Characterization of zebrafish primordial germ cells: morphology and early distribution of vasa RNA

Research into germ line development is of conceptual and biotechnologic importance. In this study, we used morphology at the level of light and electron microscope to characterize the primordial germ cells (PGCs) of the zebrafish throughout embryonic and larval development. The study was complemented by the detailed analysis of mRNA expression of a putative germ line marker vasa. By morphology alone PGCs were identified at the earliest at the 5-somite stage in the peripheral endoderm in contact with the yolk syncytial layer. Subsequently, they move from lateral to medial positions into the median mesoderm and from there by means of the dorsal mesentery into the gonadal anlage at day 5 postfertilization (pf), to establish gonads with mesenchymal cells by day 9 pf. Ultrastructural analysis of the 4-day-old zebrafish larvae demonstrates the presence of the germ line-specific structures, nuage, and annulate lamellae. vasa RNA-positive cells can be followed during zebrafish embryogenesis from the 32-cell stage onward (Yoon et al., 1997). Upon completion of gastrulation, the RNA is exclusively present in the cells of the hypoblast, which as a consequence of convergence and extension movements first arrange themselves in a V-shaped string-like conformation to end up, by late somitogenesis, as a string of cells on each side of the midline. We show that the localization of maternal vasa RNA in the ovary changes from cytoplasmic, in the previtellogenic oocytes, to cortical in the vitellogenic oocytes, to concentrate at the boundary of the yolk and cytoplasm in the one cell stage zygote. These results demonstrate that the cortical vasa RNA localization precedes its cleavage furrow-associated localization in the embryos and is presumably cytoskeleton dependent. vasa RNA localization changes from asymmetric subcellular at the sphere stage, to become entirely cytoplasmic at the dome stage. These data suggest a close resemblance in modes of segregation of the germ plasma in the frog and vasa mRNA in the fish during cleavage stages. Based on the significantly larger size and the stereotype and similar position of morphologically distinct cells, presumed to be PGCs, and their vasa RNA-positive counterparts, we conclude that vasa RNA-positive cells are the PGCs and vasa RNA represents a definitive germ line marker in the fish. Dev Dyn 1999;216:153-167.

Isolation, characterization and expression of cDNAs encoding the catfish-type and chicken-II-type gonadotropin-releasing-hormone precursors in the African catfish

The cDNAs encoding the catfish prepro-gonadotropin-releasing hormone and the chicken prepro-gonadotropin-releasing hormone II of the African catfish (Clarias gariepinus) have been isolated and sequenced. The catfish gonadotropin-releasing-hormone precursor and the chicken gonadotropin-releasing-hormone-II precursor have the same overall architecture as other gonadotropin-releasing-hormone precursors identified so far; each is composed of a signal peptide, gonadotropin-releasing hormone and a gonadotropin-releasing-hormone-associated peptide which is connected to gonadotropin-releasing hormone and chicken gonadotropin-releasing hormone II, in combination with the Gly-Lys-Arg sequence, are highly conserved during evolution when compared with the corresponding regions of mammalian, avian (chicken gonadotropin-releasing hormone I) and other fish gonadotropin-releasing-hormone precursors. However, the gonadotropin-releasing-hormone-associated peptide regions are markedly divergent. Northern-blot analysis revealed the presence of a single catfish gonadotropin-releasing-hormone mRNA species of about 470 bases, and the presence of a single chicken gonadotropin-releasing-hormone-II mRNA species of about 650 bases in the African catfish brain. In situ hybridization revealed catfish gonadotropin-releasing-hormone cell bodies rostro-caudally scattered in the olfactory nerve, along both sides of the midline of the telencephalon, in the preoptic area of the ventral hypothalamus, and in the infundibular stalk close to the pituitary. Chicken gonadotropin-releasing-hormone-II cell bodies, however, were exclusively found in the midbrain tegmentum.

