Histochemical markers reveal an unexpected heterogeneous composition of the renal embryonic collecting duct epithelium

Vascular deficiencies in renal organoids and ex vivo kidney organogenesis

Chronic kidney disease (CKD) and end stage renal disease (ESRD) are increasingly frequent and devastating conditions that have driven a surge in the need for kidney transplantation. A stark shortage of organs has fueled interest in generating viable replacement tissues ex vivo for transplantation. One promising approach has been self-organizing organoids, which mimic developmental processes and yield multicellular, organ-specific tissues. However, a recognized roadblock to this approach is that many organoid cell types fail to acquire full maturity and function. Here, we comprehensively assess the vasculature in two distinct kidney organoid models as well as in explanted embryonic kidneys. Using a variety of methods, we show that while organoids can develop a wide range of kidney cell types, as previously shown, endothelial cells (ECs) initially arise but then rapidly regress over time in culture. Vasculature of cultured embryonic kidneys exhibit similar regression. By contrast, engraftment of kidney organoids under the kidney capsule results in the formation of a stable, perfused vasculature that integrates into the organoid. This work demonstrates that kidney organoids offer a promising model system to define the complexities of vascular-nephron interactions, but the establishment and maintenance of a vascular network present unique challenges when grown ex vivo.

In Search of Imprints Left by the Impairment of Nephrogenesis

Clinical aspects dealing with the impairment of nephrogenesis in preterm and low birth weight babies were intensely researched. In this context it was shown that quite different noxae can harm nephron formation, and that the morphological damage in the fetal kidney is rather complex. Some pathological findings show that the impairment leads to changes in developing glomeruli that are restricted to the maturation zone of the outer cortex in the fetal human kidney. Other data show also imprints on the stages of nephron anlage including the niche, the pretubular aggregate, the renal vesicle, and comma- and S-shaped bodies located in the overlying nephrogenic zone of the rodent and human kidneys. During our investigations it was noticed that the stages of nephron anlage in the fetal human kidney during the phase of late gestation have not been described in detail. To contribute, these stages were recorded along with corresponding images. The initial nephron formation in the rodent kidney served as a reference. Finally, the known imprints left by the impairment in both specimens were listed and discussed. In sum, the relatively paucity of data on nephron formation in the fetal human kidney during the late phase of gestation is a call to start with intense research so that concepts for a therapeutic prolongation of nephrogenesis can be designed.

Concepts for a therapeutic prolongation of nephrogenesis in preterm and low-birth-weight babies must correspond to structural-functional properties in the nephrogenic zone

Numerous investigations are dealing with anlage of the mammalian kidney and primary development of nephrons. However, only few information is available about the last steps in kidney development leading at birth to a downregulation of morphogen activity in the nephrogenic zone and to a loss of stem cell niches aligned beyond the organ capsule. Surprisingly, these natural changes in the developmental program display similarities to processes occurring in the kidneys of preterm and low-birth-weight babies. Although those babies are born at a time with a principally intact nephrogenic zone and active niches, a high proportion of them suffers on impairment of nephrogenesis resulting in oligonephropathy, formation of atypical glomeruli, and immaturity of parenchyma. The setting points out that up to date not identified noxae in the nephrogenic zone hamper primary steps of parenchyma development. In this situation, a possible therapeutic aim is to prolong nephrogenesis by medications. However, actual data provide information that administration of drugs is problematic due to an unexpectedly complex microanatomy of the nephrogenic zone, in niches so far not considered textured extracellular matrix and peculiar contacts between mesenchymal cell projections and epithelial stem cells via tunneling nanotubes. Thus, it remains to be figured out whether disturbance of morphogen signaling altered synthesis of extracellular matrix, disturbed cell-to-cell contacts, or modified interstitial fluid impair nephrogenic activity. Due to most unanswered questions, search for eligible drugs prolonging nephrogenesis and their reliable administration is a special challenge for the future.

