Retrograde labeling of the corticopontine cells revealed layer 5-restricted elaboration of the dendritic processes.

StTTrR/FuG-C vector was injected into the pontine nuclei. (A and B) Low power views of the cortical sections stained for RFP (red) and NeuN (green). In this injection, retrogradely labeled corticopontine cells were dispersed over wide cortical areas. The areas denoted by arrows are magnified in panels C–E to show the morphology of these neurons. (C–E) Higher power views of the corticopontine cells shown in panels A and B. The break lines indicate the borders above and below layer 5, which can be accurately determined based on the NeuN staining. (F–H) Same as C–H. Only the RFP signals are shown. Note that the horizontal branching from the apical dendritic shafts are mostly contained within layer 5. (I) The images of the corticopontine cells extracted from five different regions. These images are aligned to the position of the layer 4/5 border with no size adjustment. The break lines below the cell bodies indicate the layer 5/6 borders for each cell. Bar: 250 µm for A and B; 100 µm for C–H.