Primary Studies On Sex Differentiation And The Ovary Development And Oogenesis Of Myxocyprinus Astaticus

In Mar.2012, fertilized eggs were collected from Chongqing Wanzhou Fisheries Research Institute, which were hatched and raised for studies of sex differentiation in the laboratory. From Sep.2011 to Mar.2013,49 female specimens of Myxocyprinus asiaticus were collected for studies of ovary development and oogenesis from Chongqing Wanzhou Fisheries Research Institute, Guangdong Foshan and Sichuan Luzhou.The development of ovary and oogenesis were studied by means of ordinary anatomy, histology and transmission electron microscope. Sex differentiation was studied by means of histology. The results were as follows:21 dph, genital ridge appeared which was located at the back of the digestive tract and two sides of swim bladder.25 dph, primordial germ cells moved into genital ridge and they were called primary gonads. Subsequently, the volume of primary gonads and the number of primordial germ cells increased.180 dph, gonad divided into two categories. The first gonad, we called them ovaries in which germ cells were clustering distribution and ovarian cavity appeared. Another gonad, germ cells were scattered and there were no formation of cavity in the gonads.200 dph, the volume of ovarian cavity increased and significant oogonia were observed in ovaries. Another gonad, we called them spermaries in which 3-5 germ cells aggregated and formed a cyclic which was called seminiferous lobulus.250 dph, primary oocytes appeared in the ovaries and the structure of seminiferous lobulus and lobule lumen were more pronounced.Based on the external shape, size and color, the development of ovary M. assiaticus was divided into six stages. Ovary in stage I was wirelike and transparent; we could distinguish male and female only by histology and staining. In stage II, ovary was claviform and semi-transparent, some capillaries can be seen. As for stage III, ovary was flat-cystiform and pale yellow, granulous oocytes gleamed in ovary. As for stage IV, it was round-cystiform and yellow, numerous blood vessels can be seen obviously. Eggs were evident and connected to each other in spawning plate. In stage V, the yellow eggs were dividing and would be flowing out when the abdomen was pressed. Ovary of stage VI was empty-cystiform; there are lots of congestive capillary at the surface of ovary. The undrain oocytes and follicle cells in the ovaries were absorbed slowly.According to the size of the oocytes, yolk accumulating and development of follicle cells, the oogenesis of M. assiaticus could be divided in to five phases. The first stage was phase of oogonium. Oogoniums were clustering distribution, which had a large central nuclear and a big central nucleolus. The second was phase of oocytes with single-layer follicle cells. In early period, cytoplasm was strongly basophilic, the number of nucleolus increased and yolk nucleus appeared on the edge of the nucleus. In middle period, spawning piate formed, loop of growth appeared in periphery of nuclear and transparent layer appeared in periphery of loop of growth at the same time, follicle cell formed. In later period, cytoplasm was strongly acidphilic, loop of growth and transparent layer disappeared, follicle cell grew up to single-layer. The third was phase of oocyte with yolk vesicle. In early period, yolk nucleus disappeared; yolk vesicle appeared and grew up from sporadic to single-layer in the edge of the cytoplasm. In middle period, the yolk vesicles reached to several layers gradually, with the appearance of yolk eosinophilic spheres outside of the nucleus. Follicle cells differentiated in to double-layer cells and oolemma differentiated zona pellucida, zona radiate and microvilli between oocyte and follicle cells. In later period, yolk spheres spread to the edge of the cytoplasm from the nucleus and yolk vesicles began to accumulate yolk. Zona radiate became thicker while zona pellucida was becoming more miner. The fourth was phase of oocyte was filled with yolk granules. In early period, yolk vesicles began to degenerate and its number decreased, while the number of yolk sphere increased; zona pellucida disappeared. In middle period, yolk spheres nearly filled the cytoplasm; yolk vesicles were crowded to the edge of the cytoplasm which we were called cortical alveoli. In later period, nucleus began to offset; yolk granule had the tendency to conjugate together; zone radiate got thickest; follicular cells began to degenerate. The fifth phase was the phase of mature oocyte. The volume of the oocyte was the largest, with follicle cells and oocyte separated; zone radiate degenerated and became thiner; nuclear membrane disintegrated and nucleolus disappeared; cytoplasm was concentrated at the animal pole.By the means of transmission electron microscope, the development of oogenesis was divided into five phases on the basis of vitellogenesis. First, the phase of oogonium, the oogonium had a large central nuclear which had a big central nucleolus and large nucleus-to-cytoplasm ratio. There were many oval mitochondria and vesicles of different sizes in the cytoplasm. There were many irregular sertoli cells between oogonia. Second, the phase of previtellogenesis, the number of nucleolus increased and it differentiated into large and small nucleolar. Nucleus-like bodies appeared in the periphery of the nuclear membrane. Mitochondrium aggregated, forming "mitochondria cloud". Follicle cells formed and grew up from sporadie to single-layer. Third, the phase of early period of vitellogenesis, mitochondria cloud disappeared. Yolk vesicle appeared and its number increased. There were many mitochondrium between yolk vesicles. Follicle cells differentiated into 3 layers and formed digitations by cells of the inner layer. These digitations and digitations formed by oocyte crossed and formed microvilli. Zona radiate was formed between microvilli and oocyte, which included 5 layers with microvillar channel. In phase of late-vitellogenesis, kinds of organelles including mitochondrium and endoplasmic reticulum and ribosome were activated to vitellogenesis, yolk granules began to fusing and formed yolk plate. The volume of the nucleus began to shrink, nuclear membrane waved seriously and the nucleus started to offset. Zona radiate became thicker which also included 5 layers. In phase of mature ooeyte, follicle cells degenerated and oocyte was filled with yolk plate. Organelles including mitochondria reduced significantly. Zona radiate became more thiner because zona radiate 1 and 2 degenerated.