THE ROLE OF GROWTH-HORMONE IN THE ADAPTABILITY OF ATLANTIC SALMON (SALMO-SALAR L) TO SEAWATER - EFFECTS ON THE MORPHOLOGY OF THE MUCOSA OF THE MIDDLE INTESTINE

Ultrastructural modifications of the middle intestine of the salmon, S almo salar, induced by transfer to seawater have been studied in two g roups of fish: the first group received sham treatment and the second was treated with ovine growth hormone (oGH). In sham-treated fish duri ng the first 2 days in seawater, significant distension of the interce llular spaces was observed between the apical tight junction and the b asement membrane. In the basal part of the enterocytes, tubular invagi nations in the intercellular spaces were closely associated with mitoc hondria. In oGH-implanted fish, we observed no signs of modification o f the ultrastructure of the mucosa. There were no dilatations of the i ntercellular spaces and no infoldings in the basal part of the enteroc ytes. After 7 days in seawater, the mucosa of the intestine of sham- a nd oGH-treated fish was quite similar. The effects of oGH treatment we re clear, and treatment seemed to provoke ''pre-adaptation'' of the in testinal mucosa before exposure to high salinity to maintain the morph ology of the middle intestine of Atlantic salmon abruptly transferred to seawater.