Fig. 2: Timing of reciprocal hollow- and septa-related gene expression in developing L4 barrel cortex is coincident with TCA innervation.

a, b A developmental time course of barrel formation was followed from P0 (top), up to the clear formation of barrel structures by P6 (bottom). a Specific hollow-related gene expression is up-regulated at P2 with onset of TCA innervation (indicated by arrows), as outlined by vGlut2 immunostaining (left column). This expression adopts a clustered barrel pattern by P6 (asterisks). b Septa-related gene expression (Cdh8 and Nrxn2 are shown) is broadly expressed in the developing neocortex at birth (top). Expression within the RORc⁺ L4 barrel region is progressively restricted upon TCA innervation, with inter-barrel patterns evident by P6 (arrows). Middle panels are higher magnification images of red outlined regions. c Appearance of spiny stellate neurons in barrel cortex is also associated with TCA innervation. Sparse-labeling of L4 neurons with DsRed2 was performed by IUE at E13.5. At P0, L4-destined neurons are found with an immature morphology possessing an apical process that extends to the pial surface (blue arrowheads). At P2, while most neurons appear to have an immature morphology and an extended apical dendrite, some neurons appear to have shorter apical dendrites (blue arrowheads). By P4, a clear distinction between spiny stellate (blue arrowhead) and pyramidal morphologies (red arrowhead) could be made, with progressively more spiny stellate neurons found at the edge of barrels (visualized by vGlut2 staining), having a shorter apical dendrite and asymmetrical dendritic fields directed toward the center of TCA clusters. Unless indicated, scale bars are 100 µm. Dotted lines outline the pial surface.