Glomerular density-associated changes in clinicopathological features of minimal change nephrotic syndrome in adults

Morphometric analysis of chronicity on kidney biopsy: a useful prognostic exercise

Chronic changes on kidney biopsy specimens include increasing amounts of arteriosclerosis, glomerulosclerosis, interstitial fibrosis and tubular atrophy, enlarged nephron size, and reduced nephron number. These chronic changes are difficult to accurately assess by visual inspection but are reasonably quantified using morphometry. This review describes the various patient populations that have undergone morphometric analysis of kidney biopsies. The common approaches to morphometric analysis are described. The chronic kidney disease outcomes associated with various chronic changes by morphometry are also summarized. Morphometry enriches the characterization of chronicity on a kidney biopsy and this can supplement the pathologist's diagnosis. Artificial intelligence image processing tools are needed to automate the annotations needed for practical morphometric analysis of kidney biopsy specimens in routine clinical care.

Clinical and histopathological features related to time to complete remission in adult-onset minimal change nephrotic syndrome patients with corticosteroid treatment

BACKGROUND

Minimal change nephrotic syndrome (MCNS) is a common type of nephrotic syndrome in adults, though evidence regarding its clinical and histopathological features related to time to complete remission (CR) is limited.

METHODS

This was a retrospective study of biopsy-proven, first-onset, adult MCNS patients who achieved CR after undergoing corticosteroid treatment. Body weight (BW) change rate was calculated as follows: (BW at admission - BW at discharge)/BW at discharge × 100. Histopathological examinations were performed, with particular attention given to tubulointerstitial lesions.

RESULTS

Fifty-seven patients (median 41 years old, range 22-63 years; 37 males) were diagnosed with MCNS from 2007 to 2020. Time to CR was a median 11 (8-21) days. In addition to serum creatinine and urinary protein, BW change rate also showed a positive correlation with time to CR (r_s = 0.438, p < 0.001; r_s = 0.280, p = 0.035; r_s = 0.544, p < 0.001; respectively), while multivariate Cox proportional hazards models also revealed those factors as significant predictors for longer time to CR. In MCNS patients with a higher BW change rate (n = 28), serum creatinine, urinary protein, histopathological score, and time to CR were significantly greater as compared to those with a lower BW change rate (n =29). Also, in those patients, histopathological interstitial edema was significantly associated with longer time to CR after adjustments for serum creatinine and urinary protein.

CONCLUSION

The present results indicate that BW change rate can predict time to CR in adult-onset MCNS patients. Histopathologically, interstitial edema is also an important factor for time to CR in MCNS patients with greater BW increase.

Estimating Nephron Number from Biopsies: Impact on Clinical Studies

BACKGROUND

Accumulating evidence supports an association between nephron number and susceptibility to kidney disease. However, it is not yet possible to directly measure nephron number in a clinical setting. Recent clinical studies have used glomerular density from a single biopsy and whole kidney cortical volume from imaging to estimate nephron number and single nephron glomerular filtration rate. However, the accuracy of these estimates from individual subjects is unknown. Furthermore, it is not clear how sample size or biopsy location may influence these estimates. These questions are critical to study design, and to the potential translation of these tools to estimate nephron number in individual subjects.

METHODS

We measured the variability in estimated nephron number derived from needle or virtual biopsies and cortical volume in human kidneys declined for transplantation. We performed multiple needle biopsies in the same kidney, and examined the three-dimensional spatial distribution of nephron density by magnetic resonance imaging. We determined the accuracy of a single-kidney biopsy to predict the mean nephron number estimated from multiple biopsies from the same kidney.

RESULTS

A single needle biopsy had a 15% chance and virtual biopsy had a 60% chance of being within 20% of the whole-kidney nephron number. Single needle biopsies could be used to detect differences in nephron number between large cohorts of several hundred subjects.

CONCLUSIONS

The number of subjects required to accurately detect differences in nephron number between populations can be predicted on the basis of natural intrakidney variability in glomerular density. A single biopsy is insufficient to accurately predict nephron number in individual subjects.

Assessment of nephron number and single-nephron glomerular filtration rate in a clinical setting

Total nephron counts vary widely between individuals and may affect susceptibility to certain diseases, including hypertension and chronic kidney disease. Detailed analyses of whole kidneys collected from autopsy patients remain the only method for accurately counting nephrons in humans, with no equivalent option in living subjects. Current technological advances have enabled estimations of nephron numbers in vivo, particularly the use of total nephron number and whole-kidney glomerular filtration rate to estimate the mean single-nephron glomerular filtration rate. The use of this method would allow physicians to detect dynamic changes in filtration function at the single-nephron level rather than to simply count the number of nephrons that appear to be functioning. Currently available methods for estimating total nephron number in clinical practice have the potential to overcome limitations associated with autopsy analyses and may therefore pave the way for new therapeutic interventions and improved clinical outcomes.

