One of the major features of the neocortex is the uniform organization. The six-layered structure is similar across and within cortical areas, raising the possibility that the cortical circuit is composed of repeats of homologous elementary circuits, or canonical circuits.
We have previously reported that mouse subcerebral projection neurons (SCPNs), layer 5 excitatory neurons innervating subcortical targets, are aligned radially to form columnar clusters with a radius of 1-2 somal diameters. These microcolumns are located periodically in the tangential orientation constituting a two-dimensional mosaic organization. Microcolumns of SCPNs have been found in all cortical areas investigated including the somatosensory and visual areas, and also in other mammalian species including humans.
We further found that in the developing neocortex, gap junctions specifically couple radially aligned SCPNs. This gap junctional coupling synchronizes firing activities of coupled SCPNs, and therefore likely organizes circuit formation in an activity dependent manner. Indeed, our data suggest that SCPNs in the same microcolumn specifically receive strong common synaptic inputs. Furthermore, in the mouse visual area, SCPNs in the same microcolumns showed similar neuronal activities in response to visual stimulation, suggesting that they have related functions.
These results suggest that the SCPN microcolumn is an evolutionally conserved canonical organization in layer 5. Possible roles of microcolumns in cortical information processing will be discussed.