Abstract:
Although the mammalian cerebral cortex is most often described as a hexalaminar structure, there are cortical areas (primary
motor cortex) and species (elephants, cetaceans, and hippopotami), where a cytoarchitecturally indistinct, or absent, layer
4 is noted. Thalamocortical projections from the core, or first order, thalamic system terminate primarily in layers 4/inner
3. We explored the termination sites of core thalamocortical projections in cortical areas and in species where there is no
cytoarchitecturally distinct layer 4 using the immunolocalization of vesicular glutamate transporter 2, a known marker of core
thalamocortical axon terminals, in 31 mammal species spanning the eutherian radiation. Several variations from the canonical
cortical column outline of layer 4 and core thalamocortical inputs were noted. In shrews/microchiropterans, layer 4 was present,
but many core thalamocortical projections terminated in layer 1 in addition to layers 4 and inner 3. In primate primary visual
cortex, the sublaminated layer 4 was associated with a specialized core thalamocortical projection pattern. In primate primary
motor cortex, no cytoarchitecturally distinct layer 4 was evident and the core thalamocortical projections terminated throughout
layer 3. In the African elephant, cetaceans, and river hippopotamus, no cytoarchitecturally distinct layer 4 was observed and
core thalamocortical projections terminated primarily in inner layer 3 and less densely in outer layer 3. These findings are contextualized in terms of cortical processing, perception, and the evolutionary trajectory leading to an indistinct or absent cortical layer 4.
