Once you damage it, there’s often no turning back, because the human brain, unlike other parts of the human body, does not appear to create new cells to replace those that have died or been destroyed.

“It doesn’t heal itself the way your skin does,” said Dr. Warren Tourtellotte, a professor of pathology, neurology, and neuroscience at Northwestern University.

Some animals’ brains seem to have the capacity to create new neurons, the primary cells that transmit signals through the brain, but humans seem to have lost this trait, scientists said.

That is one of the things that makes diseases like Alzheimer’s and Parkinson's, which destroy and disable portions of the brain, so permanent and debilitating.

But a group of scientists at the University of California, San Francisco has discovered a new twist to this evolutionary mystery: certain brain cells, they say, appear to be able to create new neurons in adult humans.

In a paper published Thursday in the journal Nature, the researchers announced that they had coaxed cells from a part of the brain called the subventricular zone, or SVZ, to act as stem cells under laboratory conditions, said Nader Sanai, one of the paper’s authors.

Sanai said scientists have known for a while that the SVZ contained stem cells – so-called for their ability to transform into a number of different types of cells. Most of the cells in the body, by contrast, are differentiated cells, which can only produce copies of themselves.

However, while scientists knew that some cells in the SVZ, and in another brain area called the hippocampus, could act like stem cells, they didn’t know which cells, or what by what mechanism they would operate, Sanai said.

But by studying a collection of cells collected from brain surgeries and autopsies, Sanai and his colleagues found they could stimulate cultures of cells called astrocytes to produce new neurons.

It is this ability to create new neurons that makes stem cells valuable to researchers, who believe they could be used medically. In the very long run, scientists said, understanding how the production of neurons worked could allow scientists to grow new brain tissue for people whose brains were damaged by injury or disease.

“The significance [of this research] is quite great,” said Tourtellotte, who was not an author of the study. “Because astrocytes can be easily cultured in a dish, there’s the potential of creating a neuron that could be eventually put back into someone’s brain.”

“In all fairness it’s a bit futuristic, but certainly it’s a step,” he said.

Adult neural stem cells, which might possibly be used to grow new brain tissue, are different from embryonic stem cells, which researchers believe have the potential to grow any type of human tissue.

There remains the question of why human brains don’t appear to create new neurons naturally. In the brains of rodents and other animals, including primates, SVZ astrocytes actively produce large numbers of neurons, which travel to other regions of the brain, where they constantly replace dying neurons, Sanai said.

Interestingly, Sanai’s group found no evidence of a similar cell replacement pathway in human brains.

Scientists do not yet understand the reason for this difference between humans and other classes of mammals but in a commentary in Nature, Yale University neurobiologist Pasko Rakic suggested that the complex networks of the human brain might actually resist the addition of new neurons that could disrupt established pathways.

The idea that the brain’s software is, in a sense, proprietary, could explain why neurons are not known to be replaced in humans, although the potential to do so appears to be present, Rakic writes.

Sanai said the idea was interesting, though unproven.

“When people talk about stem cells they automatically think about the potential of the cells themselves, but it’s much more complicated than that,” he said. “It’s not about simply the potential of the cell, it’s about the niche in which the cell exists. Perhaps the environment [of the brain] is repressing this potential for the greater good. Maybe the complexity of the system is so much that it’s not flexible enough to accommodate [new neurons].”

The results, Sanai said, brings up both promising results, and new questions that need to be answered before the promise of adult stem cell therapies can be realized.

“Not much is really known about human stem cells in the adult brain,” Sanai said.