Extended Data Fig. 4: Whole-brain single-neuron reconstructions reveal L4 IT projections.
From: Hierarchical organization of cortical and thalamic connectivity
a, L4 neurons are classified into at least three morphological types as shown. b, Image shows sparse labelling of L2/3 and L4 neurons in the tamoxifen-inducible Cux2-IRES-CreERT2 driver crossed with the Ai166 reporter and using a low dose of tamoxifen via oral gavage for 1 day. L4 neurons were identified on the basis of their apical dendrite and local axons, using additional anatomical context when possible. Reconstruction was performed using Vaa3D-TeraVR on the high-resolution whole-brain image stack (composed of more than 10,000 images, resolution x × y × z: 0.3 × 0.3 × 1 μm) acquired with a two-photon fMOST system. c, We identified 25 L4 neurons for complete morphological reconstruction of dendrites and axons for three cell types and three cortical areas. In this Cre line at least, spiny stellate cells (SSCs) were most frequently identified. d, Dorsal surface view shows the CC projection patterns from three anterograde tracer experiments into the predominantly L4 Cre lines for somatosensory cortex (SSp-m), visual cortex (VISp) and auditory cortex (AUD). e–k, Each panel shows two examples of reconstructed cells of the same L4 type in somatosensory, visual or auditory cortex. Local morphology for each cell is shown in the inset. Arrowheads indicate axon clusters outside local region. Red, axon; blue, basal dendrite; black, apical dendrite. Consistent with canonical descriptions, we found SSCs in the somatosensory cortex that had only local axon clusters (e). However, even in these cases, we frequently observed what appeared to be an aborted axon branch (no terminal cluster found; long arrow). We also found SSCs in somatosensory cortex that did have clear axon clusters in nearby areas (g), and, in auditory cortex, SSCs projected even to the opposite hemisphere (f). h–k, Although we identified fewer tufted pyramidal (TPC) and untufted pyramidal (UPC) cell types in this experiment, for both types we still found cells with near and long-range projections.
