Molecular regulation of nephron endowment

Magnetic resonance urography of congenital abnormalities - what the radiologist needs to know

Congenital abnormalities of the kidney and urinary tract include a wide range of malformations ranging from asymptomatic to life-threatening conditions. Although pediatric urogenital system imaging is based on the use of US (pre- and postnatal), voiding cystourethrography and scintigraphic study, magnetic resonance (MR) urography plays a fundamental role in the classification and management of congenital abnormalities of the kidney and urinary tract, giving an overview of the different clinical pictures, thanks to its panoramicity and high anatomical detail. In fact the anomalies of the urinary tract are phenotypically variable because they can affect simultaneously several segments of different embryonic derivation, with complex clinical pictures; they can appear both as isolated phenotypes or as complex malformative conditions, involving renal parenchyma, collecting system and bladder. A deep knowledge of this complex embryogenesis and its possible phenotypic patterns allows a correct interpretation of MR urography images. We describe the embryology and pathophysiology of congenital abnormalities of the kidney and urinary tract as well as MR urography technique and findings. Congenital abnormalities of the kidney and urinary tract are classified into four groups: (1) obstruction (proximal, middle and distal), (2) budding with respect to the Wolffian duct (site and number of ureter), (3) ascent and rotation (ectopia, malrotation and fusion of kidney) and (4) anomaly of metanephric differentiation (dysplasia, megapolicalycosis).

Ablation of the epithelial-specific splicing factor Esrp1 results in ureteric branching defects and reduced nephron number

Background:

Abnormalities in ureteric bud (UB) branching morphogenesis lead to congenital anomalies of the kidney and reduced nephron numbers associated with chronic kidney disease (CKD) and hypertension. Previous studies showed that the epithelial fibroblast growth factor receptor 2 (Fgfr2) IIIb splice variant supports ureteric morphogenesis in response to ligands from the metanephric mesenchyme during renal organogenesis. The epithelial‐specific splicing regulator Esrp1 is required for expression of Fgfr2‐IIIb and other epithelial‐specific splice variants. Our objective was to determine whether Esrp1 is required for normal kidney development.

Results:

Ablation of Esrp1 in mice, alone or together with its paralog Esrp2, was associated with reduced kidney size and increased incidence of renal aplasia. Three‐dimensional imaging showed that embryonic Esrp1 knockout (KO) kidneys had fewer ureteric tips and reduced nephron numbers. Analysis of alternative splicing in Esrp‐null ureteric epithelial cells by RNA‐Seq confirmed a splicing switch in Fgfr2 as well as numerous other transcripts.

Conclusions:

Our findings reveal that Esrp1‐regulated splicing in ureteric epithelial cells plays an important role in renal development. Defects in Esrp1 KO kidneys likely reflect reduced and/or absent ureteric branching, leading to decreased nephron induction secondary to incorrect Fgfr2 splicing and other splicing alterations. Developmental Dynamics 245:991–1000, 2016. © 2016 The Authors. Developmental Dynamics published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Abnormalities in Ureteric bud (UB) branching morphogenesis lead to congenital anomalies of the kidney and reduced nephron numbers associated with chronic kidney disease (CKD) and hypertension.

We investigated the consequences of ablating the epithelial‐specific splicing regulator Esrp1 on renal organogenesis and determined that Esrp1 KO mice have reduced kidney size, fewer ureteric tips, reduced nephron numbers, and increased incidence of renal aplasia.

Analysis of alternative splicing in Esrp null ureteric epithelial cells by RNA‐Seq identified numerous alterations in splicing.

These findings reveal that Esrp1 regulated splicing in ureteric epithelial cells plays an important role in the kidney and illustrate the importance of alternative splicing for normal renal organogenesis.

