The mammalian neocortex undergoes dramatic transformation during
development, from a seemingly homogenous sheet of neuroepithelial cells into a
complex structure that is tangentially divided into discrete areas. This process
is thought to be controlled by a combination of intrinsic patterning mechanisms
within the cortex and afferent axonal projections from the thalamus. However,
roles of thalamic afferents in the formation of areas are still poorly
understood. In this study, we show that genetically increasing or decreasing the
size of the lateral geniculate nucleus of the mouse thalamus resulted in a
corresponding change in the size of the primary visual area. Furthermore,
elimination of most thalamocortical projections from the outset of their
development resulted in altered areal gene expression patterns, particularly in
the primary visual and somatosensory areas, where they lost sharp boundaries
with adjacent areas. Together, these results demonstrate the critical roles of
thalamic afferents in the establishment of neocortical areas.
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