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 fis h feed and grow and silver when eels fasted and started their reproduc tive migration. This cycle is especially interesting to study the horm onal induction of oogenesis and especially vitellogenesis, important s tep 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 ultra structure, sex steroids and vitellogenin plasma levels, hepatic recept ors for estradiol (E2R)) and second, the effects of gonadotropic or st eroid treatments on the hormonal regulation of vitellogenesis by means of these criteria. Two classes of lipoproteins, VLDL (very low densit y lipoprotein) and HDL (high density lipoprotein), were found in both yellow and silver eel plasma but LDL (low density lipoprotein) was lac king. Differences in apolipoprotein composition of HDL were found betw een yellow and silver eel. In vivo estradiol priming induced an import ant increase in plasma VLDL and VHDL (very high density lipoprotein or vitellogenin) levels and suggested a role of these molecules during v itellogenesis. The ultrastructure of the eel oocyte showed the presenc e of endocytosic structure only in silver eel oocytes. This result sug gests yellow eels as pre-vitellogenic and silver eels as vitellogenic. After gonadotropic injections, plasma estradiol (E2) levels alone ros e. Vitellogenin slowly appeared after a long treatment. Vitellogenesis was much more quicker in silver eels than in yellow eels. Androgens a nd estrogens were both secreted in the plasma thus suggesting a role f or androgens in the induction of hepatic vitellogenesis. In vitro assa ys 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 dire ctly on the liver to increase vitellogenin synthesis. RE2 were identif ied; 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 s ilver eels in the responses to gonadotropins. VLDL and vitellogenin ar e implicated in the oocytes for yolk formation. Hormonal induction and control of vitellogenesis are plurihormonal.