Characterization of Skn-1a -expressing cells in the main olfactory epithelium. (A) Skn-1a-expressing cells were characterized using two-color in situ hybridization in coronal sections of the MOE at postnatal day 0 with RNA probes for Skn-1a (green) and OSN progenitor/precursor genes Mash1 (neuronal progenitors), Ngn1 (neuronal precursors), and NeuroD (differentiating/postmitotic neurons). Small populations of Skn-1a-potitive cells and Mash1-positive cells overlapped. The arrowhead indicates a co-labeled cell, and arrows indicate either Skn-1a or Mash1 single-labeled cells. None of Skn-1a-positive cells were co-labeled with Ngn1 and NeuroD (arrows). (B and C) In situ hybridization of Skn-1a (green) with OMP (mature OSNs; B, red) and Trpm5 (Trpm5-positive microvillous cells; C, red) in coronal sections of the adult MOE. Neither apical nor basal Skn-1a-expressing cells (arrows) were co-labeled with OMP signals. Trpm5 signals were co-labeled with apical Skn-1a signals (arrowheads) but not with basal Skn-1a signals (arrow). Scale bars, 25 μm. (D and E) Populations of Skn-1a-expressing cells (D) and Mash1-expressing cells (E) were analyzed by two-color in situ hybridization at postnatal day 30. Quantitative analyses revealed that 8.34 ± 2.82% (mean ± SD) of the Skn-1a-expressing cells coexpressed Mash1 (n = 3), and 77.7 ± 5.95% coexpressed Trpm5 (n = 3). In the OSN-lineage, Mash1-positive olfactory progenitors rarely expressed Skn-1a (1.41 ± 0.564%, n = 3).