Gonadogenesis analysis and sex differentiation in cultured turbot (Scophthalmus maximus)

As a flatfish, the turbot (Scophthalmus maximus) is one of the most important farmed fish species with great commercial value, which has a strong sexual dimorphism on growth rate and sexual maturity. In this study, using histology, the basic information on proliferation and migration of germ cells and early gonadal development during sex differentiation in turbot were described in detail. There were six to nine individual primordial germ cells (PGCs) with large nuclei until 15 days post-hatching (dph). The PGCs located under the mesonephric ducts undergo migration along the dorsal mesentery toward the region of the genital ridge from 0 to 15 dph. During migration, the number of PGCs was constant, and the expression of vasa had no significant changes. At 20 dph, the aggregation of somatic cells at genital ridge indicated the formation of primary gonad. Furthermore, the number of PGCs was increased to 60 and the expression of vasa was upregulated for the first time. The undifferentiated gonads with no morphological indications of sex differentiation grew larger with the increase in germ cells and somatic cells number/size from 20 to 35 dph. During 36-52 dph, cytological gonadal differentiation was observed. In presumptive testes of type I gonadal tissue (with a lance shape), the number of germ cells increased steadily and the germ cells had the same characteristics as before. Meanwhile, in presumptive ovaries of type II gonadal tissue (with a club-like shape), the germ cells proliferated and induced in two different populations of germ cells. One type had the morphological characteristics as undifferentiated germ cells, while the other type of germ cells underwent mitosis exhibiting smaller size and mottled nuclei. At 60 dph, ovarian cavity was present in the gonad of type II, which would develop into ovaries. However, spermatogonial cysts were not noticed in the gonad of type I until 90 dph, which indicated the formation of the testes.