Purkinje cell layer folding indicates the future positions of the base of each principal fissure. (a, d) At E16.5 the mouse Cb has a smooth surface, but anti-Calbindin immunostaining (red) shows a multilayer of Pcs with invaginations in the areas where fissures will form (asterisks). Yellow asterisk indicates the fissures shown in (d, e, f). (b, c, e, f) At E17.5 (b, e) and E18.5 (c, f) both the Pc layer and outer surface invaginate (foliate) simultaneously. Anti-Pax6 immunostaining shows accumulation of the gcps in the EGL, above the Pc layer invagination at E16.5 and E17.5 (inset in (d, e)), whereas by E18.5 the EGL is similar in thickness at the base and at the crown of the folia (inset in (f)). (g-g2) In R26-CreER; R26R-eYFP animals, Cre is expressed ubiquitously in precursors. Upon administration of tamoxifen at E12.5, Cre ER translocates to the nucleus, where it recombines the floxed STOP signal in the R26-eYFP locus. Sagittal sections of E17.5 Cb show that some fate mapped cells (green) colocalize with anti-RORα (red) and are, therefore, Pcs. Marked Pcs (white arrows) have round cell bodies and have extended their axons in various directions. Scale bars: (a-c) 100 μm; (d-f) 40 μm; (g-g2) 15 μm.