IMMUNOREACTIVE SOMATOLACTIN CELLS IN THE PITUITARY OF YOUNG, MIGRATING, SPAWNING AND SPENT CHINOOK SALMON, ONCORHYNCHUS-TSHAWYTSCHA

Immunocytochemical techniques using an antiserum to cod somatolactin ( SL) demonstrated the presence of SL cells in the intermediate lobe of the pituitary in Oncorhynchus tshawytscha. The cells were small in yea rling fish. Two groups of maturing fish were studied. In the spring ru n salmon collected in April and May during the upstream migration, the SL cells appeared stimulated. In September, during spawning, SL cell stimulation was maximal with indices of hypertrophy and degranulation often more marked in females than in males. In the other group, salmon of the fall run collected in the Pacific Ocean in August had well dev eloped gonads, large gonadotropes and abundant SL cells. In spawning s almon (September) the SL cells were stimulated, mainly in females. How ever, the final stimulation was less intense than in spring run spawni ng fish. The SL cells were smaller, without evident granule release, b ut still abundant in spent salmon of the fall run caught at the end of November. Various factors (time spent in rivers before spawning, star vation, decalcification, stress, hypothalamic influences) were conside red which might explain differences between spring and fall run salmon . These observations suggest that SL may play a role in the control of gonadal maturation in chinook salmon as it may also do in sockeye and chum salmon previously studied, and that SL cells may be sensitive to the ambient salinity.