Cbx1^−/− mutants exhibit defective cerebral corticogenesis and reduced proliferation of neuronal precursors. (A and B) Nissl staining of the E19 wild-type neocortex (A) shows the typical ordered arrangement of CP cells. In the E19 Cbx1^+/− neocortex (B), the ordered arrangement of CP cells is disturbed, and the boundaries at the top and bottom edges of the CP layer are blurred. (C–E) In the E19 Cbx1^−/− neocortex, the lamination of the neocortex is severely disturbed with patches of edema (C and D, arrows) and clusters of dying cells (E, arrows) scattered through the cortical layers. (F) Proliferating neural progenitors are depleted in the VZ of Cbx1^−/− brains. On E17, the layer of pKi-67–positive proliferating neural progenitors is thicker in the wild-type VZ compared with the Cbx1^−/− VZ. On E19, the pool of proliferating progenitors of the Cbx1^−/− VZ is essentially exhausted compared with the wild-type E19 VZ. (G) There is a trend toward fewer neurospheres from cultures of Cbx1^−/− brains compared with wild-type and Cbx1^+/− brains (P < 0.04); each dot represents the result from a single embryo. There is no significant difference in the numbers of neurospheres from the wild-type and Cbx1^+/− brains. Black lines represent the medians. IZ, intermediate zone; MZ, marginal zone. Bars: (A–E) 60 μm; (F) 80 μm.