TGF beta 2, LIF and FGF2 cooperate to induce nephrogenesis

SBA-positive fibers between the CD ampulla, mesenchyme, and renal capsule

During kidney development, the CD shows two peculiarities. First, the tip of the CD ampulla is always found at a specific distance from the organ capsule. Second, the CD growth occurs as a perfectly straight elongation. It is unknown whether the CD-specific growth is dependent on hormonal action or on structural elements. Histochemical experiments on neonatal rabbit kidney yielded new insight into the interface of the CD ampulla and the surrounding nephrogenic mesenchyme. Incubation of tissue sections with soybean lectin (SBA) showed the existence of fibers extending in a radial course from the ampullar tip through the mesenchyme toward the organ capsule. SBA labeling did not colocalize with collagen type I, III, IV, V, and VI, laminin, fibronectin, and tenascin. It is assumed that while the kidney increases in volume the structural fixation of the ampullar tip by the SBA-positive fibers causes CD ampullae to maintain a constant distance from the organ capsule. The connection would explain the linear extension of the CD in relation to the organ capsule. In addition, the presented data suggest that the SBA-positive fibers between ampullar tip and organ capsule create a structural microcompartmentation of the nephrogenic zone.

An epithelial precursor is regulated by the ureteric bud and by the renal stroma

Kidney epithelia develop from the metanephric mesenchyme after receiving inductive signals from the ureteric bud and from the renal stroma. However, it is not clear how these signals induce the different types of epithelia that make up the nephron. To investigate inductive signaling, we have isolated clusters of epithelial progenitors from the metanephric mesenchyme, thereby separating them from the renal stroma. When the isolated progenitors were treated with the ureteric bud factor LIF, they expressed epithelial proteins (ZO-1, E-cadherin, laminin alpha(5)) and produced nephrons (36 glomeruli with 58 tubules), indicating that they are the target of inductive signaling from the ureteric bud, and that renal stroma is not absolutely required for epithelial development in vitro. In fact, stroma-depleted epithelial progenitors produced sevenfold more glomeruli than did intact metanephric mesenchyme (5 glomeruli, 127 tubules). Conversely, when epithelial progenitors were treated with both LIF and proteins secreted from a renal stromal cell line, glomerulogenesis was abolished but tubular epithelia were expanded (0 glomeruli, 47 tubules). Hence, by isolating epithelial progenitors from the metanephric mesenchyme, we show that they are targeted by factors from the ureteric bud and from the renal stroma, and that epithelial diversification is stimulated by the ureteric bud and limited by renal stroma.

Wnt-6 is expressed in the ureter bud and induces kidney tubule development in vitro

The embryonic kidney is a classic developmental model system for studying inductive tissue interactions that govern organogenesis. We report here that Wnt-6 is expressed in the ureter bud, and that cell lines expressing Wnt-6 induce nephrogenesis in vitro. Wnt-6 cells induce tubules with similar kinetics to spinal cord (SPC) and lead to induced expression of Pax2, Pax8, Sfrp2, and E-cadherin genes, early markers of tubulogenesis. Moreover, Wnt-6 signaling rescues tubulogenesis in mesenchyme separated from Wnt-4 mutant embryos and leads to activation of Wnt-4 transcription. Wnt-6 also induces a secondary axis in early Xenopus embryos. We conclude that Wnt-6 is a candidate for the ureter epithelium-derived signal that leads to activation of kidney tubulogenesis via Wnt-4.

Growth and death in the developing mammalian kidney: signals, receptors and conversations

Because the kidney (metanephros) starts to function before completing development, its patterning and morphogenesis need to be closely integrated with its growth. This is achieved by blast cells at the kidney periphery generating new nephrons that link up to the extending collecting-duct arborisation, while earlier-formed and more internal nephrons are maturing and beginning to filter serum. This pattern of development requires that cell division and apoptosis be co-ordinated in the various kidney compartments (collecting-ducts, blast cells, metanephric mesenchyme, nephrons and vascular system). The underlying regulatory networks for cell proliferation are beginning to be unravelled, mainly through expression studies, mutation analysis and experimentation in vitro. This article summarises current knowledge of kidney growth and apoptosis, and analyses some of the 80 or so ligand-receptor pairings that seem to sustain development and growth. It also points to some unanswered questions, the most intriguing being what role does apoptosis play during normal kidney development?

Nephron induction revisited: from caps to condensates

Conversion of mesenchyme to epithelium in the metanephric kidney is clearly a multimolecular, multistep and partly redundant process. The present short review focuses on a neglected morphological aspect of kidney differentiation: the development of two transitory mesenchymal condensations that precede epithelial differentiation of nephrons. The first appearing condensate covers the tips of the collecting ducts and is termed a cap condensate. In the early kidney rudiment this structure has been referred to as a primary or early condensate. A few cells of the cap condensate (maybe only four to six cells), situated at the lateral edge of the cap, start proliferating rapidly and form a pretubular aggregate (or pretubular condensate), which converts to secretory nephron epithelia and finally segregates to different tubule segments. Throughout nephrogenesis, the cap condensates and pretubular aggregates are clearly distinguishable structures that show only partly overlapping gene expression profiles. Apart from being the source for the pretubular aggregates, the role of the cap condensate is unknown. It is now proposed that the cap regulates ureteric branching morphogenesis.

Induction of ureter branching as a response to Wnt-2b signaling during early kidney organogenesis

Epithelial-mesenchymal tissue interactions play a central role in vertebrate organogenesis, but the molecular mediators and mechanisms of these morphogenetic interactions are still not well characterized. We report here on the expression pattern of Wnt-2b during mouse organogenesis and on tests of its function in epithelial- mesenchymal interactions during kidney development. Wnt-2b is expressed in numerous developing organs in the mouse embryo, including the kidney, lung, salivary gland, gut, pancreas, adrenal gland, and genital tubercle. Additional sites of expression include the branchial arches and craniofacial placodes such as the eye and ear. The data suggest that the expression of Wnt-2b is associated with organs regulated by epithelial-mesenchymal interactions. It is typically localized in the capsular epithelium or peripheral mesenchymal cells of organ rudiments, e.g., the perinephric mesenchymal cells in the region of the presumptive renal stroma in the developing kidney at E11.5. Functional studies of the kidney demonstrate that cells expressing Wnt-2b are not capable of inducing tubule formation but instead stimulate ureter development. Incubation of isolated ureteric buds on such cells supports bud growth and branching. In addition, recombination of Wnt-2b-pretreated ureteric bud tissue with isolated nephrogenic mesenchyme results in a recovery of organogenesis and the expression of epithelial genes within the reconstituted organ explant. Lithium, a known activator of Wnt signaling (Hedgepeth et al. [1997] Dev Biol 185:82-91), is also sufficient to promote ureter branching in the reconstituted kidney in a comparable manner to Wnt-2b signaling, whereas Wnt-4, which induces tubules, neither supports the growth of a ureteric bud nor leads to reconstitution of the ureteric bud with the kidney mesenchyme. We conclude that Wnt-2b may act in the mouse kidney as an early mesenchymal signal controlling morphogenesis of epithelial tissue, and that the Wnt pathway may regulate ureter branching directly. In addition, Wnt signals in the kidney differ qualitatively and are specific to either the epithelial ureteric bud or the kidney mesenchyme.