Immunohistochemical Study of Porcine Nephroblastoma

Nephroblastoma, a relatively common renal neoplasm of young swine, represents the animal counterpart of Wilms' tumour of children. Five porcine nephroblastomas were examined histologically, and immunohistochemically with antibodies against vimentin (VIM), cytokeratins (CKs), smooth-muscle actin, Factor VIII, and laminin. Histologically all showed the three components typical of this tumour: mesenchymal blastema, epithelium (tubuli, and glomeruloid bodies) and stroma. The only antibody recognizing mesenchymal cells was VIM. One-third of tubular structures were positive for VIM. All of the tubules were positive for CK19, two-thirds expressed CK AE1/AE3, and only one-third expressed CKs 8-18. Small round tubuli, located in the stromal septa, were positive for CK7 (ureteric branches). Stromal cells expressed both VIM and actin, demonstrating myofibroblastic differentiation. The kidney originates from mesenchymal blastema, which changes to epithelium, losing VIM and acquiring CK expression. In the adult mammalian kidney, CK 19 is expressed only by the parietal epithelium of Bowman's capsule and the distal tubules. Nevertheless, CK19 is also considered a "transient" CK, expressed by different kinds of epithelia during differentiation. CK 19 was also detected in several undifferentiated neoplasms. This finding, together with the co-expression of VIM detected in some tubules, demonstrates the embryonic origin of nephroblastoma.

Embryonic renal epithelia: induction, nephrogenesis, and cell differentiation

Embryonic metanephroi, differentiating into the adult kidney, have come to be a generally accepted model system for organogenesis. Nephrogenesis implies a highly controlled series of morphogenetic and differentiation events that starts with reciprocal inductive interactions between two different primordial tissues and leads, in one of two mainstream processes, to the formation of mesenchymal condensations and aggregates. These go through the intricate process of mesenchyme-to-epithelium transition by which epithelial cell polarization is initiated, and they continue to differentiate into the highly specialized epithelial cell populations of the nephron. Each step along the developmental metanephrogenic pathway is initiated and organized by signaling molecules that are locally secreted polypeptides encoded by different gene families and regulated by transcription factors. Nephrogenesis proceeds from the deep to the outer cortex, and it is directed by a second, entirely different developmental process, the ductal branching of the ureteric bud-derived collecting tubule. Both systems, the nephrogenic (mesenchymal) and the ductogenic (ureteric), undergo a repeat series of inductive signaling that serves to organize the architecture and differentiated cell functions in a cascade of developmental gene programs. The aim of this review is to present a coherent picture of principles and mechanisms in embryonic renal epithelia.

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.

Comparison of ICAM-1 and VCAM-1 expression in various human endothelial cell types and smooth muscle cells

The diversity in the local manifestation of inflammatory vascular lesions might be partially attributable to heterogenous cell adhesion molecule (CAM) expression among endothelial cells (EC) derived from different anatomical locations. We compared basal and tumor necrosis factor-alpha (TNFalpha, 0-100 ng/ml, 0-48 h)-induced intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression in cultured human aortic EC (HAEC), vena cava EC (HVCEC), dermal microvascular EC (HMVEC), and vena cava smooth muscle cells (HVCSM), using a fluorescent ELISA and the competitive quantitative RT-PCR. We found marked differences in basal ICAM-1 expression, both at the protein and mRNA levels, such that HAEC>HVCEC approximately equal to HMVEC>>HVCSM. Basal VCAM-1 mRNA levels were significantly lower in HVCEC than in HAEC and HVCSM, while protein levels were indistinguishable. TNFalpha-induced ICAM-1 and VCAM-1 levels in all EC were similar and significantly higher than in HVCSM (2.5- and 5-fold, respectively). Dissimilar levels of basal and TNFalpha-induced CAM expression in vascular cells may explain the varied predisposition of different blood vessels to developing certain vasculopathies.

Transitional stages in the development of the rabbit renal collecting duct

The collecting duct (CD) epithelium of the mammalian kidney is an extraordinary structure with respect to its functional changes during development and its heterogeneous composition when matured. All of the different nephron epithelia of the mammalian kidney consist of one single cell type. In contrast, the differentiated CD is composed of at least three distinct cell types [principal, alpha intercalated-, and beta intercalated cells] that are responsible for the multiple physiological functions of this kidney compartment. During development the function of the CD changes: initially, the CD ampulla serves as an embryonic inducer, while the matured epithelium plays a key role in maintaining the homeostasis of body fluids. At present the process of CD maturation is not well understood. Neither the time course of development nor the morphogenic factors leading to the heterogeneously composed epithelium are known. In the present study the differentiation of the CD epithelium was investigated using newly developed monoclonal antibodies and well-characterized antisera. The morphological changes induced during differentiation were monitored by immunohistochemistry and scanning electron microscopy. The experiments were performed on neonatal and adult rabbit kidneys. Results obtained by light microscopical techniques and scanning electron microscopy revealed that the ampullary tip can be distinguished from the ampullary neck, as well as from the maturing CD. A number of proteins that were not detectable in the ampulla were detected in the neonatal CD and were found at even higher concentrations in the adult CD (PCD8, chloride/bicarbonate exchanger). Other proteins (PCD9) were downregulated during differentiation. For the first time the transient character of the differentiation stage of the neonatal CD could be demonstrated unequivocally. Furthermore, considerable heterogeneity in protein expression patterns (PCD6 and PCD9) was demonstrated within the beta IC cell population of the mature CD.

