia, since we can't do the same type of experiments in mice that we can do in humans. “We're stuck—it's not easy to understand what the function of these cells are,” says Tamás. “However, we have a chance to do that in the future by examining rosehip cells in patients with specific disorders.” Linking the gene expression profiles of the rosehip cells with the genetic markers of known diseases could illuminate how rosehips are related to certain afflictions. Besides further characterizing rosehip neurons and surveying the brain cell diversity of other cortical layers, Tamás says the team is actually gearing up for a follow-up study on the role of rosehip cells on a specific neuropsychiatric disease in human, although he would not divulge which disease.
In one way, the findings really open up more questions they they answer. “Rosehip neurons represent one of our first 'hits'” of these neurons” unique to humans, says Tamás. “There are some other emerging neurons in humans that we haven't published yet.” Our new rosy picture might really just be a harbinger for a thicket of new discoveries.