Osr1 Interacts Synergistically with Wt1 to Regulate Kidney Organogenesis

Renal hypoplasia is a common cause of pediatric renal failure and several adult-onset diseases. Recent studies have associated a variant of the OSR1 gene with reduction of newborn kidney size and function in heterozygotes and neonatal lethality with kidney defects in homozygotes. How OSR1 regulates kidney development and nephron endowment is not well understood, however. In this study, by using the recently developed CRISPR genome editing technology, we genetically labeled the endogenous Osr1 protein and show that Osr1 interacts with Wt1 in the developing kidney. Whereas mice heterozygous for either an Osr1 or Wt1 null allele have normal kidneys at birth, most mice heterozygous for both Osr1 and Wt1 exhibit defects in metanephric kidney development, including unilateral or bilateral kidney agenesis or hypoplasia. The developmental defects in the Osr1+/-Wt1+/- mouse embryos were detected as early as E10.5, during specification of the metanephric mesenchyme, with the Osr1+/-Wt1+/- mouse embryos exhibiting significantly reduced Pax2-positive and Six2-positive nephron progenitor cells. Moreover, expression of Gdnf, the major nephrogenic signal for inducing ureteric bud outgrowth, was significantly reduced in the metanephric mesenchyme in Osr1+/-Wt1+/- embryos in comparison with the Osr1+/- or Wt1+/- littermates. By E11.5, as the ureteric buds invade the metanephric mesenchyme and initiate branching morphogenesis, kidney morphogenesis was significantly impaired in the Osr1+/-Wt1+/- embryos in comparison with the Osr1+/- or Wt1+/- embryos. These results indicate that Osr1 and Wt1 act synergistically to regulate nephron endowment by controlling metanephric mesenchyme specification during early nephrogenesis.

Prenatal exposure to alcohol reduces nephron number and raises blood pressure in progeny

Prenatal ethanol exposure is teratogenic, but the effects of ethanol on kidney development and the health of offspring are incompletely understood. Our objective was to investigate the effects of acute ethanol exposure during pregnancy on nephron endowment, mean arterial pressure, and renal function in offspring. We administered ethanol or saline by gavage to pregnant Sprague-Dawley rats on embryonic days 13.5 and 14.5. At 1 month of age, the nephron number was 15% lower and 10% lower in ethanol-exposed males and females, respectively, compared with controls. Mean arterial pressure, measured in conscious animals via indwelling tail-artery catheter, was 10% higher in both ethanol-exposed males and females compared with controls. GFR was 20% higher in ethanol-exposed males but 15% lower in ethanol-exposed females; moreover, males had increased proteinuria compared with controls. Furthermore, embryonic kidneys cultured in the presence of ethanol for 48 hours had 15% fewer ureteric branch points and tips than kidneys cultured in control media. Taken together, these data demonstrate that acute prenatal ethanol exposure reduces the number of nephrons, possibly as a result of inhibited ureteric branching morphogenesis, and that these changes affect adult cardiovascular and renal function.

Regulation of kidney development by Shp2: an unbiased stereological analysis

Genes that regulate renal branching morphogenesis are likely to indirectly regulate nephron endowment, but few have been validated to do so in vivo. PTPN11, which encodes the nonreceptor protein tyrosine phosphatase Shp2, acts downstream of receptor tyrosine kinases to modulate the Ras-MAPK pathway and has been implicated in branching morphogenesis in vitro and in invertebrates, and is therefore a candidate in vivo regulator of nephron number. In this work, heterozygous null mutant Shp2(+/-) mice at postnatal days 30-35 were compared with their wild-type (WT) littermates using unbiased stereology to determine if, indeed, the former had decreased nephron number due to their 50% decrease in gene/protein dosage. Although there was a trend toward decreases in total glomerular (nephron) number and kidney volume in Shp2(+/-) mice compared with WT, neither difference was statistically significant (11310 vs. 12198 glomeruli, P = 0.22; 62.8 mm(3) vs. 66.0 mm(3) renal volume; P = 0.40). We conclude that loss of 50% gene/protein dosage of PTPN11/Shp2 is insufficient to affect glomerular (and thereby nephron) number in mouse kidneys in vivo.

