Riechepithel

Riechepithel, or olfactory epithelium, is a specialized sensory epithelium lining the superior part of the nasal cavity. It is a neuroepithelium that supports the sense of smell and serves as the first stage of the olfactory system. It is located mainly on the superior turbinate and the upper nasal septum and is covered by a thin mucus layer produced by Bowman's glands in the underlying lamina propria.

The epithelium contains three principal cell types: olfactory receptor neurons (ORNs), sustentacular (supporting) cells, and basal cells. The ORNs are bipolar neurons equipped with a dendritic knob bearing nonmotile olfactory cilia that extend into the mucus. Their axons bundle to form the olfactory nerve fibers, which pass through the cribriform plate to reach the olfactory bulb. Sustentacular cells provide structural and metabolic support, while basal cells include horizontal and globose basal cells that act as stem cells to replace ORNs and supporting cells as needed.

Each olfactory receptor neuron expresses one or a few odorant receptor proteins. Odorants dissolve in the mucus and bind to receptors on the cilia, triggering a G-protein coupled signaling cascade that depolarizes the neuron. The signal is transmitted by mitral and tufted cells in the olfactory bulb and projects to higher olfactory centers, including the primary olfactory cortex.

Regeneration is a notable feature: the epithelium continuously renews itself, with a turnover of roughly 4 to 8 weeks in humans. Basal cells drive this renewal. Damage from infection, trauma, or chronic exposure can impair olfactory function, and aging is associated with a gradual decline in smell.