Gonadal morphogenesis and sex differentiation in intraovarian embryos of the viviparous fishZoarces viviparus (Teleostei, Perciformes, Zoarcidae): A histological and ultrastructural study

Abstract

It is essential to know the timing and process of normal gonadal differentiation and development in the specific species being investigated in order to evaluate the effect of exposure to endocrine-disrupting chemicals on these processes. In the present study gonadal sex differentiation and development were investigated in embryos of a viviparous species of marine fish, the eelpout, Zoarces viviparus, during their intraovarian development (early September to January) using light and electron microscopy. In both sexes of the embryos at the time of hatching (September 20) the initially undifferentiated paired bilobed gonad contains primordial germ cells. In the female embryos, ovarian differentiation, initiated 14 days posthatch (dph), is characterized by the initial formation of the endoovarian cavity of the single ovary as well as by the presence of some early meiotic oocytes in a chromatin-nucleolus stage. By 30 dph, the endoovarian cavity has formed. By 44 dph and onward, the ovary and the oocytes grow in size and at 134 dph, just prior to birth, the majority of the oocytes are at the perinucleolar stage of primary growth and definitive follicles have formed. In the presumptive bilobed testis of the male embryos, the germ cells (spermatogonia), in contrast to the germ cells of the ovary, remain quiescent and do not enter meiosis during intraovarian development. However, other structural (somatic) changes, such as the initial formation of the sperm duct (30 dph), the presence of blood vessels in the stromal areas of the testis (30 dph), and the appearance of developing testicular lobules (102 dph), indicate testicular differentiation. Ultrastructually, the features of the primordial germ cells, oogonia, and spermatogonia are similar, including nuage, mitochondria, endoplasmic reticulum, and Golgi complexes.

Sex differentiation in Atlantic cod (Gadus morhua L.): morphological and gene expression studies

“…These presumptive males had small filament-shaped gonads, with few undifferentiated germ cells, but with large blood vessels in the proximal region of the gonad. The formation of blood vessels at the proximal or distal regions of the gonad is considered an indicator of the early testis in several fish species [46,48]. …”

Effects of exposure to the xenoestrogen octylphenol and subsequent transfer to clean water on liver and gonad ultrastructure during early development of Zoarces viviparus embryos

“…Fusion of the lobes in the abnormal gonads took place in a different way and later in the embryogenesis when compared with the normal development described in the morphology study by Rasmussen et al (2006). This fusion in the abnormal gonad was a result of a gradual disappearing of the Figure 5.…”

“…Therefore, there seems to be a different sensitivity toward exposure with OP which could be explained by the earlier sexual differentiation stage of the embryo at the onset of this experiment. The morphological differentiation of the female gonad in eelpouts starts about 14 days after hatching (Rasmussen et al, 2006), which means the ovaries from the embryos examined here had 27 days to develop. In contrast, owing to the delay of the testicular differentiation the testes from the embryos in the present had only 12 days to develop.…”

Sexual determination and differentiation in teleost fish

“…However, other structural changes that take place after hatching, such as the initial formation of the sperm duct, the presence of blood vessels in the stromal areas of the testis, and the appearance of developing testicular lobules, are indicators of testicular differentiation (Rasmussen et al 2006). …”

“…Gambusia affinis (Koya et al 2003), Cichlasoma dimerus (Meijide et al 2005), zebrafish (von , Zoarces viviparus (Rasmussen et al 2006), rainbow trout (Vizziano et al 2007), the Adriatic sturgeon Acipenser naccarii (Grandi and Chicca 2008), and Gasterostus aculeatus (Lewis et al 2008).…”