Clinical consequences of developmental programming of low nephron number

Nephron number in humans varies up to 13-fold, likely reflecting the impact of multiple factors on kidney development, including inherited body size and ethnicity, as well as maternal health and nutrition, fetal exposure to gestational diabetes or preeclampsia and other environmental factors, which may potentially be modifiable. Such conditions predispose to low or high offspring birth weight, growth restriction or preterm birth, which have all been associated with increased risks of higher blood pressures and/or kidney dysfunction in later life. Low birth weight, preterm birth, and intrauterine growth restriction are associated with reduced nephron numbers. Humans with hypertension and chronic kidney disease tend to have fewer nephrons than their counterparts with normal blood pressures or kidney function. A developmentally programmed reduction in nephron number therefore enhances an individual's susceptibility to hypertension and kidney disease in later life. A low nephron number at birth may not lead to kidney dysfunction alone except when severe, but in the face of superimposed acute or chronic kidney injury, a kidney endowed with fewer nephrons may be less able to adapt, and overt kidney disease may develop. Given that millions of babies are born either too small, too big or too soon each year, the population impact of altered renal programming is likely to be significant. Many gestational exposures are modifiable, therefore urgent attention is required to implement public health measures to optimize maternal, fetal, and child health, to prevent or mitigate the consequences of developmental programming, to improve the health future generations.

The number of nephrons in different glomerular diseases

Background

The total number of nephrons has been measured mainly from post-mortem studies and only in selected populations. Data from living subjects are scanty, and direct comparisons among different glomerular diseases are lacking. The present work exploits modern methodology to estimate the total nephron number in glomerulopathies with prevalent proteinuria/nephrotic syndrome versus glomerulopathies with nephritic syndrome (IgA nephropathy (IgAN), lupus nephritis), thus extending previous observations about the number and function of glomeruli in different physiological and pathological states.

Methods

This is a retrospective study based on one hundred and seven patients who have undergone renal biopsy. The glomerular density has been estimated from the biopsy specimens and the total cortical volume has been obtained from ultrasound recordings. Stereological methods have been applied to calculate the total number of nephrons and their volume. The correlation between clinical parameters and quantitative morphological data have studied using the Pearson correlation coefficient (r).

Results

The total number of nephrons inversely correlated with the systolic blood pressure (r = −0.4, p < 0.05). In proteinuric diseases, such as focal segmental glomerulo-sclerosis (FSGS), membranous nephropathy (MN) and diabetes, the change in estimated GFR (eGFR) directly correlated with the total number of non-sclerotic glomeruli (NSG) (r = 0.62, p < 0.01), whereas in nephritic syndrome no significant correlation was observed. The alterations in eGFR occurring in nephritic syndromes such as IgAN cannot be explained on the basis of the number of NSG.

Discussion

The fusion of the podocyte foot-processes that typically occurs in purely proteinuric diseases does not modify the glomerular filtration rate: therefore in these situations, the change in eGFR depends mainly on the number of available glomeruli. On the other side, the eGFR decrease occurring in nephritic syndromes, such as IgAN, cannot be explained simply on the basis of the number of NSG and likely depends on the substantial involvement of the mesangial axis. Future studies should verify whether these changes are reversible with appropriate therapy, thus reversing eGFR decrease.

Biopsy-based estimation of total nephron number in Japanese living kidney donors

BACKGROUND

Increasing evidence suggests that individuals with low nephron number have an increased lifetime risk of renal insufficiency, thereby emphasizing the importance of evaluating total nephron number in each individual. In recent years, new methods have been described for estimating human total nephron number using a combination of image analysis and renal biopsy, though the reproducibility and accuracy of these methods remain uncertain. This study estimated total nephron number in healthy Japanese subjects using such a method.

METHODS

Implantation biopsies from 44 living kidney donors were analyzed. Using pre-donation contrast CT angiograms, transplantation donor kidneys were three-dimensionally reconstructed, and total renal cortical volume was estimated. Total nephron number was estimated based on glomerular density in biopsy specimens and total renal cortical volume. The obtained results were analyzed in relation to clinical variables and compared with those of a previously reported Japanese autopsy study.

