Zebrafish Tbx16 regulates intermediate mesoderm cell fate by attenuating Fgf activity

Progenitors of the zebrafish pronephros, red blood and trunk endothelium all originate from the ventral mesoderm and often share lineage with one another, suggesting that their initial patterning is linked. Previous studies have shown that spadetail (spt) mutant embryos, defective in tbx16 gene function, fail to produce red blood cells, but retain the normal number of endothelial and pronephric cells. We report here that spt mutants are deficient in all the types of early blood, have fewer endothelial cells as well as far more pronephric cells compared to wildtype. In vivo cell tracing experiments reveal that blood and endothelium originate in spt mutants almost exclusive from the dorsal mesoderm whereas, pronephros and tail originate from both dorsal and ventral mesoderm. Together these findings suggest possible defects in posterior patterning. In accord with this, gene expression analysis shows that mesodermal derivatives within the trunk and tail of spt mutants have acquired more posterior identity. Secreted signaling molecules belonging to the Fgf, Wnt and Bmp families have been implicated as patterning factors of the posterior mesoderm. Further investigation demonstrates that Fgf and Wnt signaling are elevated throughout the nonaxial region of the spt gastrula. By manipulating Fgf signaling we show that Fgfs both promote pronephric fate and repress blood and endothelial fate. We conclude that Tbx16 plays an important role in regulating the balance of intermediate mesoderm fates by attenuating Fgf activity.     

Duplicate sfrp1 genes in zebrafish: sfrp1a is dynamically expressed in the developing central nervous system, gut and lateral line

The secreted frizzled-related proteins (Sfrp) are a family of soluble proteins with diverse biological functions having the capacity to bind Wnt ligands, to modulate Wnt signalling, and to signal directly via the Wnt receptor, Frizzled. In an enhancer trap screen for embryonic expression in zebrafish we identified an sfrp1 gene. Previous studies suggest an important role for sfrp1 in eye development, however, no data have been reported using the zebrafish model. In this paper, we describe duplicate sfrp1 genes in zebrafish and present a detailed analysis of the expression profile of both genes. Whole mount in situ hybridisation analyses of sfrp1a during embryonic and larval development revealed a dynamic expression profile, including: the central nervous system, where sfrp1a was regionally expressed throughout the brain and developing eye; the posterior gut, from the time of endodermal cell condensation; the lateral line, where sfrp1a was expressed in the migrating primordia and interneuromast cells that give rise to the sensory organs. Other sites included the blastoderm, segmenting mesoderm, olfactory placode, developing ear, pronephros and fin-bud. We have also analysed sfrp1b expression during embryonic development. Surprisingly this gene exhibited a divergent expression profile being limited to the yolk syncytium under the elongating tail-bud, which later covered the distal yolk extension, and transiently in the tail-bud mesenchyme. Overall, our studies provide a basis for future analyses of these developmentally important factors using the zebrafish model.     

Structure of the cumulus oophorus at the time of fertilization

Summary The paired external glomus of the fully developed pronephros has been studied in early larvae (ammocoetes) of 2 lamprey species, Lampetra fluviatilis and Petromyzon marinus, several weeks after hatching and newly hatched, by use of light-, scanning (SEM) and transmission (TEM) electron microscopy. Three weeks after hatching the glomus is a complex of capillary loops supplied by a single arteriole branching from the aorta. The glomus consists of 3 cell types: podocytes, fenestrated endothelium, and mesangial cells. A basement membrane, which has a close contact to the podocytes, is the only continuous barrier between blood and the coelomic cavity. The glomus exhibits all fine-structural elements known to be essential for function in the glomeruli of other vertebrates. We therefore assume the pronephric glomus of lampreys to be functional in ultrafiltration, with the ultrafiltrate released into the coelomic cavity. In newly hatched larvae, the structure of the glomus is not fully developed. In this earlier stage several afferent arterioles supply each glomus. The endothelial cells in the glomar capillaries still lack regular epithelial organization and resemble mesenchymal cells. However, the presence of typical podocytes stretching over a continuous basement membrane suggests that the tissue is already capable of ultrafiltration.This paper is dedicated to the memory of Professor W. Bargmann, long-time editor of Cell and Tissue Research, the author of a splendid review on the structure of the vertebrate kidney and a master of German scientific writing     

Amino acid cotransporter SLC3A2 is selectively expressed in the early proximal segment of Xenopus pronephric kidney nephrons

The transport of amino acids across membranes is critical to all cells. As amino acids freely pass through the glomerular filtration barrier of the kidney, they must be efficiently resorbed to avoid depletion of circulating amino acid reserves. Not only do defects in amino acid resorption lead to costly wastage, they also cause congenital aminoacidurias. A clone encoding Xenopus SLC3A2 was identified and shown to be expressed at high levels in the early segment of the pronephric proximal tubules in developing tadpoles. The type II membrane glycoprotein encoded by this gene can associate with a wide variety of protein partners and participates in a broad spectrum of biological processes. In this report, the first whole-mount analysis of SLC3A2 during early embryonic development is presented. The expression pattern of SLC3A2 in the early proximal segment of the Xenopus pronephros is analogous to that of a previously described SLC7A8/XAA2 amino acid transporter. In mammals, SLC3A2 and SLC7A8/XAA2 associate to form a functional neutral amino acid transporter complex and coexpression of these two genes in a small domain within the pronephric tubules indicates that this is also the situation in the developing Xenopus kidney.     

Ontogeny and osmoregulatory function of the urinary system in the Persian sturgeon,Acipenser persicus (Borodin, 1897)

The structure of the kidney and the localization of Na⁺, K⁺-ATPase (NKA) immunopositive cells were examined throughout the postembryonic development of the Persian sturgeon, Acipenser persicus, from newly hatched prelarvae (10 mm) to 20 days post hatch (20 DPH) larvae (31 mm). Investigations were conducted through histology and immunohistochemistry by using the light and immunofluorescence microscopy. The pronephros was observed in newly hatched prelarvae. The cells lining the distal pronephric tubules and their collecting ducts showed laterally expressed NKA immunofluorescence that later extended throughout the whole cytoplasm. Mesonephrogenous placodes and pre-glomeruli were distinguished at 2 DPH along the collecting ducts posteriorly. Their tubules were formed and present in kidney mesenchyma, differentiated into neck, proximal, distal and collecting segments at 7 DPH when NKA immunopositive cells were observed. Their distal and collecting tubules showed an increasing immunofluorescence throughout their cytoplasm while the glomeruli remained unstained. From D 9 to D 17, the epithelial layer of pronephric collecting duct changed along the mesonephros to form ureters. Ureters, possessing isolated strong NKA immunopositive cells, appeared as two sac-like structures hanging under the trunk kidney. Since NKA immunopositive cells were not observed on the tegument or along the digestive tract of newly hatched prelarva, and also the gills are not formed yet, the pronephros is the only osmoregulatory organ until 4 DPH. At the larval stage, the pronephros and mesonephros are functional osmoregulatory organs and actively reabsorb necessary ions from the filtrate.