Structure and Function of the Chloride Cell of Fundulus heteroclitus and Other Teleosts

Teleosts, the bony fishes, inhabit both freshwater and seawaterenvironments. Some euryhaline fish, such as Fundulus heteroclitus,alternate between the two milieux several times daily. Regardlessof adaptation, the gills of these animals possess a highly specializedcell type called the chloride cell. This cell contains numerousmitochondria and exhibits a greatly amplified basolateral cellsurface richly endowed with Na,K-ATPase. Recent studies on isolatedopercular epithelia containing chloride cells have demonstratedactive chloride secretion and passive transepithelial sodiummovements, and have established the chloride secretory roleof this cell type in seawater-adapted teleosts. Current modelssuggest that chloride transport occurs via a transcellular route.Seawater chloride cells exist in multicellular units and sharesimple, shallow tight junctions which are thought to be theroute for passive sodium movement. Freshwater chloride cells,whose function remains to be elucidated, are generally describedas existing in a unicellular configuration. However, recentobservations in Fundulus heteroclitus adapted to salinitiesas low as 1% sea water reveal that chloride cells persist inmulticellular complexes with apical crypts. Strikingly, tightjunctions between chloride cells in this freshwater environmentare deep