Characterization of zebrafish primordial germ cells: morphology and early distribution of vasa RNA

Research into germ line development is of conceptual and biotechnologic importance. In this study, we used morphology at the level of light and electron microscope to characterize the primordial germ cells (PGCs) of the zebrafish throughout embryonic and larval development. The study was complemented by the detailed analysis of mRNA expression of a putative germ line marker vasa. By morphology alone PGCs were identified at the earliest at the 5-somite stage in the peripheral endoderm in contact with the yolk syncytial layer. Subsequently, they move from lateral to medial positions into the median mesoderm and from there by means of the dorsal mesentery into the gonadal anlage at day 5 postfertilization (pf), to establish gonads with mesenchymal cells by day 9 pf. Ultrastructural analysis of the 4-day-old zebrafish larvae demonstrates the presence of the germ line-specific structures, nuage, and annulate lamellae. vasa RNA-positive cells can be followed during zebrafish embryogenesis from the 32-cell stage onward (Yoon et al., 1997). Upon completion of gastrulation, the RNA is exclusively present in the cells of the hypoblast, which as a consequence of convergence and extension movements first arrange themselves in a V-shaped string-like conformation to end up, by late somitogenesis, as a string of cells on each side of the midline. We show that the localization of maternal vasa RNA in the ovary changes from cytoplasmic, in the previtellogenic oocytes, to cortical in the vitellogenic oocytes, to concentrate at the boundary of the yolk and cytoplasm in the one cell stage zygote. These results demonstrate that the cortical vasa RNA localization precedes its cleavage furrow-associated localization in the embryos and is presumably cytoskeleton dependent. vasa RNA localization changes from asymmetric subcellular at the sphere stage, to become entirely cytoplasmic at the dome stage. These data suggest a close resemblance in modes of segregation of the germ plasma in the frog and vasa mRNA in the fish during cleavage stages. Based on the significantly larger size and the stereotype and similar position of morphologically distinct cells, presumed to be PGCs, and their vasa RNA-positive counterparts, we conclude that vasa RNA-positive cells are the PGCs and vasa RNA represents a definitive germ line marker in the fish. Dev Dyn 1999;216:153-167.

Characterization of zebrafish primordial germ cells: morphology and early distribution of vasa RNA

Research into germ line development is of conceptual and biotechnologic importance. In this study, we used morphology at the level of light and electron microscope to characterize the primordial germ cells (PGCs) of the zebrafish throughout embryonic and larval development. The study was complemented by the detailed analysis of mRNA expression of a putative germ line marker vasa. By morphology alone PGCs were identified at the earliest at the 5-somite stage in the peripheral endoderm in contact with the yolk syncytial layer. Subsequently, they move from lateral to medial positions into the median mesoderm and from there by means of the dorsal mesentery into the gonadal anlage at day 5 postfertilization (pf), to establish gonads with mesenchymal cells by day 9 pf. Ultrastructural analysis of the 4-day-old zebrafish larvae demonstrates the presence of the germ line-specific structures, nuage, and annulate lamellae. vasa RNA-positive cells can be followed during zebrafish embryogenesis from the 32-cell stage onward (Yoon et al., 1997). Upon completion of gastrulation, the RNA is exclusively present in the cells of the hypoblast, which as a consequence of convergence and extension movements first arrange themselves in a V-shaped string-like conformation to end up, by late somitogenesis, as a string of cells on each side of the midline. We show that the localization of maternal vasa RNA in the ovary changes from cytoplasmic, in the previtellogenic oocytes, to cortical in the vitellogenic oocytes, to concentrate at the boundary of the yolk and cytoplasm in the one cell stage zygote. These results demonstrate that the cortical vasa RNA localization precedes its cleavage furrow-associated localization in the embryos and is presumably cytoskeleton dependent. vasa RNA localization changes from asymmetric subcellular at the sphere stage, to become entirely cytoplasmic at the dome stage. These data suggest a close resemblance in modes of segregation of the germ plasma in the frog and vasa mRNA in the fish during cleavage stages. Based on the significantly larger size and the stereotype and similar position of morphologically distinct cells, presumed to be PGCs, and their vasa RNA-positive counterparts, we conclude that vasa RNA-positive cells are the PGCs and vasa RNA represents a definitive germ line marker in the fish. Dev Dyn 1999;216:153-167.