Transitional stages in the development of the rabbit renal collecting duct

The interface between generating renal tubules and a polyester fleece in comparison to the interstitium of the developing kidney

An increasing number of investigations is dealing with the repair of acute and chronic renal failure by the application of stem/progenitor cells. However, accurate data concerning the cell biological mechanisms controlling the process of regeneration are scarce. For that reason new implantation techniques, advanced biomaterials and morphogens supporting regeneration of renal parenchyma are under research. Special focus is directed to structural and functional features of the interface between generating tubules and the surrounding interstitial space. The aim of the present experiments was to investigate structural features of the interstitium during generation of tubules. Stem/progenitor cells were isolated from neonatal rabbit kidney and mounted between layers of a polyester fleece to create an artificial interstitium. Perfusion culture was performed for 13 days in chemically defined Iscove's Modified Dulbecco's Medium containing aldosterone (1 x 10(-7) M) as tubulogenic factor. Recordings of the artificial interstitium in comparison to the developing kidney were performed by morphometric analysis, scanning and transmission electron microscopy. The degree of differentiation was registered by immunohistochemistry. The data reveal that generated tubules are embedded in a complex network of fibers consisting of newly synthesized extracellular matrix proteins. Morphometric analysis further shows that the majority of tubules within the artificial interstitium develops in a surprisingly close distance between 5 and 25 mum to each other. The abundance of synthesized extracellular matrix acts obviously as a spacer keeping generated tubules in distance. For comparison, the same principle of construction is found in the developing parenchyma of the neonatal kidney. Most astonishingly, scanning electron microscopy reveals that the composition of interstitial matrix is not homogeneous but differs along a cortico-medullary axis of proceeding tubule development.

Replication of segment-specific and intercalated cells in the mouse renal collecting system

The renal collecting system (CS) is composed of segment-specific (SS) and intercalated (IC) cells. The latter comprise at least two subtypes (type A and non-type A IC). The origin and maintenance of cellular heterogeneity in the CS is unclear. Among other hypotheses, it was proposed that one subtype of IC cells represents a stem cell population from which all cell types in the CS may arise. In the present study, we tested this stem cell hypothesis for the adult kidney by assessing DNA synthesis as a marker for cell replication. SS and IC cells were identified by their characteristic expressions of sodium- (epithelial sodium channel, Na-K-ATPase), water- (aquaporin-2) and acid/base- (H+ -ATPase, anion exchanger AE1) transporting proteins. Immunostaining for bromodeoxyuridine (BrdU) and for the proliferating cell nuclear antigen (PCNA) was used to reveal DNA synthesis in CS epithelium. BrdU- and PCNA-immunostaining as well as mitotic figures were seen in all subtypes of CS cells. Dividing cells retained the cell-type specific expression of marker molecules. Treatment of mice with bumetanide combined with a high oral salt intake, which increases the tubular salt load in the CS, profoundly increased the DNA-synthesis rate in SS and non-type A IC cells, but reduced it in type A IC cells. Thus, our data show that DNA synthesis and cell replication occur in each cell lineage of the CS and in differentiated cells. The replication rate in each cell type can be differently modulated by functional stimulation. Independent proliferation of each cell lineage might contribute to maintain the cellular heterogeneity of the CS of the adult kidney and may also add to the adaptation of the CS to altered functional requirements.

Urea Restrains Aldosterone-Induced Development of Peanut Agglutinin–Binding on Embryonic Renal Collecting Duct Epithelia

ABSTRACT. Peanut agglutinin (PNA) represents a commonly used marker for β-type intercalated (IC) cells and their distribution in the corticomedullary course of the collecting duct (CD) in the mature rabbit kidney. It has been shown that aldosterone is able to generate >90% of PNA-binding cells in an embryonic CD epithelium in vitro. In adult kidney, a maximum of only 25% PNA-positive cells is found in the cortical segment of the CD, and PNA-binding completely disappears in the inner-medullary CD. Molecules that regulate the gradual development of CD-specific cells during organ growth are unknown. In the present experiments, it was found that addition of physiologic concentrations of urea to the culture medium is able to restrain the action of aldosterone in embryonic CD epithelia. Urea antagonizes in a concentration-dependent manner the action of aldosterone finally leading to only 10% of PNA-binding cells. The data point to a urea-specific effect, because osmolytes such as NaCl and mannitol did not affect PNA binding. In addition, urea did not influence expression of principal-cell typical markers such as AQP2 and 3. The findings may explain that a higher number of PNA-positive cells is found in the cortical region of the kidney correlated with a low concentration of urea as compared with only few PNA-binding cells in the medullary CD, where a high concentration of urea occurs. Thus, an increasing concentration of urea may trigger the number of PNA-positive cells in the cortical-medullary course of the CD during organ development. E-mail: karl.schumacher@vkl.uni-regensburg.de

The influence of culture media on embryonic renal collecting duct cell differentiation

During kidney development the embryonic ampullar collecting duct (CD) epithelium changes its function. The capability for nephron induction is lost and the epithelium develops into a heterogeneously composed epithelium consisting of principal and intercalated cells. Part of this development can be mimicked under in vitro conditions, when embryonic collecting duct epithelia are isolated from neonatal rabbit kidneys and kept under perfusion culture. The differentiation pattern is quite different when the embryonic collecting duct epithelia are cultured in standard Iscove's modified Dulbecco's medium as compared to medium supplemented with additional NaCl. Thus, the differentiation behavior of embryonic CD epithelia is unexpectedly sensitive. To obtain more information about how much influence the medium has on cell differentiation, we tested medium 199, basal medium Eagle, Williams' medium E, McCoys 5A medium, and Dulbecco's modified Eagle medium under serum-free conditions. The experiments show that in general, all of the tested media are suitable for culturing embryonic collecting duct epithelia. According to morphological criteria, there is no difference in morphological epithelial cell preservation. The immunohistochemical data reveal two groups of expressed antigens. Constitutively expressed antigens such as cytokeratin 19, P CD 9, Na/K ATPase, and laminin are present in all cells of the epithelia independent of the culture media used. In contrast, a group of antigens detected by mab 703, mab 503, and PNA is found only in individual series. Thus, each culture medium produces epithelia with a very specific cell differentiation pattern.