Histological Studies On Gonadal Ontogenesis, Differentiation And Development In Flounder (Paralichthys Olivaceus)

From 2005 to 2007,adolescent flound were cultivated to sex-maturation in Beidaihe central experiment station,chinese academy of fishery sciences.The gonadal samples were randomly collected in process of Paralichthys olivaceus cultivation.Using histological method,the research was about gonadogenesis and sex differentiation and development within Paralichthys olivaceus, which was allround and systematic investigation in micro-level.To gain foundament data,the data was provided for Paralichthys olivaceus biology and artifical propagation investigate and so on. The results showed as follows:One elliptical PGC was observed at 10th day post-hatch(dph).On the 20 th dph,gonad anlage was made of PGC and genital ridge.Germinal epithelium was found in the gonad anlage.Gonad emerged and remained undifferentiated at 30th dph.The anlage of the efferent duct was found when the fish was 50th dph.The gonad was differentiated.It was testis.The testis was filled with spermatogonium in 95th dph.And the testis wasⅠstage testis.On the 65th dph,the dorsal and ventral edges of these gonads began to extend laterally and they eventually fused to form the presumptive ovarian cavities along the lateral sides of the gonads.The gonadal differentiation was ovary.Oocytes in early meiosis were first observed from the ovary of 105th dph.And the gonad was stageⅠovary.So the differentiation of testis was earlier than that of ovary.Befor testis differentiation,there was an leakage at the ventral aspect of gonad.In the future, it can develop into the dictus deferens.During the differentiation of ovary,the ovary cavity was first formed.Therefore,the ovary cavity and ductus were the symbols of the gonad differentiation of Paralichthys olivaceuswas.The ovary and testis of Paralichthys olivaceus was came from the same gonad and kept the same sexual distinction in its all life,ao it was belonged to the gonochorism differentiation.The testis of Paralichthys olivaceus was belong to microphyll type testis.The development of testis in Paralichthys olivaceuswas was divided into six stage.Spermatogenesis might be divided into five phases by histology.In stageⅠ(at the age of 2～3 months),the testis was transparent, flesh.The structure of seminiferous lobulus could be found.The sex couldn't be distinguished form the appearance.In stageⅡ(at the age of 4～6 months),the testis was divided symmetrical and "upper-down" double leaves.In the testis could observate the spermatogenic cyst.Primary spermatocyte was fast multiplication.In stageⅢ(at the age of 7～9 months),mono-extremitas of testis was elongation toward trail.There was consideral primary spermatocyte.In stageⅣ(at the age of 10～11 months),anterior extremity of the testis was extremely thickness and posterior extremity was slender.Some of spermatogenic cysts were full of primary spermatocyte,some secondary spermatocyte,some spermid.In stageⅤ(at the age of 12～14 months),Iobule lumen was filled with mature sperm.In stageⅥ(at the age of 15 months),the volume of testis decreased because of discharging of apermatic fliud.In the lobule lumen,there were some of degenerating sperm,which were becoming dim.Spermatogenesis was conspicuous asynchronization in development process of the testis.This asynchronization determined that spermatozoa was gradually mature and evacuation.It was a mutual adaptation with type of batch oviposition.In idiophase,GSI sustained higher numerus all the time.But there was only a peak in the process.On the basis of shape,external color and size of the ovary development in Paralichthys olivaceuswas was divided into six stages.The development oocyte of Paralichthys olivaceuswas could be divided into six phases according to the changes of morphocytology and histology.The ovary ofⅠstage(at the age of 3～5 months) only emerge one time Paralichthys olivaceuswas's whole life.Oogonium was fast increase.In stageⅡ(at the age of 4～17 months),gonad was triangle shape from micro girdle-shaped.Oocyte was surrounded by unilaminar follicle cell.Oil and hyperbasophilic yolk nucleus emerge in cytoplasm.As for the stageⅢ(at the age of 18～21 months),outside of the oocyte,there were two layers of follicle cells.Cytoplasm was filled by yolk vacuoles.The primary egg envelope was formed At the same time,the yolk granules was observated.In stageⅣ(at the age of 22～23 months),peeling ovum was difficult in earlier period. But it was easy in advanced stage.Oocyte was filled with yolk granules.As for the stageⅤ(at the age of 24～25 months),oocyte were full maturity,the ovaries entered ovlation period.The last stage was stageⅥ(at the age of 26 months).Ovary was chalasis cystic form.Oocyte was degeneration.The development of primary oocyte is nonsynchronized fromⅢtoⅣand fromⅣtoⅤ.After the first time oviposition,histologic structure of ovary is stageⅢ.So Paralichthys olivaceu swas belonge to type of batch oviposition.In idiophase,GSI sustained higher mumerus all the time.And it has one time per year.