EFFECT OF TRIIODTHYRONINE ON IN-VITRO MATURATION OF VENDACE (COREGONUS-ALBULA) OOCYTES UNDER UNFAVORABLE INFLUENCES

The ultimate control of fish oocyte final maturation by gonadotropin a ppears to be mediated through follicular synthesis of a steroid mediat or. The mediator, identified in most teleosts as 17 alpha-hydroxy- 20 beta-dihydroprogesterone (17,20P), is synthesized in follicular (granu losa) cells and it directly acts on the oocyte, triggering nuclear and cytoplasmic transformations leading to a fertilizable egg (Dettlaff 1 977, Masui & Clarke 1979, Nagahama 1987abc). Both of these hormones, a cting on the different structures of the follicle (oocyte + follicular envelope), can induce oocyte final maturation in a variety of fish sp ecies in vivo and in vitro (review: Saat 1985b). This allows experimen tal investigation of the responsiveness of each follicular component a t different conditions. This approach has been used to study the diffe rential sensitivity of follicular components to unfavourable influence s in the chondrostean, Acipenser stellatus Pall. (Davydova 1972, Dettl aff & Davydova 1974, 1979ab, 1981) and in the warm-water teleost, Cara ssius auratus gibelio (Bloch) (Saat 1985a). In both these species, the follicular epithelium cells lost their capacity to respond to hypophy seal hormones earlier than oocytes to progesterone, suggesting that th e formers are more sensitive to unfavourable influences. In the stella te sturgeon A. stellatus, the follicular response to gonadotropin can be restored by the administration of triiodthyronine (T-3) (Dettlaff & Davydova 1974, 1979ab, 1981). Thyroid hormones have been shown to enh ance the oocyte final maturation also in teleosts (Hurlburt 1977, Eple r & Bieniarz 1983, Sullivan et al. 1989), and therefore one can expect the same phenomenon in these fish. The aim of the present study was t o investigate the effect of unfavourable influences and T-3 on the fol licular sensitivity to hormones in the cold-water teleost, Coregonus a lbula (L.), in vitro.