Sonic hedgehog-Expressing Basal Cells Are General Post-Mitotic Precursors of Functional Taste Receptor Cells

Background: Taste buds contain similar to 60 elongate cells and several basal cells. Elongate cells comprise three functional taste cell types: I, glial cells; II, bitter/sweet/umami receptor cells; and III, sour detectors. Although taste cells are continuously renewed, lineage relationships among cell types are ill-defined. Basal cells have been proposed as taste bud stem cells, a subset of which express Sonic hedgehog (Shh). However, Shh1 basal cells turn over rapidly suggesting that Shh1 cells are post-mitotic precursors of some or all taste cell types. Results: To fate map Shh-expressing cells, mice carrying ShhCreER(T2) and a high (CAG-CAT-EGFP) or low (R26RLacZ) efficiency reporter allele were given tamoxifen to activate Cre in Shh1 cells. Using R26RLacZ, lineage-labeled cells occur singly within buds, supporting a post-mitotic state for Shh1 cells. Using either reporter, we show that Shh1 cells differentiate into all three taste cell types, in proportions reflecting cell type ratios in taste buds (I > II > III). Conclusions: Shh1 cells are not stem cells, but are post-mitotic, immediate precursors of taste cells. Shh1 cells differentiate into each of the three taste cell types, and the choice of a specific taste cell fate is regulated to maintain the proper ratio within buds. (C) 2014 Wiley Periodicals, Inc.