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

Research into germ line development is of conceptual and biotechnologic imp ortance, In this study, we used morphology at the level of light and electr on microscope to characterize the primordial germ cells (PGCs) of the zebra fish throughout embryonic and larval development, The study was complemente d by the detailed analysis of mRNA expression of a putative germ line marke r vasa, By morphology alone PGCs were identified at the earliest at the B-s omite stage in the peripheral endoderm in contact with the yolk syncytial l ayer, Subsequently; they move from lateral to medial positions into the med ian mesoderm and from there by means of the dorsal mesentery into the gonad al anlage at day 5 postfertilization (pf), to establish gonads with mesench ymal cells by day 9 pf, Ultrastructural analysis of the 4-day-old zebrafish larvae demonstrates the presence of the germ line-specific structures, nua ge, and annulate lamellae, vasa RNA-positive cells can be followed during z ebrafish embryogenesis from the 32-cell stage onward (Yoon et al,, 1997), U pon completion of gastrulation, the RNA is exclusively present in the cells of the hypoblast, which as a consequence of convergence and extension move ments first arrange themselves in a V-shaped string-like conformation to en d up, by late somitogenesis, as a string of cells on each side of the midli ne, We show that the localization of maternal vasa RNA in the ovary changes from cytoplasmic, in the previtellogenic oocytes, to cortical in the vitel logenic oocytes, to concentrate at the boundary of the yolk and cytoplasm i n the one cell stage zygote, These results demonstrate that the cortical va sa RNA localization precedes its cleavage furrow-associated localization in the embryos and is presumably cytoskeleton dependent, vasa RNA localizatio n changes from asymmetric subcellular at the sphere stage, to become entire ly cytoplasmic at the dome stage, These data suggest a close resemblance in modes of segregation of the germ plasm in the frog and vasa mRNA in the fi sh during cleavage stages, Based on the significantly larger size and the s tereotype 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., (C) 1999 Wiley-Liss, I nc.