Brush cells, the newly identified gatekeepers of the urinary tract.

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A specialized epithelial cell with chemosensory properties of taste cells known from the mouth has been newly identified in the urethra and linked to pathogen recognition. We here describe its properties and its link to defence mechanisms, showing parallels to similar sentinel cells in the respiratory and gastrointestinal tract. In the urethra, slender epithelial cells with apical microvilli ('brush cells') express bitter and umami taste receptors and the downstream signalling cascade known from oropharyngeal gustation, utilizing it to monitor for bacterial products and bacterial growth facilitating conditions. Upon stimulation, they release acetylcholine, and their sensitivity is subjected to cholinergic feedback. They are approached by cholinoceptive sensory nerve fibres, and intraurethral bitter application evokes reflex detrusor activity. Similar cells in the respiratory and gastrointestinal mucosa additionally regulate immune function through local neurogenic inflammation and cytokine release, triggered by bacterial products and parasites. This cell is interpreted to serve as chemosensory sentinel for potential hazardous compounds in the urethral lumen, triggering a protective mechanism (flushing through micturition) against further ascent. Dysfunction may be related to higher risk of infection or inadequate detrusor activity, pharmacological intervention may be considered to combat infection or detrusor overactivity.