A Tentative Study Of The Characterization Of Development And Differentiation At The Early Life Stages Of The Cynoglossus Semilaevis

Histological and ultrastructural observations on the developmental characters of Cynoglossus semilaevis during the early life stages, including the embryonic development, organ differentiation, the development of hatching gland cells and the way of yolk absorption. Summary of the results are as follows:1. The embryonic development and organ differentiation of C. semilaevis were observed with tissue slice method. Notochord, optic vesicle, mesoderm, spinal cord floor-plate, neural tube, gut, otic vesicle, oropharyngeal membrane, tubular heart, neuromasts, kuppfer's vesicle, primordial germ cells, swimbladder, crown-like larval fin and brain of C. semilaevis embryo were described for the first time. The result indicated that, during neurula period, myomeres are initial differentiated at the back of the embryo after formation of the optic primordium. Then the neural keel is enlarged anteriorly to form the brain rudiment. Blastopore block out period, after formation of the embryo, the otic vesicle is present behind the brain. The notochord is ventral to the brain. During the development of the embryo, the cells of the notochord are vacuolated and beneath the notochord is the gut. Embryo encircled 3/4 of the yolk, dorsal to the embryo is a row of cuboidal cells, the spinal cord floor plate. This plate is ventral to the neural tube. The floor-plate extends anteriorly to the hindbrain. The heart is a thin-walled tube containing erythrocytes. It is ventral to the head paralleled with diencephalon. The neuromasts are present to the ventral head. Embryo encircled 4/5 of the yolk, the kuppfer's vesicle is at the joint between the embryo back-end and yolk sac. Embryo encircled the yolk, the primordial germ cells are near the back-gut. Hatching larvae, the dorsal and ventral fin membrane are present, pigment layer is widely distributed within the ectoderm covering the head, body, tail, and surface of the yolk sac near its attachment to the embryo. A pneumatic duct connects the swimbladder to the digestive tract and swimbladder is not inflation at the larva 4. At the larva 5, the digestive system is finished and the yolk is working out. The gallbladder started to develop from day 8 after hatching. The brain is composed of telencephalon, rhinencephalon, diencephalon, midbrain, cerebellum and encephalon,12 days after hatching out.2. The development of the hatching gland in C. semilaevis were examined with microscopy. The hatching gland with monolayered cells originates from the ectoderm. The hatching gland appeared first at the stage of lens formation; the hatching gland cells were located in the ventral part of the head and in the joint region between the head and the yolk sac. With the development of the embryo, the HG with monolayered cells was still distributed on the outer surface of embryonic body and yolk sac. At the same time, the number and the volume of HGCs increased significantly, HGCs spread more widely. When the development of embryo finished, the number of HGCs reached about 500-800. HGCs were located in the lateral parts and the ventral part of the head, the joint region between the head and the yolk sac, the anterior part and the ventral part of the yolk sac. The HGCs in C. semilaevis were oval cells 6-8μm in short diameter and 7-11μm in long diameter. HGCs were darkish pink when stained with eosin. HGCs disappeared from epidermis about 2 days after hatching.3. Both growth and the way of yolk's absorption during post-embryo development in utilizing morphology and organize slice. The results showed:The yolk ball absorbs firstly, and then fat ballet, the absorbed way is from outside to inside.4. In mid-embryogenesis, the zygote comes to be comprised of three distinct districts:the embryo proper, the yolk sac and the perivitelline fluid. The yolk sac was examined ultrastructurally in the mid to late embryonic stages of C. semilaevis. The results show that, the yolk sac encloses the yolk sac membrane and its content of yolk. During embryonic development, the yolk sac declines in protein concentration due to vitellin polypeptides undergoing limited proteolysis to yield a number of vitellin cleavage products of lower molecular weights. While they are gradually displaced in the cytosolic spaces of the vitellophages and transformed into yolk granules by intervention of vitellophages. The yolk granules assembled into yolk granule vesicles at the inner epithelium of the yolk sac and eventually, yolk granules are transferred to the perivitelline fluid via transcytosis through the yolk sac membrane.Our research in developmental biology can be used for artificial culture to improve the survival ratio and economic benefit. On the other hand, our results be useful for the further study on the biology of reproduction of C. semilaevis and also important to the developmental regulation, heredity breeding of C. semilaevis and providing correlative basic information on the wildlife conservation.