Dark cells are specialized nonsensory epithelial cells found on either side of the vestibular organs, and lining the endolymphatic space. These dark-cell areas in the vestibular organ are structures involved in the production of endolymph, an inner ear fluid, secreting potassium towards the endolymphatic fluid. Dark cells take part in fluid homeostasis to preserve the unique high-potassium and low-sodium content of the endolymph and also maintain the calcium homeostasis of the inner ear. Morphological and immunohistochemical studies in several species have indicated that these dark cell areas also form a single layer resting on top of pigmented cells at the base of the cristae ampullaris in the semi-circular canals and around the utricular macula. Structure Dark cells are morphologically and functionally similar to marginal cells of the stria vascularis as they both display characteristics of fluid transport tissue; however, studies indicate an earlier histological and immunohistological maturity in the dark-cell areas compared to the stria vascularis. The dark cell epithelium consists of cells with a multitude of pinocytotic vesicles near their luminal surface. A numerable portion of infoldings occurs at the basal end of the dark cell toward the basal membrane. These infoldings contain a high quantity of mitochondria. The nucleus of the dark cell is displaced toward the surface. Function Vestibular dark cells transport potassium ions into the inner ear endolymph, a potassium-rich fluid whose homeostasis is essential for hearing and balance. Dark cell regions of the vestibular system are involved in active (energy consuming) ion transport to maintain the unusual endolymph composition. In other words, dark cells utilize the Na+/K+-ATPase pump in order to transport potassium. The basolateral membranes of vestibular dark cells are highly folded, allowing the enclosure of the numerous large mitochondria, and they contain high levels of Na+/K+-ATPase in both alpha and beta isoforms, transporting potassium into the cell in exchange for sodium while consuming ATP. The infoldings also create large surface area over which ion exchange can take place and the plethora of mitochondria enclosed provides the needed energy source of ATP for active transport. The basolateral membrane also contains a Na+/K+/Cl—-co-transporter, (NKCC1) which transports all three ions into the cell. The transport of sodium ions into the cell enhances the effect of the Na+/K+-ATPase pump by stimulating the outward transport of Na+, and therefore, the inward transport of K+. Na-K-Cl cotransporter is the therapeutic target of action for loop diuretics in the kidney