Estradiol-17β silastic implants suppress oocyte development in the brook trout, Salvelinus fontinalis

A technique for long-term implantation (i.e., 6 months) of steroid-containing Silastic capsules in brook trout was developed along with a method for longitudinal study of ovarian dynamics in individual females. Estradiol-17β implantation, extending from spring during early oogenesis through November, suppressed overall ovarian development in a dose-dependent manner, as determined by follicle size analysis and gonosomatic index, and reduced the peak of plasma estradiol-17β normally observed prior to ovulation in the fall. Frozen sections of follicles revealed that estradiol implants retarded the progression of vitellogenesis from primary to secondary stages in the largest size class of follicles. In particular, sections of ovarian biopsies from animals with three estradiol implants had only small follicles in September which cytologically were no more advanced than those in May. By contrast, animals receiving steroid-free implants had significantly larger follicles in September which entered the secondary stage of vitellogenesis and became increasingly competent to respond to progestogen treatment in vitro through October and November. Image analysis of ovarian biopsies indicated that estradiol implants deranged the normal groupsynchronous growth pattern of the brook trout ovary and tended to produce asynchronous growth, especially in animals with a single estradiol implant.