Three-dimensional imaging reveals ureteric and mesenchymal defects in Fgfr2-mutant kidneys

Current techniques to morphologically characterize the processes of nephrogenesis and ureteric branching during kidney development have many limitations. Here, we used in vivo three-dimensional analysis to study renal development in mice lacking fibroblast growth factor receptor 2 in the ureteric bud (Fgfr2(UB-/-)) and in littermate controls. We found that Fgfr2(UB-/-) mice have more severe defects in ureteric branching morphogenesis than previously reported, including significantly fewer branches and tips than control mice. Furthermore, these mice had decreased ureteric volume and surface area and longer ureteric segments than control mice. We also observed previously unrecognized abnormalities in nephrogenesis, including a gradual increase in volume and surface area during maturation from renal vesicles to mature nephrons, in the mutant mice. Finally, we quantified many events of normal renal development that are either difficult or impossible to measure without this three-dimensional technique. In summary, the three-dimensional approach is a powerful and quantitative means to characterize branching morphogenesis and nephrogenesis.

Deficiency in Six2 during prenatal development is associated with reduced nephron number, chronic renal failure, and hypertension in Br/+ adult mice

The Br/+ mutant mouse displays decreased embryological expression of the homeobox transcription factor Six2, resulting in hertitable renal hypoplasia. The purpose of this study was to characterize the renal physiological consequences of embryonic haploinsuffiency of Six2 by analyzing renal morphology and function in the adult Br heterozygous mutant. Adult Br/+ kidneys weighed 50% less than those from wild-type mice and displayed glomerulopathy. Stereological analysis of renal glomeruli showed that Br/+ kidneys had an average of 88% fewer glomeruli than +/+ kidneys, whereas individual glomeruli in Br/+ mice maintained an average volume increase of 180% compared with normal nephrons. Immunostaining revealed increased levels of endothelin-1 (ET-1), endothelin receptors A (ET(A)) and B (ET(B)), and Na-K-ATPase were present in the dilated renal tubules of mutant mice. Physiological features of chronic renal failure (CRF) including elevated mean arterial pressure, increased plasma creatinine, and dilute urine excretion were measured in Br/+ mutant mice. Electron microscopy of the Br/+ glomeruli revealed pathological alterations such as hypercellularity, extracellular matrix accumulation, and a thick irregular glomerular basement membrane. These results indicate that adult Br/+ mice suffer from CRF associated with reduced nephron number and renal hypoplasia, as well as glomerulopathy. Defects are associated with embryological deficiencies of Six2, suggesting that proper levels of this protein during nephrogenesis are critical for normal glomerular development and adult renal function.

Associations between age, body size and nephron number with individual glomerular volumes in urban West African males

BACKGROUND

Glomerulomegaly has been associated with an increased risk of renal disease. Few reports have investigated the heterogeneity of glomerular size within kidneys and associated risk factors. This study measured the individual glomerular volume (IGV) of 720 non-sclerotic glomeruli in kidneys of adult West African males, and investigated associations of IGV with age, total glomerular (nephron) number and body surface area (BSA).

METHODS

IGVs were determined in the kidneys of 24 Senegalese males from two age groups (12 subjects aged 20- 30 years and 12 subjects aged 50-70 years). Subjects were randomly chosen at autopsies performed at Le Dantec Hospital in Dakar. Volumes of 30 glomeruli per subject were determined using the disector/Cavalieri stereological method.

RESULTS

IGVs ranged from 1.31 x 10(6) microm3 to 12.40 x 10(6) microm3 (a 9.4-fold variation). IGV varied up to 5.3-fold within single kidneys. The trimmed range of IGV within subjects (10th to 90th percentile of IGV) was directly correlated with median glomerular size. The mean and standard deviation (SD) of IGV did not differ significantly between age groups or between subjects with higher (> or =1.78 m2) and lower BSA (<1.78 m2). In older subjects the SD of IGV was significantly and directly correlated with BSA. Kidneys with less than 1 million nephrons had significantly larger mean IGV than kidneys with more than 1 million nephrons, and the trimmed range of IGVs within subjects was inversely correlated with total glomerular number.

