foxj1 expression correlates with elongation of monocilia and the emergence of multiple cilia. In situ hybridization and scanning electron microscopy (SEM) on explanted brains at stage 35. (A) Explant shown in dorsal (d) and ventral (v) view. Strong expression in the ventral midline and the subcommissural organ (SCO; arrowhead). (B) SEM picture of brain explant dissected sagittally with view onto the ventricular surface, the zona limitans intrathalamica (ZLI) is delimited with orange dashed line and boundaries between brain regions are indicated by white dashed lines. Bar represents 200 μm. (C) Overview and enlargements show short primary cilia on cells in the prosencephalic region (arrowhead in (C’ )), and elongated monocilia on cells within the ZLI (outlined arrowhead in (C” )). (D) Close-up view onto the ventral aspect of a single hindbrain rhombomere with indicated boundaries and midline (ml). Enlargements showing short cilia on the rhombomere (arrowhead in (D’ )) as well as elongated cilia in the ventral ml (outlined arrowhead in (D” )). (E) Close-up view onto the rhombencephalon (rhomb) roof with elongated monocilia (outlined arrowhead) and first MCCs (arrowhead). a = anterior; E = epiphysis; l = left; mes = mesencephalon; p = posterior; Po = preoptic region; pros = prosencephalon; r = right.