Kappe neurons are tubulin-negative and actin-positive. Double labeling of Go-ir-positive cells with anti-tubulin or anti-actin antibody is analysed in horizontal cryostat sections of olfactory epithelia. (a) Double fluorescent labeling of Go-ir (green) and tubulin (red) shows absence of co-localization; scale bar 10µm. The insets represent magnified images of single neurons taken at 100× magnification, 0.1µm optical sections. (b) Double fluorescent labeling of Go-ir (green) and actin (red) shows co-localization: Go-ir positive neurons exhibit highly localized actin staining at the apical surface of their cell bodies, the expected position for microvilli. Scale bar 10µm. The insets show single neurons, images taken at 100× magnification, 0.1µm optical sections. (c) Quantification of co-label for Go-ir and actin or tubulin, respectively, shows over 90% co-label (yellow) for actin, but less than 10% co-label for tubulin. The small number of Go-ir/tubulin co-labeled cells is likely to result from the dense packing of cells, dendrites, cilia and microvilli, at the limit of light-microscopic resolution. (d) Schematic representation of four types of olfactory sensory neurons with their laminar position. Ciliated neurons (orange) have round somata and slender dendrites that terminate in bundles of cilia on the epithelial surface. They constitue the most basal layer of olfactory sensory neuron. Microvillous neurons (blue) have bundles of microvilli on their apical surface. Crypt neurons (red) are globular-shaped and carry both microvilli and cilia on their apical surface. They are located more apical than microvillous neurons. Go-ir-positive kappe neurons (green) are pear-shaped with an apical appendage resembling a cap (German: Kappe), have no cilia, and are located even more apical than crypt neurons. Kappe neurons (green) constitute a novel olfactory sensory neuron population.
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