Gonadal morphogenesis and sex differentiation in cultured chub mackerel, Scomber japonicus

摘要

The timing of primordial germ-cell (PGC) migration with regard to the gonadal anlagen, gonad formation and sex differentiation was examined histologically in the chub mackerel (Scomber japonicus) at 5–190?days post hatching (dph). At 5?dph, PGCs appeared on the peritoneal epithelium surface or in the mesentery, on the dorsal side of the abdominal cavity. By 10?dph, stromal cells around the PGCs proliferated. The gonadal primordium was formed by 15?dph. The gonadosomatic index was 0.01% at 30?dph and increased thereafter (0.32% in females and 0.04% in males at 160?dph). Ovarian differentiation occurred at 30–40?dph, indicated by ovarian cavity formation (elongation and fusion of the upper and lower ovarian edges). Meiosis was subsequently initiated. A few meiotic oocytes surrounded the cavity at 50?dph; most were in the perinucleolus stage at 60?dph and attained a diameter of 60–70?μm at 190?dph. Testicular differentiation occurred at 30?dph, indicated by the formation of the sperm duct primordium. Spermatogonia gradually proliferated, developing into spermatocytes at the chromatin–nucleolus stage (after 90?dph) and subsequently into spermatids and spermatozoa (160?dph). These data could aid the development of seeding and cell-engineering technologies for scombrid fish.