Brain can repair itself, study in mice suggests

In experiments with mice, researchers damaged a specific set of mature nerve cells in the cerebral cortex, and found that primitive cells known as neural precursors began to divide in an effort to replace the damaged cells.

Although any clinical application of the findings is years, if not decades, away, the results lay the groundwork for a new approach to treating brain and spinal cord injuries as well as degenerative diseases like Alzheimer's.

If the human brain can be made to launch enough precursor cells to replace dying ones, the brain may be able heal itself from the inside, according to the researchers led by Dr. Jeffrey D. Macklis of Harvard Medical School in Boston. They report their findings in the June 22nd issue of Nature. The neural precursor cells share some characteristics with stem cells, the type of cell that has the capacity to give rise to many different types of cells.

In an interview with Reuters Health, Macklis said that in the next 10 to 30 years, scientists may be able to "repopulate" damaged areas of the adult brain with new, functioning cells born of the person's own precursor cells. When and how well such a tactic would work will depend on the type of injury. For instance, according to Macklis, some spinal cord injuries may be repaired with a relatively small number of neuronal connections and should be easier to tackle than a complex brain disease like Alzheimer's.

Neural precursor cells are active during foetal development and less than 10 years ago, researchers still believed the adult brain had no precursors left and was thus unable to generate new nerve cells. (interview) Scientists now know that precursors exist at low levels throughout the adult brain, and that new cells do indeed develop in two brain areas--one responsible for our sense of smell, the other for the rapid turnover of memory.

The cerebral cortex, however, does not spontaneously sprout new cells to replace dead ones. The cerebral cortex is the thin layer of grey matter on the surface of the brain that controls higher mental functions, general movement, perception and behavioural reactions.

Macklis and his colleagues speculated that this is because the right "signals" do not reach precursors in this area, and not because the precursors are incapable of growing into mature neurons. (interview)

In their experiments, the Harvard researchers injected microscopic beads into the brains of mice and then two weeks later exposed the mice to laser light that activated the beads--prompting a natural cell-suicide program that killed a specific set of cells.

They found that some neural precursors did indeed multiply, grow up into mature neurons, and reach out to connect with other neurons in the damaged area. (interview & PR, pg. 2, par 2)

Macklis stressed that no one would ever suggest treating brain injuries by selectively killing brain cells. Instead, this experiment illustrates the principle that cerebral cortex precursors can be stimulated to replace dying mature cells.

But if precursors exist in the adult human brain, why do they not naturally repair injury or disease? In the case of brain injury, Macklis pointed out, the damage involves a more extensive area than just a group of cells. With degenerative diseases, such as Alzheimer's or Parkinson's, small numbers of cells die sporadically--perhaps not of a magnitude that would signal precursors to assist.

Alternatively, Macklis said, precursors may indeed naturally respond to brain damage, but at levels too low to detect. Weeding out what controls the precursor response is one goal of future research.(interview)