Effect of decreased environmental salinity on growth hormone cells in the gilthead sea bream (Sparus aurata)

The effect of decreased environmental salinity on growth hormone producing cells (GH cells) of the adenohypophysial proximal pars distalis has been studied in the gilthead sea bream ( Sparus aurata L.) adapted to sea water (SW, 980 mosmol kg −1 ) and brackish water (BW, 200 mosmol kg −1 ). A combined immunocytochemical, morphometric and electron microscopic study was carried out. GH cells offish adapted to BW occupied a greater hypophysial volume (about 21% of the total hypophysial volume in BW, 17% in SW) and had a larger nuclear area (mean 16 μm 2 in BW, 13 μm 2 in SW) than GH cells of SW‐adapted fish. The immunoreactivity against a salmon GH‐antiserum was lower in BW (mean optical density 142 in BW, 159 in SW). Ultrastructural characteristics of GH cells of BW‐adapted fish were distended rough endo‐plasmatic reticulum and large secretory granules (about 216 nm in diameter for BW, 209 nm in SW). Volumetric, densitometric and ultrastructural evidence suggested that the synthesis and release of GH were activated in S. aurata adapted to hypo‐osmotic environment.