Brändli AW (1999), Towards a molecular anatomy of the Xenopus pron...

XB-ART-12086

Int J Dev Biol 1999 Jan 01;435:381-95.

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Towards a molecular anatomy of the Xenopus pronephric kidney.

Brändli AW .

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Kidney development is distinguished by the sequential formation of three structures of putatively equivalent function from the intermediate mesoderm, the pronephros, mesonephros, and metanephros. While these organs differ morphologically, their basic structural organization exhibits important similarities. The earliest form of the kidney, the pronephros, is the primary blood filtration and osmoregulatory organ of fish and amphibian larvae. Simple organization and rapid formation render the Xenopus pronephric kidney an ideal model for research on the molecular and cellular mechanisms dictating early kidney organogenesis. A prerequisite for this is the identification of genes critical for pronephric kidney development. This review describes the emerging framework of genes that act to establish the basic components of the pronephric kidney: the corpuscle, tubules, and the duct. Systematic analysis of marker gene expression, in temporal and spatial resolution, has begun to reveal the molecular anatomy underlying pronephric kidney development. Furthermore, the emerging evidence indicates extensive conservation of gene expression between pronephric and metanephric kidneys, underscoring the importance of the Xenopus pronephric kidney as a simple model for nephrogenesis. Given that Xenopus embryos allow for easy testing of gene function, the pathways that direct cell fate decisions in the intermediate mesoderm to make the diverse spectrum of cell types of the pronephric kidney may become unraveled in the future.

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Species referenced: Xenopus
Genes referenced: aplnr dll1 irx1 irx3 pax2 pax8 pc.1 sall1 wnt4 wt1

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	Fig. 3. Selected examples of marker gene expression in the developing Xenopus pronephric kidney. Lateral views are shown anterior to the right. Transcripts were detected by wholemount in situ hybridization. (A) Pax-8, stage 32. Expression is detected in the emerging pronephric tubules (arrowheads) and at reduced levels in the pronephric duct. (B) Pax-2, stage 36. All epithelia of the pronephric kidney including the nephrostomes (arrowheads) are stained. (C) Delta-1, stage 30. Staining is confined to the developing pronephric tubule anlage. (D) Wnt-4, stage 32. Expression is largely restricted to the nephrostomes (arrowheads). (E) Iro-3, stage 36. Transcripts are present in the common tubule only. Anterior (arrowhead) and posterior (arrow) ends of the expression domain are indicated. (F) Sal-1, stage 38. Expression is found in the common tubule (arrowhead) and the anterior segment of the pronephric duct (arrow indicates posterior end of the expression domain). (G) WT-1, stage 30. Staining is evident in the pronephric capsule. (H) Msr, stage 34. Staining marks the developing vasculature. The major blood vessels in the region of the pronephric kidney are indicated. In situ hybridization and photography was performed as described elsewhere (Heller and Brändli, 1997, 1999). Abbreviations: ACV, anterior cardinal vein; CCV, common cardinal vein; D, pronephric duct; PC, pronephric capsule; PCV, posterior cardinal vein; PS, pronephric sinus; T, pronephric tubules.
	pax8 (paired box 8) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 32, lateral view, anterior left, dorsal up. Expression is detected in the emerging pronephric tubules (arrowheads) and at reduced levels in the pronephric duct.

	dll1 (delta-like 1) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 30, lateral view, anterior left, dorsal up. gene expression in seen in the pronephric mesenchyme, which will develop into the pronephric tubules (T), as well as CNS and head tissues.
	wnt4 (Wnt family member 4 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 32, lateral view, anterior left, dorsal up. In the developing pronephric kidney, expression is largely restricted to the nephrostomes (arrow heads), as well as the brain, trigeminal nerve and pharyngeal region.

	sall1 (spalt-like transcription factor 1 ) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 38, lateral view, anterior left, dorsal up.
	wt1 (Wilms tumor 1) gene expression in Xenopus laevis embryo, assayed via in situ hybridization, NF stage 30, lateral view, anterior left, dorsal up.

	Fig. 1. Basic organization of the Xenopus pronephric kidney. (A) Schematic representation of the Xenopus pronephric kidney. Blood vessels branching from the dorsal aorta form the glomus, the capillary network of the pronephric corpuscle. Blood travels from the dorsal aorta to the glomus where it undergoes size-selective pressure filtration. The plasma filtrate is collected in the nephrocoel and passes via ciliated nephrostomal funnels into the pronephric tubules. The tubular epithelia perform selective reabsorption of nutrients and salts. Blood from the posterior cardinal vein flows through the pronephric sinus, an ill-defined capillary network between the tubules, to return resorbed solutes to the circulatory system. The remaining excretory products pass into the pronephric duct, which empties into the cloaca. Abbreviations: acv, anterior cardinal vein; ccv, common cardinal vein; pcv, posterior cardinal vein
	Fig. 1. Basic organization of the Xenopus pronephric kidney. (B) Segmental organization of pronephric nephrons. The pronephric capsule is comprised of visceral and parietal epithelia. The visceral (or podocyte) layer (green) is a specialized segment of the splanchnic mesoderm. It contacts the vasculature of the glomus and is continuous with the epithelium of the parietal layer. Pronephric tubules (red) are subdivided into nephrostomes, connecting tubules, and the common tubule. Finally, the pronephric duct (blue) is comprised of at least two parts, the anterior and posterior duct segments. Abbreviations: acv, anterior cardinal vein; ccv, common cardinal vein; pcv, posterior cardinal vein.