Fig. 5. Cerebellar midline fusion and variations in the expression of roof plate markers at the MHB junction. (A) Sagittal section of the cerebellum of a newborn mouse. PCs and GCPs are labeled by in situ hybridization for the detection of Ror (red) and Math1 (blue) transcripts, respectively. (B,C) Math1 is expressed in the hindbrain rhombic lip including the cerebellar midline (arrowhead in B) until E13.5. Math1 expression is downregulated on the cerebellar midline at E14.5 (arrowhead in C) when the GCPs begin to form the external granular layer. (D,E) The proliferating GCPs, labeled by incorporation of BrdU, extend rostrally between E14.5 and E15.5, in particular in the midline region (arrowheads). (F-I) PCs are detected at successive developmental stages (top left of each picture) in whole-mount cerebellum (F) or transverse vibratome sections (G-I) by Ror in situ hybridization (F-H) or CaBP (calbindin, I) immunocytochemistry. PCs are at first excluded from the midline region (arrowhead in F,G), which becomes secondarily colonized by E15.5 (arrowhead in H,I). (J) In mouse the cerebellar midline expresses high levels of GAD67 transcripts suggesting its specialization in the production of GABAergic interneurons. (K-N) Expression of dorsal midline markers is distinctly modulated in the MHB region (arrowheads) of chick (K,L) and mouse (M,N) embryos.