Department of Molecular Physiology

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The olfactory epithelium covers a complicated system of turbinates in the posterior part of the nasal cavity. The turbinates are made of cartilage and provide a large surface for the contact of the inhaled air with the sensory epithelium. A coronal section of the nose (perpendicular to the body axis) reveals the intricate structure of these turbinates. The figure shows only a small part of a coronal section, stained blue with DAPI to vizualize the nuclei of all cell types. The red line marks the sensory surface – the chemosensory cilia of the olfactory sensory neurons. These cilia detect the presence of odorants in the air that fills the (black) spaces between the turbinates.  The red colour results from immunostaining the enzyme adenylyl cyclase III (AC III) with a red-fluorescent dye. AC III is a key enzyme in the olfactory transduction cascade: activated by odorant receptor proteins via a G-protein, it synthesizes cAMP, the second messenger of olfactory signal transduction.

The olfactory epithelium is a stratified tissue, covered with a thin layer of chemosensory cilia which are stained red here by immunolabeling the ciliary enzyme AC III. Directly underneath the cilia, a single layer of epithelial supporting cells (blue DAPI stain) serve as barrier between the tissue and the air outside. The next layer is formed by mature olfactory sensory neurons which are labeled green here by the green fluorescent protein (GFP) which is expressed under the control of the OMP-promoter in this transgenic mouse from the lab of Peter Mombaerts (Columbia University, New York). OMP, the olfactory marker protein, is expressed exclusively in mature olfactory neurons. Since olfactory sensory neurons live for only a few weeks, they have to be replaced constantly by newly differentiating neurons. The nuclei of roughly five to seven layers of such immature neurons can be seen under the green cells layer. The layer with a few green blots is the submucosa, a tissue layer that consists of connective tissue, blood vessels and nerve fascicles of olfactory neurons (stained green by GFP). The bottom layer is cartilage with the nuclei of chondrocytes stained by DAPI.

The turbinates are not completely covered with olfactory epithelium. In some regions, no olfactory sensory neurons are expressed – the respiratory epithelial cells in these regions carry rapidly moving microvilli. The figure on the right shows a region of transition between respiratory (left) and olfactory (right) epithelia. The tissue is only 2-3 cell layers thick, mature olfactory neurons are marked by GFP. The inset shows expression of the GTP-binding protein G_olf, immunostained red. G_olf is expressed inn the cilia, but also in dendrites, somata and axons of olfactory neurons.