Researchers have found a stem cell population that exclusively leads to the birth of nerve cells responsible for intelligence in higher mammals, especially humans. Researchers say that this discovery could help them grow nerve cells in a lab and could in turn help treat brain disorders.

The present research shows that not all brain cells come from the same type of the stem cell, called radial glial cells, as was earlier believed. Some cells like those in the cerebral cortex are born from a separate stem cell.

"The cerebral cortex is the seat of higher brain function, where information gets integrated and where we form memories and consciousness. If we want to understand who we are, we need to understand this area where everything comes together and forms our impression of the world," said Ulrich Mueller, a professor and director of the Dorris Neuroscience Center at Scripps Research.

The cerebral cortex is made up of many uneven layers like that of an onion. The layers on the inside are small and host important life functions like breathing while the upper layer are stretched and hold functions related to information processing.

It is these upper layers that help humans think rationally and hold abstract ideas and are the "relatively young invention"which have become more efficient during primate evolution.

The stem cells were discovered in mice in which the growth of the brain could be tracked. The researchers used genetic markers to see how the cells in the upper layer of the cortex are born. They followed a gene, Cux2, that is exclusively present in the upper layers and tagged it with an enzyme called Cre recombinase. This enzyme turns another gene on that glows fluorescent red.

Until now, researchers believed that the brain develops from inside out meaning that nerve cells in the inner region of the cortex are born from the original stem cells first and then the other layers are added in a time specific manner (the cells born first invariably become cells in the inner region and cells that come later become cells of the upper regions). But, the present study showed that the biomarker gene glowed in the first few days of brain development, even before any cells in the upper layer of the cortex were born.

Researchers concluded that regardless of time, stem cells that were positive for Cux2 always became the upper layer nerve cells.

"Advanced functions like consciousness, thought, and creativity require a lot of different neuronal cell types and a central question has been how all this diversity is produced in the cortex. Our study shows this diversity already exists in the progenitor cells," said Santos Franco, first author of the paper, in a press release.

In many disorders, the nerve cells in the upper layers of the cortex aren't connected properly.

"This opens a door now to try to make the upper-layer neurons, which are frequently affected in psychiatric disorders," said Mueller. Researchers were, until now, trying to grow upper layer cells from stem cells that gave rise to lower level cells.

The study is to be published in the journalĀ Science.