Vitellogenin Incorporation into Oocytes of Rainbow Trout, Oncorhynchus mykiss, in Vitro: Effect of Hormones on Denuded Oocytes: vitellogenin/oocyte growth/insulin/thyroxine/rainbow trout

An in vitro culture procedure to measure vitellogenin (VTG) incorporation into oocytes without follicle cell layers was developed using oocytes of the rainbow trout, Oncorhynchus mykiss. Oocytes incorporated VTG specifically and linearly for up to 24 hr. The maximum incorporation observed was 314 μg/24 hr/oocyte, using vitellogenic (3.6 mm diameter) oocytes. The effect of hormones was examined by adding insulin, growth hormone, prolactin, gonadotropins (GTH‐I, GTH‐II), thyroid hormones, testosterone, estradiol‐17β, or 17α, 20β‐dihydroxy‐4‐pregnen‐3‐one to the medium. The results indicated that insulin and thyroxine stimulated uptake of VTG by 13% and 12%, respectively. Insulin specifically stimulated VTG incorporation and did not cause any change in background uptake of insulin. The lack of an effect of gonadotropins hormones on denuded oocytes suggests that the previously observed stimulation of VTG incorporation into follicle cell‐enclosed oocytes in vivo and in vitro by GTH‐I is most likely mediated by the somatic cells of the ovarian follicle.