Fig. 4. Notch activity outside sensory patches can induce ectopic hair-cell differentiation. Whole-mount immunostaining in auditory and vestibular epithelia of E10 embryos electroporated at stage HH 13 (E2) with N^ICD-IRES-GFP. (A) At low magnification, the overall pattern of HCA staining is normal in the basilar papilla and lagenar macula (compare with Fig. 2A), but several patches of ectopic hair cells (arrows) have differentiated in the region inferior to the basilar papilla. (B) Higher magnification view of clusters of ectopic hair cells neighbouring the distalmost region of the basilar papilla (white box in A). The ectopic hair cells lie within GFP-positive groups of cells (arrowheads). (C) Low magnification view of sensory epithelium of a crista and the surrounding non-sensory tissues. At some distance from the crista, clusters of GFP-positive cells are present (white box). (D) Higher magnification reveals that some members of each cluster of GFP-positive cells have differentiated as ectopic hair cells (asterisks). The optical section here is transverse to the ectopic sensory epithelium, and the hair bundles (HCA-positive) are on the apical surface. (E) Two compact groups of GFP-positive cells forming spherical or vesicular aggregates. One of these structures contains hair cells (asterisk); the other does not. (F) An ectopic sensory-organ-like structure located at the proximal end of the cochlear duct. GFP-positive cells form a tubular protrusion (arrowheads) ending in a hollow epithelial pouch containing hair cells (asterisk), whose HCA-positive hair bundles face the central lumen. BP, basilar papilla; L, lagenar macula.