ionocyte

An ionocyte is a specialized epithelial cell dedicated to active transport of ions across membranes, contributing to osmoregulation and acid–base balance. Ionocytes are found in tissues that interface with the external environment or fluids requiring tight ionic control, including the gills and kidneys of aquatic vertebrates, as well as the skin of some amphibians and certain fish epithelia.

In teleost fish, gill ionocytes are often referred to as chloride cells and are distinguished by abundant mitochondria and the expression of ion transport proteins such as Na+/K+-ATPase, the Na+/K+/2Cl− cotransporter (NKCC) where present, a chloride channel (CFTR), and carbonic anhydrases. A second major type, H+-ATPase–rich ionocytes, uses V-ATPase to acidify the boundary layer and to drive sodium uptake. These cells regulate absorption and secretion of Na+, Cl−, K+, and H+, supporting osmoregulation as fish move between freshwater and seawater.

Formation, distribution, and activity of ionocytes are modulated by environmental salinity and developmental stage; hormones such as cortisol and prolactin have been shown to influence ionocyte abundance and function in several species. In humans and other terrestrial vertebrates, similar ion-transporting cells regulate epithelial ion balance in the kidney and airway epithelia; however, the term ionocyte is primarily used in the context of aquatic osmoregulation. Defects in related transporters can contribute to disorders of salt balance and acid-base homeostasis.