We humans are born in a feeble
state, very much needing our parents to help us survive. They also play a role
in shaping our unripe brains for social life. A major new discovery offers
insight into these remarkable neural developments in the first few months of
life—and highlights the peculiar evolutionary strategy that allows us to have
such big brains.
Scientists have a clear sense of how the brain grows in the
womb. A deep “progenitor” zone creates new cells, called neurons, which are
guided by molecular signals to various specialized regions. We have long
thought that this differentiation in the brain was largely complete before
birth.
The new
study—published in October in the journal Science and led by Mercedes
Paredes of the University of California, San Francisco—changes this
thinking. In fact, new neurons fan out all over the frontal lobes after birth. The frontal lobes carry out our most
distinctively human functions—speech, reason, planning, the regulation of emotions.
Neurons formed in the progenitor zone must migrate across the
brain to the spots where they are needed to form connections. This happens with
help from proteins on the cell’s outer walls. The proteins allow a neuron to
recognize the surfaces and cells that it needs to slink over—and where it must
stop to connect. This circuit formation is key to brain development.
Why are these new cells needed? The scientists identified the
presence of gamma-aminobutyric acid (GABA), which suggests one of the roles of
this cell migration. In the brain, GABA functions chemically to inhibit a cell
from its activities—acting as a kind of “off” switch.
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