CONCLUSION

There was a considerable variation in IGV within kidneys of Senegalese males at autopsy. The heterogeneity of IGV was increased in association with low nephron number and increased BSA, with more pronounced effects in older subjects.

A common RET variant is associated with reduced newborn kidney size and function

Congenital nephron number varies five-fold among normal humans, and individuals at the lower end of this range may have an increased lifetime risk for essential hypertension or renal insufficiency; however, the mechanisms that determine nephron number are unknown. This study tested the hypothesis that common hypomorphic variants of the RET gene, which encodes a tyrosine kinase receptor critical for renal branching morphogenesis, might account for subtle renal hypoplasia in some normal newborns. A common single-nucleotide polymorphism (rs1800860 G/A) was identified within an exonic splicing enhancer in exon 7. The adenosine variant at mRNA position 1476 reduced affinity for spliceosome proteins, enhanced the likelihood of aberrant mRNA splicing, and diminished the level of functional transcript in human cells. In vivo, normal white newborns with an rs1800860(1476A) allele had kidney volumes 10% smaller and cord blood cystatin C levels 9% higher than those with the rs1800860(1476G) allele. These findings suggest that the RET(1476A) allele, in combination with other common polymorphic developmental genes, may account for subtle renal hypoplasia in a significant proportion of the white population. Whether this gene variant affects clinical outcomes requires further study.

Evaluation of reflux nephropathy, pyelonephritis and renal dysplasia

MR urography has the potential to significantly improve our understanding of the relationship between reflux nephropathy, pyelonephritis, vesicoureteric reflux and renal dysplasia. MR urography utilizes multiple parameters to assess both renal anatomy and function and provides a more complete characterization of acquired and congenital disease. Pyelonephritis and renal scarring can be distinguished by assessing the parenchymal contours and signal intensity. Characteristic imaging features of renal dysplasia include small size, subcortical cysts, disorganized architecture, decreased and patchy contrast enhancement as well as a dysmorphic pelvicalyceal system. Because of its ability to subdivide and categorize this heterogeneous group of disorders, it seems inevitable that MR urography will replace DMSA renal scintigraphy as the gold standard for assessment of pyelonephritis and renal scarring. MR urography will contribute to our understanding of renal dysplasia and its relationship to reflux nephropathy.

Effects of dexamethasone exposure on rat metanephric development: in vitro and in vivo studies

Maternal administration of dexamethasone (DEX) for 48 h early in rat kidney development results in offspring with a reduced nephron endowment. However, the mechanism through which DEX inhibits nephrogenesis is unknown. In this study, we hypothesized that DEX may indirectly inhibit nephrogenesis by inhibiting ureteric branching morphogenesis. Whole metanephroi from embryonic day 14.5 (E14.5) rat embryos were cultured in the presence of DEX. DEX (10(-5) M) exposure for 2 days significantly inhibited ureteric branching compared with metanephroi grown in control media or DEX (10(-7) M). Culturing metanephroi for a further 3 days (in control media only) reduced total glomerular number in metanephroi previously exposed to DEX (10(-5) M) or (10(-7) M) compared with control cultures. Expression of genes known to regulate ureteric branching morphogenesis was determined by real-time PCR in metanephroi after 2 days in culture. DEX exposure in vitro decreased expression of glial cell line-derived neurotrophic factor (GDNF) and increased expression of bone morphogenetic protein-4 (BMP-4) and transforming growth factor-beta1 (TGF-beta1). Similar gene expression changes were found in E16.5 metanephroi in which the dam had been exposed to 2 days of DEX (0.2 mg.kg(-1).day(-1)) at E14.5/15.5 in vivo. However, in kidneys collected at E20.5 after in vivo exposure for 2 days, GDNF expression was increased and BMP-4 and TGF-beta1 expression decreased suggesting a biphasic response in gene expression to DEX exposure. These results show for the first time that inhibition of ureteric branching morphogenesis may be a key mechanism through which DEX exposure results in a reduced nephron endowment.

