The mutual patterning between the developing nephron and its covering tissues—valid reasons to rethink the search for traces left by impaired nephrogenesis

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The interstitium at the developing nephron in the fetal kidney during advanced pregnancy — a microanatomical inventory

Background A series of noxae can evoke the termination of nephron formation in preterm and low birth weight babies. This results in oligonephropathy with severe consequences for health in the later life. Although the clinical parameters have been extensively investigated, little is known about the initial damage. Previous pathological findings indicate the reduction in width of the nephrogenic zone and the lack of S-shaped bodies. Current morphological investigations suggest that due to the mutual patterning beside the forming nephron, also its structural neighbors, particularly the interjacent interstitium, must be affected. However, beside the findings on integrative and mastering functions, systematic microanatomical data explaining the configuration of the interstitium at the developing nephron in the fetal kidney during advanced pregnancy is not available. Therefore, this work explains the typical features. Results The generated data depicts that the progenitor cells, nephrogenic niche, pretubular aggregate, and mesenchymal-to-epithelial transition are restricted to the subcapsular interstitium. During the proceeding development, only the distal pole of the renal vesicles and comma- and S-shaped bodies stays in further contact with it. The respective proximal pole is positioned opposite the peritubular interstitium at the connecting tubule of an underlying but previously formed nephron. The related medial aspect faces the narrow peritubular interstitium of a collecting duct (CD) ampulla first only at its tip, then at its head, conus, and neck, and finally at the differentiating CD tubule. The lateral aspect starts at the subcapsular interstitium, but then it is positioned along the wide perivascular interstitium of the neighboring ascending perforating radiate artery. When the nephron matures, the interstitial configuration changes again. Conclusions The present investigation illustrates that the interstitium at the forming nephron in the fetal kidney consists of existing, transient, stage-specific, and differently far matured compartments. According to the developmental needs, it changes its shape by formation, degradation, fusion, and rebuilding

Cell Projections and Extracellular Matrix Cross the Interstitial Interface within the Renal Stem/Progenitor Cell Niche: Accidental, Structural or Functional Cues?

Background: During nephron induction, morphogenetic molecules are reciprocally exchanged between epithelial and mesenchymal stem/progenitor cells within the renal stem/progenitor cell niche. That these molecules remain concentrated, it is assumed that both cell populations stand in close contact to each other. However, recently published data illustrate that epithelial and mesenchymal cells are separated by an astonishingly wide interstitial interface. Methods: To gain deeper morphological insights into the spatial distribution of mesenchymal and epithelial stem/progenitor cells, the embryonic zone of neonatal rabbit kidney was fixed either with glutaraldehyde (GA) or in a combination with cupromeronic blue, ruthenium red or tannic acid. Transmission electron microscopy was then performed on exactly orientated sections. Results: Conventional fixation with GA illustrates that epithelial and mesenchymal stem/progenitor cells are separated by a bright but inconspicuously looking interstitial interface. In contrast, fixation of specimens in GA containing cupromeronic blue, ruthenium red or tannic acid elucidates that part of the interstitial interface exhibits a special extracellular matrix extending like woven strands between mesenchymal and epithelial stem/progenitor cells. In parallel, filigree projections from mesenchymal stem/progenitor cells cross the interstitial interface to penetrate the basal lamina of epithelial cells. Fusion of the plasma membranes cannot be observed. Instead, touching mesenchymal cell projections form a cone at the contact site with tunneling nanotubes. Conclusions: The results demonstrate that the contact between mesenchymal and epithelial stem/progenitor cells does not form accidentally but physiologically and appears to belong to a suspected system involved in the exchange of morphogenetic information

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

P CDAmpl, 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 CDAmpl 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 CDAmpl 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 CDAmpl 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 P CDAmpl is a characteristic feature of the embryonic parts of the collecting duct epithelium. It may play a pivotal role during nephron induction

Microanatomy of the developing nephron in the fetal human kidney during late gestation

