Ionocyte distribution in gills of the euryhaline milkfish, Chanos chanos (Forsskål, 1775)

As a marine euryhaline teleost, the milkfish, Chanos chanos, is considered a good model fish for studies of ionoregulation. In preparation for studies of the effects of seawater-freshwater adaptation on the biochemistry and morphology of the milkfish gill, the present investigation describes different distributions of ionocytes on the gill epithelium of seawater- versus freshwater-adapted fish. Paraffin sections of filaments revealed afferent, interlamellar, and efferent regions of the epithelium. Round eosinophilic epithelial cells (5-10 μm in diameter) were exhibited in the interlamellar epithelium near the afferent side of the filament. Due to their identical size and location, histochemical (osmium-zinc iodide) and immunofluorescent (Na, K-ATPase) staining was used to demonstrate that these eosinophilic, Na, K-ATPase-immunoreactive (NKIR) cells are ionocytes. It is thus hard to examine the apical openings of ionocytes by scanning electron microscopy in the gill epithelium of the milkfish as compared to those in the other teleosts, because most ionocytes are distributed in the interlamellar regions of the filaments. Further confocal micrographs showed abundant NKIR cells on the filamental epithelia of both seawater- and freshwater-adapted fish. However, NKIR cells were rarely observed on the lamellar epithelia of gills in seawater-adapted individuals, while they were commonly found in freshwater-adapted milkfish.