title: Is TrpM5 a reliable marker for chemosensory cells? Multiple types of microvillous cells in the main olfactory epithelium of mice.
abstract: BACKGROUND
  In the past, ciliated receptor neurons, basal cells, and supporting cells were considered the principal components of the main olfactory epithelium. Several studies reported the presence of microvillous cells but their function is unknown. A recent report showed cells in the main olfactory epithelium that express the transient receptor potential channel TrpM5 claiming that these cells are chemosensory and that TrpM5 is an intrinsic signaling component of mammalian chemosensory organs. We asked whether the TrpM5-positive cells in the olfactory epithelium are microvillous and whether they belong to a chemosensory system, i.e. are olfactory neurons or trigeminally-innervated solitary chemosensory cells.
  
  
  RESULTS
  We investigated the main olfactory epithelium of mice at the light and electron microscopic level and describe several subpopulations of microvillous cells. The ultrastructure of the microvillous cells reveals at least three morphologically different types two of which express the TrpM5 channel. None of these cells have an axon that projects to the olfactory bulb. Tests with a large panel of cell markers indicate that the TrpM5-positive cells are not sensory since they express neither neuronal markers nor are contacted by trigeminal nerve fibers.
  
  
  CONCLUSIONS
  We conclude that TrpM5 is not a reliable marker for chemosensory cells. The TrpM5-positive cells of the olfactory epithelium are microvillous and may be chemoresponsive albeit not part of the sensory apparatus. Activity of these microvillous cells may however influence functionality of local elements of the olfactory system.
authors.affiliation: Rocky Mountain Taste and Smell Center, Dept. of Cell and Developmental Biology, School of Medicine, University of Colorado Denver, Aurora, 80045, USA. Anne.Hansen@uchsc.edu
authors.forename: Anne
authors.initials: A
authors.lastname: Hansen
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Finger
mesh_terms: D000818:Animals; D001369:Axons / Q000648:ultrastructure; D015415:Biomarkers / Q000032:analysis*; D002628:Chemoreceptor Cells / Q000737:chemistry / Q000166:cytology / Q000648:ultrastructure; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D008871:Microvilli / Q000648:ultrastructure; D009831:Olfactory Mucosa / Q000737:chemistry* / Q000166:cytology / Q000294:innervation* / Q000648:ultrastructure; D050053:TRPM Cation Channels / Q000032:analysis* / Q000235:genetics
chemical_list: D015415:Biomarkers; D050053:TRPM Cation Channels; C498617:Trpm5 protein, mouse