Key features of the nephrogenic zone in the fetal human kidney—hardly known but relevant for the detection of first traces impairing nephrogenesis

Installation of the developing nephron in the fetal human kidney during advanced pregnancy

BACKGROUND

The kidneys of preterm and low birth weight babies reflect vulnerability, since several noxae can evoke the termination of nephron formation. This again leads to oligonephropathy with severe consequences for health in the later life. While the clinical parameters have been intensely investigated, only little is known about the initial traces left by the noxae. For the fetal human kidney, solely the lack of basophilic S-shaped bodies and the reduction in width of the nephrogenic zone were registered. It is not known in how far also the involved progenitor cells, the earlier nephron stages, the collecting duct (CD) ampullae, and the local interstitium are collaterally harmed.

AIM

The interstitium at the forming nephron is heterogeneously structured. Thereby, it fulfills quite different mastering and integrative tasks. Since data dealing with the installation of a nephron is not available, the microanatomical features were recorded.

RESULTS

The microscopic specimens show that the installation of the transient stages of nephron anlage is not synchronized. Instead, it is controlled within a nephrogenic compartment of the nephrogenic zone. It starts near the renal capsule by positioning the nephrogenic niche so that the nephrogenic progenitor cells face the epithelial progenitor cell at the tip of a CD ampulla. Then, the induced nephrogenic progenitor cells assimilate in the pretubular aggregate. While its medial part remains opposite the head of the CD ampulla, at its proximal end, the primitive renal vesicle is formed. Only a part of it separates to stick to the section border between the head and conus of the CD ampulla. This marks the link with the future connecting tubule at the distal pole of the extending renal vesicle. Meanwhile, the proximal pole is mounted next to the connecting tubule of an earlier developed nephron. The resulting two-point mounting serves a common elongation of the conus at the CD ampulla and the medial aspect of the comma-shaped body. In the S-shaped body, it supports to defoliate the arising glomerulus and to link it with the perforating radiate artery at its deep lateral aspect.

CONCLUSIONS

The investigation depicts that the installation is an interactive process between the stages of nephron anlage and its structural neighbors. A special meaning has the interjacent interstitium. It is vital for the positioning, shaping, and physiological integration. Due to its special location, this is mainly exposed to noxae.

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

Background

The impairment of nephrogenesis can cause the termination of nephron formation in preterm and low birth weight babies. This leads to oligonephropathy with severe health consequences in later life. Although many clinical parameters are known, surprisingly little information is available regarding the initial damage on the developing nephron. Equally astounding, the first morphological data regarding the specifics of nephron formation in the nephrogenic zone of the fetal human kidney during late gestation has only been published within the past few years. In this context, it was observed that each stage of nephron anlage is surrounded by a specific set of tissues. Although highly relevant for the normal progress of nephron formation, the mutual patterning has not been systematically described.

Results

To contribute, the different stages of nephron anlage in the nephrogenic zone of the fetal human kidney during late gestation were screened by the optical microscope and documented by images. Following this, magnifications (28 × 18 cm) were produced to trace the contours of the developing nephron and its covering tissues. The resulting sketches, almost true to scale, were scanned, edited, and processed by a design program. As a base, first the individual position, size, and shape of the nephrogenic niche, pretubular aggregate, renal vesicles, comma- and S-shaped bodies are presented. Secondly, their structural relations to the renal capsule, collecting duct ampulla, perforating radiate artery, and expanding interstitium are shown. Third of all, the focus is on less considered configurations, such as site-specific approximation, local distancing, punctual adhesion, integration, separation, delamination, formation of congruent and divergent surfaces, and folding and opening of interstitial clefts.

Conclusions

The present contribution illuminates the mutual patterning between the developing nephron and its covering tissues. It is indispensable to know about the microanatomical relations, in order to identify whether the noxae impairing nephrogenesis targets only the developing nephron or also its covering tissues as interacting and controlling instances.

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.

Kidney-Detrimental Factors and Estimated Glomerular Filtration Rate in Preterm Newborns: The Role of Nutrition

: Background: Kidney function in preterm newborns may be impaired by many factors.

METHODS

71 newborns with gestational age (GA) < 32 weeks were enrolled. Serum creatinine (sCr), cystatin C (CysC), beta-trace protein (BTP) and urea were measured at T0 (3^rd day of life) and T36 (GA 36 weeks), and estimated glomerular filtration rate (eGFR) was calculated according to different formulas at T36. Pre-natal and post-natal kidney injury risk scores were calculated.

RESULTS

Newborns with GA ≤ 28 weeks had higher sCr at T0, and lower sCr, BTP and higher urea levels at T36 (p = 0.007, p = 0.005 and p = 0.029, respectively). eGFR values were not different according to GA when calculated by the formulas using only CysC, but were higher in subjects with GA ≤ 28 weeks according to the other formulas. The post-natal score was positively correlated with eGFR according to sCr-based formulas, but the correlations did not persist when adjusted for urea levels and GA.

CONCLUSIONS

CysC-based eGFR values are not influenced by GA. Post-natal score shows a direct correlation with eGFR according to sCr-based formulas, not persisting after adjustment for GA and urea levels, implying the importance of the nutritional status, since more premature subjects receive a more aggressive nutritional regimen, testified by higher urea levels.

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.

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.

Proximal Tubular Development Is Impaired with Downregulation of MAPK/ERK Signaling, HIF-1α, and Catalase by Hyperoxia Exposure in Neonatal Rats

Supplemental oxygen therapy (hyperoxia) is a widely used treatment for alveolar hypoxia in preterm infants. Despite being closely monitored, hyperoxia exposure is believed to undermine neonatal nephrogenesis and renal function caused by elevated oxidative stress. Previous studies have mostly focused on the hyperoxia-induced impairment of glomerular development, while the long-term impact of neonatal hyperoxia on tubular development and the regulatory component involved in this process remain to be clarified. Here, we examined tubular histology and apoptosis, along with the expression profile of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling, hypoxia-inducible factor 1α (HIF-1α), and catalase, following hyperoxia exposure in neonatal rats. Hematoxylin and eosin (H&E) staining revealed the early disappearance of the nephrogenic zone, as well as dilated lumens and reduced epithelial cells, of mature proximal tubules following neonatal hyperoxia. A robust increase in tubular cell apoptosis caused by neonatal hyperoxia was found using a TUNEL assay. Moreover, neonatal hyperoxia altered renal MAPK/ERK signaling activity and downregulated the expression of HIF-1α and catalase in the proximal tubules throughout nephrogenesis from S-shaped bodies to mature proximal tubules. Cell apoptosis in the proximal tubules was positively correlated with HIF-1α expression on the 14th postnatal day. Our data indicates that proximal tubular development is impaired by neonatal hyperoxia, which is accompanied by altered MAPK/ERK signaling as well as downregulated HIF-1α and catalase. Therapeutic management that targets MAPK/ERK signaling, HIF-1α, or catalase may serve as a protective agent against hyperoxia-induced oxidative damage to neonatal proximal tubules.