Variations in the volumes of parenchyma and stroma of the liver and in the cytology of hepatocytes are associated with gonadal stages in female Ohrid trout (Salmo letnica)

Quantitative microscopic studies of season- and breeding-related changes in the liver of farm-raised fish are very scarce, and those of wild fish populations are virtually nonexistent. Moreover, none of the available studies investigated breeding-related changes in hepatic stroma, although it is already known that changes might exist in the parenchyma. To the best of our knowledge, ours is the first study using wild adult Ohrid trout females to evaluate quantitative influences of the breeding cycle on the volumes of the two liver compartments—parenchyma and stroma. Quantitative microscopy (stereology) using light microscopy was supplemented with semi-quantitative and quantitative analyses of hepatocyte cytology to express in numbers the seasonal changes in the extent of vacuolated versus basophilic cytoplasm. The liver volume and that of each compartment changed from that at the time of pre-vitellogenesis to the end of the spawning season. The changes in total volumes of parenchyma and stroma—increasing from pre-vitellogenesis to late vitellogenesis—followed the changes in total liver volume. Despite all the changes in total volumes, no differences existed in the relative proportions between the two major compartments. After spawning, the stromal volume continued to increase while that of the parenchyma reduced despite no detectable statistically significant impacts on the liver volume. Changes in both the liver and parenchyma volumes were correlated with the gonadosomatic index and plasma oestradiol levels. With this study, we first establish that fish liver remodelling might also occur in the stroma. When comparing Ohrid trout with other fish species, we noted interspecies similarities and differences with regard to the hepatocyte cytology during the breeding cycle. In our study, the semi-quantitative and stereological studies revealed that, from pre-vitellogenesis to post-spawning, although cytoplasmic vacuolation of the hepatocytes decreased, the basophilia increased. Overall, these cytological changes were only partially in agreement with the data from other trout studies. We believe that this is because of intrinsic interspecies differences in association with natural conditions. Thus, establishing normal patterns is crucial, at least for flagship species, namely to support interpretation of histopathological changes in biomonitoring.