Dual activation of Shh and Notch signaling induces dramatic enlargement of neocortical surface area

The expansion of the neocortex represents a characteristic event over the course of mammalian evolution. Gyrencephalic mammals that have the larger brains with many folds (gyri and sulci) seem to have acquired higher intelligence, reflective of the enlargement of the neocortical surface area. In this process, germinal layers containing neural stem cells (NSCs) and neural progenitors expanded in number, leading to an increase in the total number of cortical neurons. In this study, we sought to expand neural stem/progenitor cells and enlarge the neocortical surface area by the dual activation of Shh and Notch signaling in transgenic (Tg) mice, promoting the proliferation of neural stem/progenitor cells by the Shh signaling effector while maintaining the undifferentiated state of NSCs by the Notch signaling effector. In the neocortical region of the Tg embryos, NSCs increased in number, and the ventricles, ventricular zone, and neocortical surface area were dramatically expanded. Furthermore, we observed that folds/wrinkles on the neocortical surface were progressively formed, accompanied by the vascular formation. These findings suggest that Shh and Notch signaling may be key regulators of mammalian brain evolution.