OOGENESIS IN THE EEL (ANGUILLA-ANGUILLA L) - OOCYTE ULTRASTRUCTURE AT DIFFERENT STAGES OF DEVELOPMENT AND ROLE PLAYED BY LIPOPROTEINS DURING VITELLOGENESIS

Eels belong to two stages of development : yellow during which the fish feed and grow and silver when eels fasted and started their reproductive migration. This cycle is especially interesting to study the hormonal induction of oogenesis and especially vitellogenesis, important step in which oocytes incorporate vitellogenin, the precursor molecule for yolk. First, this paper describes different available criteria for vitellogenesis in the eel (lipoprotein characterization, oocyte ultrastructure, sex steroids and vitellogenin plasma levels, hepatic receptors for estradiol (E2R)) and second, the effects of gonadotropic or steroid treatments on the hormonal regulation of vitellogenesis by means of these criteria. Two classes of lipoproteins, VLDL (very low density lipoprotein) and HDL (high density lipoprotein), were found in both yellow and silver eel plasma but LDL (low density lipoprotein) was lacking. Differences in apolipoprotein composition of HDL were found between yellow and silver eel. In vivo estradiol priming induced an important increase in plasma VLDL and VHDL (very high density lipoprotein or vitellogenin) levels and suggested a role of these molecules during vitellogenesis. The ultrastructure of the eel oocyte showed the presence of endocytosic structure only in silver eel oocytes. This result suggests yellow eels as pre-vitellogenic and silver eels as vitellogenic. After gonadotropic injections, plasma estradiol (E2) levels alone rose. Vitellogenin slowly appeared after a long treatment. Vitellogenesis was much more quicker in silver eels than in yellow eels. Androgens and estrogens were both secreted in the plasma thus suggesting a role for androgens in the induction of hepatic vitellogenesis. In vitro assays on eel hepatocytes in primary culture shown a stimulatory effect on vitellogenin synthesis with E2 and high dosages of androgens. In vivo studies had shown a role of growth hormone (GH) during vitellogenesis. The action of GH was confirmed in vitro suggesting that GH acts directly on the liver to increase vitellogenin synthesis. RE2 were identified; they belong to the nuclear steroid receptor family. However they are present in the eel liver at a low concentration. GH seems to play a role on the synthesis of RE2. Differences exist between yellow and silver eels in the responses to gonadotropins. VLDL and vitellogenin are implicated in the oocytes for yolk formation. Hormonal induction and control of vitellogenesis are plurihormonal.