Fig. 1. Expression of Fgf8 and neuronal cell markers in developing OE. (A) Five successive images show in situ hybridization for Fgf8 (full-length ORF probe) and OE neuronal lineage markers in invaginating nasal pit (NP) at E10.5. In whole-mount in situ hybridization, Fgf8 is detected in commissural plate and olfactory placode (white asterisk), branchial arches, mid-hindbrain junction, and limb and tail buds. Scale bar: 1 mm. In serial sections, locations of neuronal lineage markers within the OE are shown: arrowheads indicate Mash1-expressing cells, arrow indicates Ncam1-expressing neurons. Scale bar: 200 µm. (B) Double label in situ hybridization for Fgf8 (full-length ORF probe, orange) and Sox2 (blue) demonstrates overlap of the two markers in a small rim of surface ectoderm and adjacent invaginating neuroepithelium (brackets). Scale bar: 50 µm. (C) In situ hybridization for unprocessed, intronic RNA (Fgf8int probe) expression and processed Fgf8 mRNA (Fgf8ex2,3 probe) expression. Red and blue arrowheads indicate the basolateral extent of intronic RNA versus processed mRNA expression, respectively. The domain of mRNA expression subsumes that of intronic expression. Scale bars: 50 µm. LNP, lateral nasal process; MNP, medial nasal process. (D) Model of peripheral-to-central process of neuronal differentiation in developing OE and origin of Sox2-expressing neural stem cells from Fgf8-expressing ectoderm.