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Using a sparse-labeling in utero electroporation method, we investigated the axonal outgrowth of these neurons in mice and correlated the data with axonal projections in adults.Examination of lateral cortex neurons labeled during the main period of cortical neurogenesis (E11.5–E15.5) indicated that axonal outgrowth commonly occurs in the intermediate zone.2007): callosal projection neurons, which extend axons across the corpus callosum and reside in the supragranular (II/III) and infragranular layers (V and VI), and subcortical projection neurons, which extend axons away from the cortex and exclusively reside in the infragranular layers.The diverse projections of these neurons develop in 3 phases: primary axon extension, delayed collateral branch formation, and selective axon elimination (O'Leary and Koester 1993).These neurons include the pyramidal neurons of layers II/III, V, and VI and the spiny stellate neurons of layer IV (Feldman 1984; Jones 1984; De Felipe and Farinas 1992; Lübke and Feldmeyer 2010).They are classified into subtypes on the basis of their highly diverse axonal projections; however, most can be broadly divided into two principal populations (Molyneaux et al.Pregnant mice were injected intraperitoneally with 5-ethynyl-2′-deoxyuridine (Ed U, 25 mg/kg; Invitrogen, Carlsbad, CA, USA; Salic and Mitchison 2008) 3 times (4-h intervals) from the time at 0, 24, or 48 h after electroporation at E12.5.Heads of E13.5–15.5 or brains of E16.5–18.5 mice were immersion-fixed in 4% paraformaldehyde (PFA) in 0.1 M sodium phosphate buffer (p H 7.4) or PBS.

This output system is primarily formed by excitatory cortical neurons, the major neuronal type in the cerebral cortex.

Excitatory cortical neurons project to various subcortical and intracortical regions, and exhibit diversity in their axonal connections.

Although this diversity may develop from primary axons, how many types of axons initially occur remains unknown.

Conversely, the axonal direction varied; neurons labeled before E12.5 and the earliest cortical plate neurons labeled at E12.5 projected laterally, whereas neurons labeled thereafter projected medially.

The expression of Ctip2 and Satb2 and the layer destinations of these neurons support the view that lateral and medial projection neurons are groups of prospective subcortical and callosal projection neurons, respectively.

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