Morphohistochemical changes in hepatocytes during the life cycle of the European eel.
The comparative analysis of morphological, histochemical and cytochemical patterns of eel (Anguilla anguilla L.) hepatocytes reveals clear differences between two stages of its life cycle, i.e. the trophic stage (yellow eel) and reproductive stage (silver eel). The storage of glycogen prevails in the yellow eel, whilst lipids appear to be remarkably increased in the silver eel, in which some hepatocytes also show glycogen-rich areas. Generally, in the silver eel dehydrogenase and acid phosphatase activities seem greater and different distribution of the reaction products is present; on the contrary, a lower G6PDH activity is observed. The electron microscopy characteristics and distribution of both cellular organelles and reserve materials reflect the modifications found at light microscopy. The ultrastructural patterns provide further evidence for the heterogeneity of liver parenchyma in silver eel. In particular, the coexistence of nuclei showing a different degree of chromatin compactness is also accounted for by the quantitative cytochemical data on the nuclear DNA after Feulgen reaction and intercalation with propidium iodide at low and high concentrations. With regard to the DNA content, the hepatocytes in the silver eel as well as in the yellow eel are mainly 2c. However, some 4c values are also found, which according to the literature can be ascribed to cells in G2 phase. The present data may express the onset of different functional requirements during the reproductive stage in comparison with the trophic one. Moreover, our results are consistent with modifications found by other authors as a consequence of interruption of nourishment and during gonad maturation, i.e. two phenomena characterizing the transition from yellow to silver eel.