Effects of maternal vitamin A status on kidney development: a pilot study

Nephron endowment ranges widely in normal human populations. Recent autopsy studies have drawn attention to the possibility that subtle congenital nephron deficits may be associated with increased risk of developing hypertension later in life. Since modest maternal vitamin A deficiency reduces nephron number in rats, we designed a pilot study to determine the prevalence of maternal vitamin A deficiency in Montreal (Canada) and Bangalore (India) and the usefulness of newborn renal volume as a surrogate for nephron endowment. Among 48 pregnant Montreal women, two (4%) had one isolated mid-gestation retinol level slightly below the accepted limit of normal (0.9 mumol/L), whereas 25 (55%) of 46 pregnant women in Bangalore had at least one sample below this limit. Average estimated retinoid intake was correlated with mean serum retinol in pregnant women from Bangalore. In Montreal where maternal vitamin A deficiency was negligible, we found that newborn renal volume (estimated by renal ultrasonography at 2-6 weeks of age) was correlated with surface area at birth and was inversely correlated with serum creatinine at 1 month. Interestingly, renal volume adjusted for body surface area in Montreal (184+/-44 ml/m(2)) was significantly greater than in Bangalore (114+/-33 ml/m(2)) (p<0.01). Definitive studies are needed to establish whether maternal vitamin A deficiency accounts for subtle renal hypoplasia in Indian newborns. If so, there may be important public health implications for regions of the world where maternal vitamin A deficiency is prevalent.

Spatial gene expression in the T-stage mouse metanephros

The E11.5 mouse metanephros is comprised of a T-stage ureteric epithelial tubule sub-divided into tip and trunk cells surrounded by metanephric mesenchyme (MM). Tip cells are induced to undergo branching morphogenesis by the MM. In contrast, signals within the mesenchyme surrounding the trunk prevent ectopic branching of this region. In order to identify novel genes involved in the molecular regulation of branching morphogenesis we compared the gene expression profiles of isolated tip, trunk and MM cells using Compugen mouse long oligo microarrays. We identified genes enriched in the tip epithelium, sim-1, Arg2, Tacstd1, Crlf-1 and BMP7; genes enriched in the trunk epithelium, Innp1, Itm2b, Mkrn1, SPARC, Emu2 and Gsta3 and genes spatially restricted to the mesenchyme surrounding the trunk, CSPG2 and CV-2, with overlapping and complimentary expression to BMP4, respectively. This study has identified genes spatially expressed in regions of the developing kidney involved in branching morphogenesis, nephrogenesis and the development of the collecting duct system, calyces, renal pelvis and ureter.

MafB is essential for renal development and F4/80 expression in macrophages

MafB is a member of the large Maf family of transcription factors that share similar basic region/leucine zipper DNA binding motifs and N-terminal activation domains. Although it is well known that MafB is specifically expressed in glomerular epithelial cells (podocytes) and macrophages, characterization of the null mutant phenotype in these tissues has not been previously reported. To investigate suspected MafB functions in the kidney and in macrophages, we generated mafB/green fluorescent protein (GFP) knock-in null mutant mice. MafB homozygous mutants displayed renal dysgenesis with abnormal podocyte differentiation as well as tubular apoptosis. Interestingly, these kidney phenotypes were associated with diminished expression of several kidney disease-related genes. In hematopoietic cells, GFP fluorescence was observed in both Mac-1- and F4/80-expressing macrophages in the fetal liver. Interestingly, F4/80 expression in macrophages was suppressed in the homozygous mutant, although development of the Mac-1-positive macrophage population was unaffected. In primary cultures of fetal liver hematopoietic cells, MafB deficiency was found to dramatically suppress F4/80 expression in nonadherent macrophages, whereas the Mac-1-positive macrophage population developed normally. These results demonstrate that MafB is essential for podocyte differentiation, renal tubule survival, and F4/80 maturation in a distinct subpopulation of nonadherent mature macrophages.