In vitro development and preservation of specific features of collecting duct epithelial cells from embryonic rabbit kidney are regulated by the electrolyte environment

During kidney development the embryonic collecting duct (CD) epithelium changes its function. The capability for nephron induction is lost and the epithelium develops into functional principal (P) and intercalated (IC) cells. Aldosterone is able to modulate this differentiation. Consequently we investigated whether increased concentrations of extracellular NaCl or Na gluconate may also have an influence on the development of individual CD cell features. Embryonic CD epithelia were isolated from neonatal rabbit kidneys, placed on tissue carriers and cultured in gradient containers, which were constantly perfused with medium for 13 days. Isotonic culture conditions could be mimicked, when on both the luminal and basal side standard Iscove's Modified Dulbecco's Medium (IMDM) was used. In another set of experiments, gradient culture was performed. Standard IMDM was applied on the basal side and IMDM supplemented with 12 mM NaCl and 17 mM Na gluconate on the luminal side. This adaptation of IMDM led to the same Na concentrations as found in the serum of neonatal rabbits. The development of CD cell features was monitored by cellular markers such as the monoclonal antibodies (Mabs) 703 and 503 recognizing P and IC cell features respectively. Epithelia cultured under isotonic conditions showed less than 5% Mab 703- and 503-immunopositive cells. In contrast, epithelia cultured in a luminal-basal medium gradient revealed more than 80% positive cells. Immunoreactivity started to develop after a long lag period of 4 days, then increased continuously during the following 5 days and reached a maximum at day 14. When the medium gradient was then changed to an isotonic environment for another 5 days immunoreactivity for Mab 703 remained stable, while the number of Mab 503-positive cells was found to be decreased to 10%. Thus, the extra-cellular electrolyte environment not only induces but also preserves individual cell features.

PCDAmp1, a new antigen at the interface of the embryonic collecting duct epithelium and the nephrogenic mesenchyme

In the neonatal rabbit kidney nephrogenesis is not yet terminated. The ampullar collecting duct epithelium acts as an inducer that generates the nephron anlagen, however, to date the morphogenic mechanisms involved are unknown. A presupposition for successful nephron induction is the close tissue interaction between the basal aspect of the ampullar collecting duct epithelium and the surrounding mesenchyme. To gain new insights in this area we raised monoclonal antibodies (mabs), to identify specific structures localized at the tissue interface. With the generated mab CDAmp1 we found an intensive immunohistochemical reaction between the basal aspect of the ampullar collecting duct epithelium and the mesenchyme. The label was most concentrated at the ampullar tip and continuously decreased in the shaft region. In the maturing collecting duct of the neonatal kidney and in the adult renal collecting duct no immunohistochemical reaction was found. The binding pattern of mab CDAmp1 is different from that of all known collecting duct cell markers and from antibodies against known basement membrane compounds such as laminin or collagen type IV. Under in vitro conditions immunoreactivity with mab CDAmp1 was obtained using embryonic collecting duct epithelia and perfusion culture. The antigen was present in specimens treated with Iscove's modified Dulbecco's Medium (IMDM) containing 10% fetal bovine serum. Omittance of serum or hormonal treatment with aldosterone, insulin or vitamin D3 led to the disappearance of the newly detected antigen, while characteristics of the differentiated collecting duct cells were up-regulated. We conclude that the expression of PCDAmp1 is a characteristic feature of the embryonic parts of the collecting duct epithelium. It may play a pivotal role during nephron induction.

