| Turbot (Scophthalmus maximus) is one of the important marine food fish species which contribute to the highly fish yield and valuable commercial benefit in the worldwide aquaculture, including China. In this paper, the development of primordial gonad and the sex differentiation of turbot are studied by histological staining methods, and the techniques of early gonad's sex identification are investigated. The main results and conclusions are as follows:1. Histological study on the development of primordial gonad in turbotTurbot, 0 to 40 days old, 2.5mm to 32.2mm in total length (TL), were processed by histological staining method. The primordial gonad was observed under microscope. The primordial cell (PGC) migrated into germinal ridge after hatching. In larva 10 days after hatching, TL 6.14mm, it is found that PGC migrated into the germinal ridge, which locates between the gut and the mesonephric, and begin to form the primordial gonad together. At this phase, PGC is c. 5.1μm-6.5μm in diameter, and its nuclear is c.2.4μm-3.3μm in diameter. In larva 15 days old, TL 8.41mm, one end of the primordial gonad, with several PGCs, were fixed on the abdominal cavity membrane nearly the kidney. In larva 20 days old, TL 14.55mm, the young tuebot presented metamorphosis process, and the ends of the primordial gonad are fixed on the abdominal cavity membrane. In larva 25 days after hatching, TL 17.76mm, the primordial gonad begins to move towards to the bottom of the abdominal cavity away from the kidney. In lavra 30 days, TL 24.21mm, the primordial gonad has arrived at the bottom of the abdominal cavity. In the 40 days, average TL 32.2mm, the primordial gonad has already moved to the bottom edge of abdominal cavity, with one end of it frees in it. There is no distinct evidence of sexual differentiation in primordial gonad at this time.2. Histological studies on the Ovarian differentiation in turbot ?After a long period of undifferentiation stage, some of the primordial gonads in Turbot enter to the sexual differentiation phase. It is the evidence of ovarian differentiation in the cytology that oogonia form oogonia clusters through mitosis, while the evidence in the anatomy is the formation of Ovarian cavity. Oogonias form clusters of PGCs in a 35.2mm TL young turbot fish. At the same time, the somatic cells from the abdominal cavity integrate into one side of the gonad to form a cavity, which indicate the ovarian primal differentiating of turbot. Ovarian cavity appears in a 40.3mm TL of turbot young fish, and enlarge with the development of primordial gonads till the formation of mature ovarian When the turbot reaches 69.3mm TL, ovarian begins to form gibbosity which will stretch into the caudal muscle. In 113.4mm TL, early perinucleolus stage oocytes are observed, which indicate the completion of ovarian differentiation in turbot fish.3. Histological studies on the Testicular differentiation in turbot ?Testicular differentiation of turbot is posterior to ovarian differentiation, which is similar with other fish species. At a 47.6mm TL, we find some cystic structures that content several spermatogonias in the gonad. This specific structure is named as spermatogenic cysts. At a 55.2mm TL, the spermatogenic cysts can be seen more clearly, and seminiferous lobula generate primarily. At a 64.2mm TL, the spermatogonias begin to shrink obviously, which means they are transforming to spermatocytes.After this, there is no obvious structural changes in the development process untilt testicular differentiation finishes.4. Studies on techniques of early gonads sex identification in turbot ? the gonad of Turbot, 90mm to 150mm TL, 10g to 90g weight, would be found after dissected, but cannot identify directly. In this study, the gonads are removed, placed on a glass slide, covered with few drops of aceto-carmine and squashed with a cover slip. The slide will be examined under the microscope. The staining result shows that: Ovarian has several obvious characteristics after stained with aceto-carmine. Ovarian tissues could be easily identified by the presence of large numbers of previtellogenic oocytes, since the bulky oogonias can be observed under the microscope very clearly. Compared to ovarian, testicular structures stained by aceto-carmine are not found any specific features. . Therefore, ovarian and testicular can be early distinguished under the microscope after stained with aceto-carmine, which can develop to a rapid, convenient and accurate technology for early gonad's sex identification in turbot. |
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