Chloride cells are cells in the gills of teleost fishes which pump excessive sodium and chloride ions out into the sea against a concentration gradient in marine fish. Alternatively, in the gills of freshwater teleost fish, they pump sodium and chloride ions into the fish, also against a concentration gradient.
Mechanism of action
Marine teleost fishes consume large quantities of seawater to reduce osmotic dehydration. The excess of ions absorbed from seawater is pumped out of the teleost fishes via the chloride cells. These cells use active transport on the basolateral (internal) surface to accumulate chloride, which then diffuses out of the apical (external) surface and into the surrounding environment. Such mitochondria-rich cells are found in both the gill lamellae and filaments of teleost fish. Using a similar mechanism, freshwater teleost fish use these cells to take in salt from their dilute environment to prevent hyponatremia from water diffusing into the fish. 
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