The neocortex consists of diverse neurons that are classified into excitatory projection neurons and inhibitory interneurons. Of these, excitatory projection neurons are produced from progenitor cells in a stereotypical temporal order, where deep-layer (DL) neurons are generated first, followed by more superficial upper-layer (UL) projection neurons. These UL neurons play crucial roles in integrating higher-order information within the two cerebral hemispheres, whereas perturbations in their development result in associative dysfunctions such as autism spectrum disorders. Despite their functional importance, the mechanisms by which UL projection neurons are generated at correct timing and in appropriate numbers remain elusive. This is particularly important for understanding the principles of the assembly and function of neocortex in mammalian vertebrates, which have undergone significant expansion during evolution.
To explore these questions, we utilized a system in which the timing and extent of neocortical neurogenesis can be manipulated in vivo. Using a temporal inducible system, we demonstrate that the onset of Foxg1 expression triggers the sequential generation of DL and UL neurons in part through suppression of Tbr1 expression, and triggers downstream gene program. We further established a non-biased genetic ablation system using Neurog2 expression to ablate early post-mitotic DL neurons. Our results demonstrate that sequential acquisition of UL competence requires negative feedback propagated from the post-mitotic DL neurons. These results indicate that neocortical progenitors integrate both intrinsic and extrinsic cues to generate UL neurons, a regulatory system that controls the sequence of deep- and upper-layer neurogenesis, and scale the production of UL neurons with DL counterparts during neocortical development and evolution.