Around 85% of humans are right-handed, but genetic and environmental
factors may provide alternative paths of development, such as
left-handedness or two-handedness.
Our nervous systems have left-right differences that are important
for correct functioning. Handedness is probably the best known asymmetry
arising from the development of the nervous system. This is observed
very early on: embryos of eight weeks already tend to move their right
arms more often than their left arms.
‘Embryos of eight weeks already tend to move their right arms more often than their left arms. At this 'age' signals are not sent from the brain to the arms yet, but only from the spinal cord.’
At this 'age' signals are not sent
from the brain to the arms yet, but only from the spinal cord. A few
weeks later, left-right differences also become visible in the shape and
size of the premature brain.
A team of scientists from the Netherlands, the UK and China searched
for genes that contribute to left-right differences in the nervous
system, in the period between four and eight weeks after fertilization.
The genetic analysis showed that the left and right sides of the spinal
cord develop at different paces.
The left side of the spinal cord matures slightly faster than the
right side. Sets of key genes that control growth and maturity were
found to reach a more advanced profile of activity on the left side than
the right. In the hindbrain, an area which is the predecessor for some
adult parts of the brain, this was the other way around.
"This seems logical, since many nerve fibers cross over from one
side to the other at the boundary between the hindbrain and spinal
cord," says Carolien de Kovel, lead author of the study and researcher
at the Max Plank Institute for Psycholinguistics (MPI). "How exactly
this left-right genetic difference in the spinal cord leads to
right-handedness is, however, not yet clear."
Clyde Francks, head of the MPI research group 'Brain and behavioral
asymmetries' and Research Fellow at the Donders Institute at the Radboud
University, explains, "We think that these very early left-right
differences in the spinal cord may act to trigger some of the later
asymmetries of the brain, such as the eventual dominance of the left
hemisphere for language functions in most adults'.
Asymmetry and schizophrenia
"Right-handedness seems the standard in
human development," De Kovel adds, "but some people may exhibit
left-handedness or two-handedness. Interestingly, disturbances in such
asymmetries seem to be more common in people with psychiatric conditions
such as schizophrenia."
Hence, De Kovel and her colleagues also compared the results of
their study with genetic factors that influence the risk of
schizophrenia. It was found that genes which exhibit the largest
left-right differences in the embryos also tended to be involved in the
risk of schizophrenia.
"The findings do not prove directly that these
genes cause schizophrenia by their actions in the spinal cord, because
the same genes are also active in the grown up brain. However this does
provide us with clues on which we can base further research," De Kovel