Ectopic granule cell proliferation in Plexin-B2-deficient mice. All sections are sagittal. A, B, E, P13 Plexin-B2−/− cerebellum immunostained for Ki67 and Hoechst (B) and Pax6 (E). Ki67-labeled proliferating cells are found in the EGL (arrows in A, B) but also in the periphery of Purkinje cell islands (asterisks) at the interface with granule cells (arrowheads). Most Ki67-positive cells are also labeled for Pax6 (arrowheads in E). C, D, P11 Plexin-B2+/− (C) and Plexin-B2−/− (D) cerebella hybridized with a Math1 riboprobe. Math1 expression is only detected in the EGL (arrows). In Plexin-B2−/−, gaps in Math1 expression are also observed (arrowheads in D). F–H, P13 Plexin-B2+/− (F) and Plexin-B2−/− (G, H) cerebellum hybridized with Barhl1 riboprobe. In H, Barhl1 signal has been converted to red color with Photoshop and superposed to CaBP immunostaining and Hoechst staining. In Plexin-B2+/−, Barlh1 mRNA is highly expressed in granule cell progenitors in the EGL (arrow) and at a low level in the molecular layer (ML) and IGL. Barhl1 expression is also very high in the EGL of Plexin-B2−/− (arrow) and at a low level the IGL and at the periphery (arrowheads) of the ectopic islands of CaBP-positive Purkinje cells (asterisks). I–K, Quantifications of the number of proliferating cells at P11 (I) or P15 (J, K) revealed by short-term BrdU pulse labeling (I, J; see Materials and Methods) or H3P immunostaining (K). Number of cells are expressed by millimeters of sections (J, K) or by 60× microscope field (I; see Materials and Methods). Statistics are based on n = 2 animals for each cases. Scale bars: A, 150 μm; B, 60 μm; C, D, 30 μm; E, 20 μm; F–H, 40 μm.