Hepatic ulstrastructure changes associated with the parr‐smolt transformation of Atlantic salmon (Salmo salar)

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Liver samples of two groups of juvenile Atlantic salmon (Salmo salar) were collected fall through spring and examined by light and transmission electron microscopy. One group of fish was maintained under simulated natural photoperiod (SNP), the other under an 8 month advanced, phase‐shifted photoperiod (8MA). As indicated by changes in branchial Na+/K+ATPase activity, salinity tolerance, and morphological characteristics, fish in the SNP group underwent parr‐smolt transformation in the spring. Smoltification‐associated changes in ultrastructure were observed in liver parenchymal cells only. Hepatocytes of SNP smolts were smaller with markedly decreased glycogen content as compared to cells of SNP parr. An increase in mitochondrial size and a decrease in numbers of mitochondria per hepatocyte were also seen during smoltification. In smolt hepatocytes, the nucleus and mitochondria occupied a greater percent of the cell volume than in parr liver parenchymal cells. Rough and smooth endoplasmic reticulum, dense bodies, and Golgi profiles also appeared to occupy more of the cell volume of smaller hepatocytes seen during smoltification. No smoltification‐associated changes were seen in hepatocyte nuclear size, chromatin state, or nucleolar frequency. During the fall, Atlantic salmon in the 8MA group underwent some morphological and osmoregulatory changes associated with smoltification. Hepatic fine structure of these fish in the fall resembled that seen in SNP smolts. In spring, 8MA fish exhibited no indications of smoltification, and hepatocyte ultrastructure of these fish was similar to that of SNP parr. Biological implications of observations are discussed, particularly in light of other investigations concerning the liver and smoltification. Copyright © 1991 Wiley‐Liss, Inc., A Wiley Company

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Journal of Experimental Zoology