Sex differentiation and aspects of gametogenesis in shortnose sturgeon Acipenser brevirostrum Lesueur
Shortnose sturgeon Acipenser brevirostrum gonad samples were collected from industry-reared fish and wild broodstock at various developmental stages to elucidate patterns of gonadal differentiation and maturation. Genital ridges, containing germ cells, were present in 26 day-old fish and distinct gonads were present by day 54. Sturgeon gonads are known to consist of two tissue types (adipose and gametogenic) and both were present at 72 day. Anatomical differentiation of gonads occurred by 6 months and was advanced by 15 months. Ovaries had distinct lamellae while testes remained non-lamellate. Gonial proliferation had occurred by 15 months, but the cells were not identifiable as spermatogonia or oogonia. Small white ‘pinhead’ oocytes were macroscopically visible in ovaries as early as 36 months. At 43 months ovaries were clearly organized, with some areas containing only immature oocytes and other containing oocytes apparently developing as cohorts. Individual fish showed considerable variation: the level of development remained unchanged at 84 months in some females, while others showed clear progression towards sexual maturation at 48 months. Sperm cells were present in males as early as 52 months. Advanced development of ovarian follicles was observed only in biopsies of re-conditioned broodstock of wild origin. In the year before spawning, the most advanced oocytes became pigmented, the chorion thickened, the nucleus (germinal vesicle) migrated towards the micropyle complex at the animal pole, and ovulation occurred in May under appropriate environmental conditions.
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