Fig. 1. Midline defects in the cerebellum and cerebellar primordium of Gbx2^neo homozygotes. (A-C) Dorsal view of posterior E17.5 brains. The cerebellar midline (asterisks) in Gbx2^neo homozygotes (B) is absent compared with control (A), whereas the entire cerebellum is absent in Gbx2^hb homozygotes (C). Insets in A,B show whole-mount ISH of Math1 expression at E11.5. In controls, Math1 is expressed in proliferative granule cell precursors in the cerebellar primordium (CEP; black arrow), including the vermis primordium (white arrowhead, A inset). Math1 expression is downregulated throughout the CEP and is absent along the vermis primordium of Gbx2^neo homozygotes (inset in B). (D,E) Hematoxylin- and Eosin-stained coronal sections of E14.5 cerebella. The vermis primordium is greatly reduced in size and the vermal EGL is much thinner in Gbx2^neo homozygotes (E) compared with controls (D). (F,G) Darkfield images of anti-Phospho-Histone H3 staining (white) on coronal sections of E14.5 cerebella. Mitotically active granule cell precursors are greatly reduced throughout the vermal EGL of Gbx2^neo homozygotes (G). Insets in F,G show whole-mount ISH of Math1 expression at E13.5, illustrating the loss of midline granule cell precursors (arrowhead). Cb, cerebellum; Cp, choroid plexus; Mb, midbrain; Md, medulla; hEGL, hemispheric external germinal layer; ic, inferior colliculus; vEGL, vermal external germinal layer.