NEW YORK, Jun 22 (Reuters Health) - Adding to a growing body of research
showing that the adult brain can indeed grow new cells, Harvard investigators
have the first evidence that the brain tries to repair itself after injury.
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 fetal 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 gray matter on the
surface of the brain that controls higher mental functions, general movement,
perception and behavioral 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)
