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As well in light as in transmission electron microscopy can be seen that the renal stem/progenitor cell niche shows a special arrangement of two different kinds of stem/progenitor cells. Epithelial cells are found in the tip of an ureteric bud derived CD ampulla encircled by a special basal lamina. Mesenchymal cells are separated from them by a striking interstitial interface. Specimens fixed by conventional glutaraldehyde solution show that the interface looks bright and unremarkable. In contrast, fixation of specimens with glutaraldehyde in combination with cupromeronic blue, ruthenium red, or tannic acid illustrates that the interface contains a remarkable network of extracellular matrix spanning between epithelial and mesenchymal stem/progenitor cells. After unpacking this particular extracellular matrix for electron microscopy, elaboration of related functions such as structural composition of contained molecules, binding of morphogenetic factors, and influence on parenchyma development is under current experimental work.

Repair of parenchyma by stem/progenitor cells is seen as a possible alternative to cure acute and chronic renal failure in future. To learn about this therapeutic purpose, the formation of nephrons during organ growth is under focus of present research. This process is triggered by numerous morphogenetic interactions between epithelial and mesenchymal cells within the renal stem/progenitor cell niche. Recent data demonstrate that an astonishingly wide interstitial interface separates both types of stem/progenitor cells probably controlling coordinated cell-to-cell communication. Since conventional fixation by glutaraldehyde (GA) does not declare in transmission electron microscopy the spatial separation, improved contrasting procedures were applied. As a consequence, the embryonic cortex of neonatal rabbit kidneys was fixed in solutions containing glutaraldehyde in combination with cupromeronic blue, ruthenium red or tannic acid. To obtain a comparable view to the renal stem/progenitor cell niche, the specimens had to be orientated along the cortico-medullary axis of lining collecting ducts. Analysis of tissue samples fixed with GA, in combination with cupromeronic blue, demonstrates demasked extracellular matrix. Numerous braces of proteoglycans cover, as well, the basal lamina of epithelial stem/progenitor cells as projections of mesenchymal stem/progenitor cells crossing the interstitial interface. Fixation with GA containing ruthenium red or tannic acid illustrates strands of extracellular matrix that originate from the basal lamina of epithelial stem/progenitor cells and line through the interstitial interface. Thus, for the first time, improved contrasting techniques make it possible to analyze in detail a microheterogeneous composition of the interstitial interface within the renal stem/progenitor cell niche.

Stem/progenitor cells are promising candidates for the regeneration of parenchyma in acute and chronic renal failure. After an implantation stem/progenitor cells must migrate through the interstitial space to concentrate at the site of damage. However, information is lacking to what extent the interstitial interface is influencing the development of stem/progenitor cells into nephron structures. In consequence, tubule regeneration within an artificial polyester interstitium was analyzed by electron microscopy in comparison with the interstitial interface of matured tubules and the interstitium within the renal stem/progenitor cell niche. The experiments demonstrate that fixation of specimens with glutaraldehyde (GA) is leading in all cases to inconspicuously looking interstitial interfaces. In contrast, fixation of regenerating tubules in GA containing ruthenium red and tannic acid shows a dense network of fibers lining along the basal lamina. In contrast, matured tubules reveal after ruthenium red label an extremely thickened basal lamina, while only a punctate pattern is obtained after tannic acid treatment. Finally, within the renal stem/progenitor cell niche ruthenium red and tannic acid label reveals large amounts of extracellular matrix spanning through the interstitium. Thus, fixation of tissue in GA containing ruthenium red and tannic acid exhibits an unexpectedly regional heterogeneity of the renal interstitial interface. This fact has to be considered for an optimal therapeutic repair of parenchyma, since contacts between stem/progenitor cells with the interstitial interface influence further development., (Copyright © 2012 Wiley Periodicals, Inc.)

The development of the nephron is piloted by interactions between epithelial and surrounding mesenchymal stem/progenitor cells. Data show that an astonishingly wide interstitial space separates both kinds of stem/progenitor cells. A simple contrasting procedure was applied to visualize features that keep renal epithelial and mesenchymal stem/progenitor cells in distance. The kidney of neonatal rabbits was fixed in solutions containing glutaraldehyde (GA) in combination with alcian blue, lanthanum, ruthenium red, or tannic acid. To obtain a comparable view to the renal stem/progenitor cell niche, the tissue was exactly orientated along the axis of collecting ducts. Fixation with GA or in combination with alcian blue or lanthanum revealed an inconspicuous interstitial space. In contrast, fixation with GA containing ruthenium red exhibits strands of extracellular matrix lining from epithelial stem/progenitor cells through the interstitium up to the surface of mesenchymal stem/progenitor cells. Fixation with GA containing tannic acid shows that the basal lamina of epithelial stem/progenitor cells, the adjacent interstitial space and also the surface of mesenchymal stem/progenitor cells are connected over a net of extracellular matrix. The applied technique appears to be a suitable method to illuminate the interstitium in stem/progenitor cell niches of specialized tissues, the microenvironment of tumors and extension of degeneration.

The cell biological mechanism controlling the regeneration of renal tubules in renal failure after application of stem/progenitor cells is subject of actual research. Unsolved issues are the integration of stem/progenitor cells in a diseased organ environment, the differentiation into epithelial tissue and the formation of tubules in a spatial environment. Following this therapeutic strategy new biomaterials have to be found promoting spatial development of tubules. To obtain new information about the growth of tubules renal stem/progenitor cells from neonatal rabbit kidney were isolated and mounted in a tissue carrier between a selection of commercially available polyester fleeces. This procedure replaces coating by extracellular matrix proteins and creates an artificial interstitium supporting development of tubules. Perfusion culture was performed with chemically defined IMDM containing aldosterone as tubulogenic factor. Polyester fleeces were investigated by scanning electron microscopy. The spatial development of tubules was registered on whole-mount specimens and on cryosections labeled with SBA and antibodies indicating tubule differentiation. It is found that some polyester fleeces promote the spatial development of tubules between the fibers, whereat each of them produces its individual growth pattern.