Neuronal apoptosis inhibitory protein is expressed in developing kidney and is regulated by PAX2

During fetal kidney development, the extent of ureteric bud (UB) branching will determine final nephron endowment for life. Nephron number varies widely among normal humans and those who are born at the low end of the nephron number spectrum may be at risk for essential hypertension in adulthood. Little is known about how nephron number is set. However, we previously showed that the transcription factor, Pax2, suppresses apoptosis in UB cells during kidney development and optimizes branching morphogenesis. Here, we report that PAX2 directly binds to a specific recognition motif in the human neuronal apoptosis inhibitory protein (NAIP) gene promoter. NAIP is an endogenous inhibitor of apoptosis, inactivating caspase-3 and caspase-7 in neuronal tissues. PAX2 activates NAIP gene transcription (7-fold) in vitro and NAIP transcript level is increased fourfold in HEK293 cells stably transfected with PAX2. We show that Naip is expressed in embryonic day 15 (E15) fetal kidney tissue (RT-PCR) and NAIP protein is demonstrated by immunohistochemistry in E15 mouse kidney collecting ducts and P1 proximal tubules. Naip mRNA is significantly reduced (50%) in heterozygous Pax2 mutant mice. Finally, we show that an antisense Naip1 cDNA transfected into murine collecting duct cells doubles caspase-3/7 activity induced by Baxalpha. These observations suggest that the powerful effects of PAX2 on renal branching morphogenesis and final nephron number may be mediated by activation of Naip which then suppresses apoptosis in UB cells.

The reaper-binding protein scythe modulates apoptosis and proliferation during mammalian development

Scythe (BAT3 [HLA-B-associated transcript 3]) is a nuclear protein that has been implicated in apoptosis, as it can modulate Reaper, a central apoptotic regulator in Drosophila melanogaster. While Scythe can markedly affect Reaper-dependent apoptosis in Xenopus laevis cell extracts, the function of Scythe in mammals is unknown. Here, we report that inactivation of Scythe in the mouse results in lethality associated with pronounced developmental defects in the lung, kidney, and brain. In all cases, these developmental defects were associated with dysregulation of apoptosis and cellular proliferation. Scythe-/- cells were also more resistant to apoptosis induced by menadione and thapsigargin. These data show that Scythe is critical for viability and normal development, probably via regulation of programmed cell death and cellular proliferation.

Foxd1-dependent signals control cellularity in the renal capsule, a structure required for normal renal development

Development of the metanephric kidney involves the establishment of discrete zones of induction and differentiation that are crucial to the future radial patterning of the organ. Genetic deletion of the forkhead transcription factor, Foxd1, results in striking renal abnormalities, including the loss of these discrete zones and pelvic fused kidneys. We have investigated the molecular and cellular basis of the kidney phenotypes displayed by Foxd1-null embryos and report here that they are likely to be caused by a failure in the correct formation of the renal capsule. Unlike the single layer of Foxd1-positive stroma that comprises the normal renal capsule, the mutant capsule contains heterogeneous layers of cells, including Bmp4-expressing cells, which induce ectopic phospho-Smad1 signaling in nephron progenitors. This missignaling disrupts their early patterning,which, in turn, causes mispatterning of the ureteric tree, while delaying and disorganizing nephrogenesis. In addition, the defects in capsule formation prevent the kidneys from detaching from the body wall, thus explaining their fusion and pelvic location. For the first time, functions have been ascribed to the renal capsule that include delineation of the organ and acting as a barrier to inappropriate exogenous signals, while providing a source of endogenous signals that are crucial to the establishment of the correct zones of induction and differentiation.