Background: Clinical experiences reveal that the kidneys of preterm and low birth weight infants are highly vulnerable. Noxae of various molecular composition can damage the outer renal cortex, resulting in an early termination of nephron formation. However, in contrast to what is known about the rodent kidney, with reference to the damage on the early stages of nephron anlage such as the comma-shaped body, renal vesicles, pretubular aggregate or nephrogenic niche, this information in the fetal human kidney is not available. The few documented pathological alterations in the fetal human kidney during late gestation are glomeruli with a dilated Bowman's space and a shrunken tuft, the reduction in width of the nephrogenic zone and the lack of here contained S-shaped bodies. The latter points out that the shaping, folding or expansion of the developing nephron must be disrupted. Since these specific aspects have been little investigated, the aim of the present microanatomical contribution is to highlight it. Methods: Firstly, the individual stages of nephron anlage in the fetal human kidney during late gestation were documented by microscopic images. Then, as a stylistic tool for the pointing to specific sites of the running developmental process, a series of true to scale sketches were produced. Results: The generated sketches depict the spatial expansion of the transiently appearing stages of nephron anlage. These are restricted to the nephrogenic zone and are framed by the inner side of the renal capsule, the related collecting duct ampulla and a perforating radiate artery. Practical hints and a consequent nomenclature explain the developmental course and help us to identify the precise location of the proximal - distal poles, medial - lateral profiles, connecting points, adhesion sites or folds at the developing nephron on microscopic specimens. Conclusions: The impairment of nephrogenesis in preterm and low birth weight babies is an unsolved biomedical issue. To contribute, by provided true to scale sketches, numerous practical hints and a consequent nomenclature typical features of nephron formation in the fetal human kidney at late gestation are demonstrated. (C) 2021 Elsevier GmbH. All rights reserved

The rabbit nephrogenic zone in culture: past, present and future as a model to investigate causes of impaired nephrogenesis

In preterm infants, intrauterine as well as extrauterine influences are held responsible for causing prematurity of renal parenchyma and impaired nephrogenesis, leading to a high incidence of severe kidney diseases later in life. Although involved noxae and resulting molecular effects are quite different, all of them converge to the nephrogenic zone which is restricted to the outer cortex of a developing kidney. Covered by the organ capsule, it consists of aligned ureteric bud-derived collecting duct (CD) ampullae containing epithelial stem cells, nephrogenic mesenchymal stem cells, renal vesicles and S-shaped bodies. Owing to the complex composition of the nephrogenic zone and the different noxae, it is appropriate to investigate impaired nephrogenesis with an adequate in vitro system. In this case, isolation and culture of the nephrogenic zone from neonatal rabbit kidney is particularly well-suited. As compared to human specimens, it exhibits to a large extend a comparable microarchitecture. However, a decisive advantage is that it can be easily and quickly isolated in original composition with microsurgical techniques. Thus, pieces of the explant are available to a variety of advanced culture experiments. Formation of renal spheroids can be used for drug toxicity testing. Mounting in a tissue carrier makes it possible to register functional differentiation of the CD epithelium. Perfusion culture within an artificial interstitium enables investigation of spatial development of tubules. The present article has been written to inform about past and present results, recognized risks and future challenges

Shaping of the nephron – a complex, vulnerable, and poorly explored backdrop for noxae impairing nephrogenesis in the fetal human kidney

Background The impairment of nephrogenesis is caused by noxae, all of which are significantly different in molecular composition. These can cause an early termination of nephron development in preterm and low birth weight babies resulting in oligonephropathy. For the fetal human kidney, there was no negative effect reported on the early stages of nephron anlage such as the niche, pretubular aggregate, renal vesicle, or comma-shaped body. In contrast, pathological alterations were identified on subsequently developing S-shaped bodies and glomeruli. While the atypical glomeruli were closely analyzed, the S-shaped bodies and the pre-stages received little attention even though passing the process of nephron shaping. Since micrographs and an explanation about this substantial developmental period were missing, the shaping of the nephron in the fetal human kidney during the phase of late gestation was recorded from a microanatomical point of view. Results The nephron shaping starts with the primitive renal vesicle, which is still part of the pretubular aggregate at this point. Then, during extension of the renal vesicle, a complex separation is observed. The medial part of its distal pole is fixed on the collecting duct ampulla, while the lateral part remains connected with the pretubular aggregate via a progenitor cell strand. A final separation occurs, when the extended renal vesicle develops into the comma-shaped body. Henceforth, internal epithelial folding generates the tubule and glomerulus anlagen. Arising clefts at the medial and lateral aspect indicate an asymmetrical expansion of the S-shaped body. This leads to development of the glomerulus at the proximal pole, whereas in the center and at the distal pole, it results in elongation of the tubule segments. Conclusions The present investigation deals with the shaping of the nephron in the fetal human kidney. In this important developmental phase, the positioning, orientation, and folding of the nephron occur. The demonstration of previously unknown morphological details supports the search for traces left by the impairment of nephrogenesis, enables to refine the assessment in molecular pathology, and provides input for the design of therapeutic concepts prolonging nephrogenesis