[Vital long-term preservation of human gingiva in perfusion culture]

Perfusion culture offers the advantage of keeping gingiva alive for a long time as an stable explant according to cell biological parameters. To investigate the suitability of cultured human gingival explants for transplantations the biopsies were put into a newly developed perfusion chamber and cultured for at least 21 days. Gingiva explants were derived from healthy donors undergoing surgical removal of molar teeth. The tissue pieces were cultured without prior proteolytic desintegration or subculture. Immediately after excision a morphological and immunohistochemical analysis of the tissue was carried out and the distribution pattern of cytokeratin and vimentin was examined. Gingival explants cultured for 7, 14 and 21 days in serum-free keratinocyte growth medium in perfusion culture were analyzed in the same way. The morphology of the cultured explant (21 days) was well preserved from stratum basale up to stratum corneum. As proved by immunohistochemical incubation with antibodies to CK 5/6, CK 14 and CK 19, a tissue-specific cytokeratin (CK) expression pattern was maintained during the whole perfusion period. After 7 days of culture vimentin was synthesized in the fibroblast layer and was found in small quantities in each layer of the epithelium. In contrast to conventional cultures, where dissociation of the tissue and a subculture interruption is usually needed for long-term culture, this is not necessary for perfusion cultured tissue. The use of perfusion-cultured gingival explants as autogenous transplants is investigated herein.

Plasticity in epithelial polarity of renal intercalated cells: targeting of the H(+)-ATPase and band 3

The intercalated cell is an epithelial cell of the renal collecting tubule that is specialized for H+ and HCO3- transport. These cells exist as two types, alpha and beta. The alpha-cell secretes H+ into the lumen by an apical H(+)-ATPase and a basolateral Cl-/HCO3- exchanger that is a form of band 3 protein (AE1). The beta-cell secretes HCO3- into the lumen by an apical Cl-/HCO3- exchanger and a basolateral H(+)-ATPase. In a previous study, it was suggested that a reversal in epithelial polarity of these cells occurs during the response of the kidney to an acid load (G.J. Schwartz, J. Barasch, and Q. Al-Awqati. Nature Lond. 318: 368-371, 1985). Recent studies, however have shown that there are many other subtypes where the distribution of these two proteins does not fit into this neat bipolar classification. This group of investigators recently generated an immortalized cell line of the beta-intercalated cell and found that the apical Cl-/HCO3- exchanger is also AE1. Furthermore, when these cells were seeded at high densities, the polarized targeting of the apical band 3 was reversed to the basolateral membrane. This was produced by the secretion of extracellular matrix protein that by themselves were capable of reversing the polarity of band 3 (J. S. van Adelsberg, J. C. Edwards, J. Takito, B. Kiss, and Q. Al-Awqati. Cell 76: 1053-1061, 1995). A large new extracellular matrix protein, hensin, was identified and found to be present exclusively in the collecting tubule. The extensive recent literature on the biology of alpha- and beta-intercalated cells is reviewed here and found to be compatible with the idea of the reversal of polarity as a mechanism for the regulation of H+ secretion by the tubule.

Aldosterone modulates PNA binding cell isoforms within renal collecting duct epithelium

To investigate the differentiation of the ampullary collecting duct cells into adult principal and intercalated cells, the embryonic cortex of newborn New Zealand rabbit kidney was isolated and brought in culture. With this culture technique the ampullary cells formed a polarized collecting duct epithelium which was kept under permanent exchange of medium and in the presence of aldosterone, arginine vasopressin and/or insulin. After 14 days of perfusion culture the epithelia showed light and dark cells resembling the principal and intercalated cells of the adult collecting duct. The differentiation from embryonic into adult collecting duct cells was controlled by applying the monoclonal antibody CD 7. Independent of the hormonal treatment all of the epithelial cells matured in culture and expressed the CD 7 antigen. This corresponded with the situation found within the adult kidney, where the CD 7 antigen was localized in all principal and intercalated (IC) cells, whereas the embryonic ampullary epithelium in the neonatal kidney remained negative. A differentiation feature of the beta-type intercalated cell was investigated by labeling the cultured epithelia with peanut agglutinin (PNA). In contrast to the CD 7 antigen the development of PNA binding was highly dependent of time and individual hormone administration. While in control epithelia only 8% of PNA positive cells were found, aldosterone induced epithelia revealed 72% PNA labeled cells. The combination of aldosterone and insulin increased the number of PNA-positive cells to 90%. By scanning electron microscopy it could further be shown that several isoforms of cells were reactive with PNA. Thus, in culture the PNA label is not restricted to the typical beta-type IC cells.