Scientists find homolog of mammalian neocortex in bird brain

Oct 01, 2012

A seemingly unique part of the human and mammalian brain is the neocortex, a layered structure on the outer surface of the organ where most higher-order processing is thought to occur. But new research at the University of Chicago has found the cells similar to those of the mammalian neocortex in the brains of birds, sitting in a vastly different anatomical structure.

The work, published in , confirms a 50-year-old hypothesis about the identity of a mysterious structure in the bird brain that has provoked decades of scientific debate. The research also sheds new light on the evolution of the brain and opens up new animal models for studying the neocortex.

"If you want to study or dopamine cells, which are biomedically important, you can study them in mammals, in chick embryos, in zebrafish. But for these neurons of the , we could only do that in mammals before," said Clifton Ragsdale, PhD, associate professor of neurobiology at University of Chicago Biological Sciences and senior author of the study. "Now, we can take advantage of these other experimental systems to ask how they are specified, can they regenerate, and other questions."

Both the mammalian neocortex and a structure in the bird brain called the dorsal ventricular ridge (DVR) originate from an embryonic region called the telencephalon. But the two regions mature into very different shapes, with the neocortex made up of six distinct cortical layers while the DVR contains large clusters of neurons called nuclei.

Because of this divergent anatomy, many scientists proposed that the bird DVR does not correspond to the mammalian cortex, but is analogous to another mammalian called the amygdala.

"All mammals have a neocortex, and it's virtually identical across all of them," said Jennifer Dugas-Ford, PhD, postdoctoral researcher at the University of Chicago and first author on the paper. "But when you go to the next closest group, the birds and reptiles, they don't have anything that looks remotely similar to neocortex."

But in the 1960s, neuroscientist Harvey Karten studied the neural inputs and outputs of the DVR, finding that they were remarkably similar to the pathways traveling to and from the neocortex in mammals. As a result, he proposed that the DVR performs a similar function to the neocortex despite its dramatically different anatomy.

Dugas-Ford, Ragsdale and co-author Joanna Rowell decided to test Karten's hypothesis by using recently discovered sets of molecular markers that can identify specific layers of mammalian cortex: the layer 4 "input" neurons or layer 5 "output" neurons. The researchers then looked for whether these marker genes were expressed in the DVR nuclei.

In two different bird species—chicken and zebra finch—the level 4 and 5 markers were expressed by distinct nuclei of the DVR, supporting Karten's hypothesis that the structure contains cells homologous to those of mammalian neocortex.

"Here was a completely different line of evidence," Ragsdale said. "There were molecular markers that picked out specific layers of cortex; whereas the original Karten theory was based just on connections, and some people dismissed that. But in two very distant birds, all of the gene expression fits together very nicely with the connections."

Dugas-Ford called the evidence "really incredible."

"All of our markers were exactly where they thought they would be in the DVR when you're comparing them to the neocortex," she said.

A similar experiment was conducted in a species of turtle, and revealed yet another anatomical possibility for these neocortex-like cells. Instead of a six-layer neocortex or a cluster of nuclei, the turtle brain had layer 4- and 5-like cells distributed along a single layer of the species' dorsal cortex.

"I think that's the interesting part, that you can have all these different morphologies built with the same cell types, just in different conformations," Rowell said. "It's a neocortex or a big clump of nuclei, and then in reptiles they have an unusual dorsal cortex unlike either of those."

Future experiments will test the developmental steps that shape these neurons into various structures, and the relative pros and cons of these anatomical differences. The complex language and tool-use of some bird species suggests that the nuclear organization of this pathway is also capable of supporting advanced functions—and even may offer advantages over the .

"If you wanted to have a special nuclear processing center in Broca's area to carry out language processing, you can't do that in a mammal," Ragsdale said. "But in a bird they have these special nuclei that are involved in vocalization. It's as if you have additional flexibility: You can have shorter circuits, longer circuits, you can have specialized processing centers."

Beyond the structural differences, the discovery of homologous cell types will allow scientists to study cortical neurons in bird species such as the chicken, a common model used for examining embryonic development. Such research could help scientists more easily study the neurons lost in paralysis, deafness, blindness, and other neurological conditions.

Explore further: Noted researchers warn that biomedical research system in US is unsustainable

Related Stories

Our brains are more like birds' than we thought

Jul 02, 2010

For more than a century, neuroscientists believed that the brains of humans and other mammals differed from the brains of other animals, such as birds (and so were presumably better). This belief was based, in part, upon ...

The evolution of brain wiring: Navigating to the neocortex

Mar 23, 2011

A new study is providing fascinating insight into how projections conveying sensory information in the brain are guided to their appropriate targets in different species. The research, published by Cell Press in the March ...

Why evolutionarily ancient brain areas are important

Nov 30, 2011

Structures in the midbrain that developed early in evolution can be responsible for functions in newborns which in adults are taken over by the cerebral cortex. New evidence for this theory has been found in the visual system ...

Control of brain waves from the brain surface

Jun 15, 2012

Whether or not a neuron transmits an electrical impulse is a function of many factors. European research is using a heady mixture of techniques – molecular, microscopy and electrophysiological – ...

Researchers identify 'Facebook neurons'

Jan 10, 2011

Carnegie Mellon University researchers have found that within the brain's neocortex lies a subnetwork of highly active neurons that behave much like people in social networks. Like Facebook, these neuronal ...

Recommended for you

Chimpanzees prefer firm, stable beds

9 hours ago

Chimpanzees may select a certain type of wood, Ugandan Ironwood, over other options for its firm, stable, and resilient properties to make their bed, according to a study published April 16, 2014 in the open-access ...

For cells, internal stress leads to unique shapes

10 hours ago

From far away, the top of a leaf looks like one seamless surface; however, up close, that smooth exterior is actually made up of a patchwork of cells in a variety of shapes and sizes. Interested in how these ...

Adventurous bacteria

11 hours ago

To reproduce or to conquer the world? Surprisingly, bacteria also face this problem. Theoretical biophysicists at Ludwig-Maximilians-Universitaet (LMU) in Munich have now shown how these organisms should ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

VendicarD
1.8 / 5 (5) Oct 01, 2012
Don't say "homo". It is impolite, and makes the gay Republicans nervous.

More news stories

Revealing camouflaged bacteria

A research team at the Biozentrum of the University of Basel has discovered an protein family that plays a central role in the fight against the bacterial pathogen Salmonella within the cells. The so cal ...

Chimpanzees prefer firm, stable beds

Chimpanzees may select a certain type of wood, Ugandan Ironwood, over other options for its firm, stable, and resilient properties to make their bed, according to a study published April 16, 2014 in the open-access ...