Study On The Development Of Gonad At Early Stages In Cynoglossus Semilaevis

Primordial germ cells are the precursors of fish germ cell during early stages of gonad. In this paper, embryos and larvae and juvenile of all periods in Cynoglossus semilaevis were observed and studied by using histological technique. Electron microscopy techniques and immunohistochemistry were employed to trace the origin and migration of PGCs, gonadal development in C. semilaevis. The study provided basic information on developmental biology and a reference with cultivation and aquaculture production practices for C. semilaevis.According to the microstructure of tissue section of the embryo, early germ cells and the original gonad were observed during different phases. The results showed that PGCs appear in the yolk sac near the blastoderm during gastrulation period. Multi-yolk syncytial bodies were observed which were related to the emergence of PGCs. The PGCs of embryos followed notochord wall during muscle stage and migrated to the vicinity of intestinal primordium in the preincubation. PGCs migrated along the dorsal midline of the body in the larval phase and proliferated with the migration. Primitive genital ridge finally appeared between the intestine and kidney tubes. Gonad began histological differentiation during 60 days of juvenile period. It emerged two forms of gonad:one had fissure cavity radiating around and would be ovarian cavity prototype, the other one had no crack and would develop into testis.The result of ultrastructure showed that PGCs were oval and had the large nuclear, prominent cell boundaries. In comparison with somatic cells, PGCs were large size. Cytoplasm contained annular lamellar structure which linked with the rough endoplasmic reticulum, mitochondria and a larger number of electron-dense bodies. Most of the electron-dense bodies concentrated in the nucleus, and some distributed along the outer of nuclear membrane. During the migration period, PGCs extended pseudopodia. Mitochondria significantly increased in quantity and volume, and some showed rod-like. The results of immunohistochemistry showed that Nanosl protein expressed specifically in PGCs. The protein could be used as the marker protein of PGCs. It showed that genital ridge firstly appeared between the intestine and nephridial duct. PGCs migrated along the visceral mesoderm and continued to proliferate with the migration. They both constituted the original gland. The original gonad was influenced by the environmental factors such as temperature during the fingerling period. It was the important period of transition toward development of the juvenile period.