The Where, What and Why of the Developing Renal Stroma

In recent years, a great deal has been learnt about the molecular regulation of kidney development. While most research has focused on the molecular regulation of ureteric branching morphogenesis and nephron formation, significant insights into the definition and functions of the renal stroma have emerged. Many molecules expressed in the developing renal stroma are now known to play significant regulatory roles in kidney development. However, the term 'renal stroma' continues to have different meanings to different researchers. This review clarifies this situation and defines the derivation, location and functions of the stroma in the developing metanephros.

Hepatocyte growth factor-mediated renal epithelial branching morphogenesis is regulated by glypican-4 expression

The glypican (Gpc) family of cell surface heparan sulfate proteoglycans are expressed in a tissue-specific and developmentally regulated fashion. To determine if individual Gpcs can modulate heparin-binding growth factor signaling, we examined hepatocyte growth factor (HGF)-stimulated mitogenic, motogenic, and morphogenic responses of renal tubular cells expressing different Gpcs. Adult inner medullary collecting duct (IMCD) cells were found to express primarily Gpc4 and to proliferate, migrate, and form tubules with HGF, correlating with sustained extracellular signal-regulated kinase (ERK) activation. Embryonic IMCD cells expressing predominantly Gpc3 proliferated and migrated in response to HGF but activated ERK only transiently and failed to form tubules. Overexpressing Gpc-4 but not Gpc-3 or Gpc-1 led to sustained HGF-stimulated ERK activation and rescued the tubulogenic response in these cells. These results demonstrate that both signaling and phenotypic responses to HGF can be regulated by specific Gpc expression patterns.

Nephron endowment and blood pressure: what do we really know?

It has been hypothesized that a reduced number of nephrons at birth contributes to the development of essential hypertension. Nephron number in normal human kidneys has been shown to vary up to eightfold. Therefore, a significant proportion of the population appears to be at risk for developing hypertension. Furthermore, nephron deficits might explain why some racial groups have a higher incidence of hypertension and end-stage renal disease than others. Animal studies have demonstrated that maternal limitations in nutrient supply, both gross and nutrient-specific; exposure to elevated levels of hormones or toxins; and genetic factors can lead to permanent deficits in nephron number and, when examined, elevated blood pressure. In this review, maternal and genetic factors influencing nephron endowment and the implications of nephron deficit for hypertension and renal disease in humans are discussed.

Compensatory renal growth after unilateral nephrectomy in the ovine fetus

Unilateral nephrectomy of the adult animal results in compensatory renal growth but does not involve formation of new nephrons. It is not clear whether compensatory growth can occur during the period of active nephrogenesis in utero and if so, whether more nephrons can be formed. Male ovine fetuses (n = 20) underwent unilateral nephrectomy (n = 10) or sham nephrectomy (n = 10) at 100 d of gestation (term, 150 d). After 27 to 34 d, ewes and fetuses were killed and the right kidney of each fetus was removed and weighed. The wet weight of the right kidney was greater in the unilaterally nephrectomized fetuses (16.3 +/- 1.3 g compared with 12.2 +/- 0.7 g; mean +/- SEM, P < 0.05) as was the kidney to body weight ratio (5.2 +/- 0.3 g/kg compared with 3.8 +/- 0.2 g/kg; P < 0.001). Nephron number in the right kidney was estimated by an unbiased stereologic technique. There was a 45% increase in the number of nephrons in the kidneys from unilaterally nephrectomized animals compared with the kidneys from sham-operated animals (530,763 +/- 37,136 nephrons in the unilaterally nephrectomized group compared with 365,672 +/- 36,016 nephrons in the sham-operated group; P < 0.01). Mean glomerular volume was lower in the unilaterally nephrectomized group; however, total glomerular volume per kidney was not different between groups. This study demonstrates that there is a significant amount of compensatory growth and nephron endowment in a remaining kidney after unilateral nephrectomy during the period of active nephrogenesis in the sheep. This is the first time such events have been shown to occur in utero.