RESULTS

The estimated non-sclerotic and total numbers of glomeruli in this cohort were 650,000 ± 220,000 and 710,000 ± 220,000 (mean ± SD) per kidney. Non-sclerotic glomerular number ranged from 280,000 to 1,220,000 per kidney (4.4-fold) and correlated directly with eGFR (r = 0.328, p = 0.030) and inversely with age (r = - 0.355, p = 0.018).

CONCLUSION

The estimated total nephron number obtained in the present study was 25% less than that reported in American living kidney donors obtained using the same procedure and similar to that obtained in a previous Japanese autopsy study using the disector/fractionator method. These results confirm the feasibility of a combined CT angiography and biopsy-based method to estimate total nephron number in humans.

Structural and Functional Changes in Human Kidneys with Healthy Aging

Aging is associated with significant changes in structure and function of the kidney, even in the absence of age-related comorbidities. On the macrostructural level, kidney cortical volume decreases, surface roughness increases, and the number and size of simple renal cysts increase with age. On the microstructural level, the histologic signs of nephrosclerosis (arteriosclerosis/arteriolosclerosis, global glomerulosclerosis, interstitial fibrosis, and tubular atrophy) all increase with age. The decline of nephron number is accompanied by a comparable reduction in measured whole-kidney GFR. However, single-nephron GFR remains relatively constant with healthy aging as does glomerular volume. Only when glomerulosclerosis and arteriosclerosis exceed that expected for age is there an increase in single-nephron GFR. In the absence of albuminuria, age-related reduction in GFR with the corresponding increase in CKD (defined by an eGFR<60 ml/min per 1.73 m²) has been shown to associate with a very modest to no increase in age-standardized mortality risk or ESRD. These findings raise the question of whether disease labeling of an age-related decline in GFR is appropriate. These findings also emphasize the need for a different management approach for many elderly individuals considered to have CKD by current criteria.

Glomerular Density and Volume in Renal Biopsy Specimens of Children with Proteinuria Relative to Preterm Birth and Gestational Age

BACKGROUND AND OBJECTIVES

A low total nephron number, which is associated with low birth weight (LBW), may indicate increased susceptibility to early-onset renal diseases in children. However, few studies have assessed renal biopsy findings in LBW children. We examined the relationship between LBW and glomerular density (GD) and/or glomerular volume (GV) in renal biopsy samples as a surrogate for total nephron number.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Renal biopsy findings of children of LBW were compared with those of age-matched control subjects of normal birth weight (NBW) who were histopathologically diagnosed with FSGS or minimal change nephrotic syndrome (MCNS) from 1995 to 2011. The GD and GV were estimated on the basis of measurements obtained by computerized image analysis.

RESULTS

A total of 31 subjects (mean age 11 years; eight with low birth weight-FSGS [LBW-FSGS], 10 with normal birth weight-FSGS [NBW-FSGS], and 13 with normal birth weight-minimal change nephrotic syndrome [NBW-MCNS]) were analyzed. The mean birth weight of each group was 777 g (629-1000), 3110 g (2888-3358), and 3120 g (2748-3398), respectively (median [25th-75th percentile]). Age, body mass index, BP, and degrees of globally sclerotic glomeruli at biopsy were comparable between the groups. The GD was lower (LBW-FSGS, 1.4±0.6/mm2; NBW-FSGS, 3.3±1.2/mm2; and NBW-MCNS, 3.6±1.1/mm2; P<0.05) and the GV was greater (LBW-FSGS, 4.1 [3.1-5.1]×106µm3; NBW-FSGS, 1.6 [1.5-2.1]×106µm3; and NBW-MCNS, 1.3 [1.1-1.8]×106µm3 [median, (25th-75th percentile)]; P<0.05) in patients with LBW-FSGS than in the other patient groups. The GD showed close positive correlations with birth weight (r=0.48) and gestational age (r=0.54), independent of renal function and degree of global glomerular sclerosis.

CONCLUSIONS

A low GD together with marked glomerular enlargement characterizes renal biopsy samples of children born with a LBW at an early stage of gestation.

Counting glomeruli and podocytes: rationale and methodologies

PURPOSE OF REVIEW

There is currently much interest in the numbers of both glomeruli and podocytes. This interest stems from a greater understanding of the effects of suboptimal fetal events on nephron endowment, the associations between low nephron number and chronic cardiovascular and kidney disease in adults, and the emergence of the podocyte depletion hypothesis.

RECENT FINDINGS

Obtaining accurate and precise estimates of glomerular and podocyte number has proven surprisingly difficult. When whole kidneys or large tissue samples are available, design-based stereological methods are considered gold standard because they are based on principles that negate systematic bias. However, these methods are often tedious and time consuming, and oftentimes inapplicable when dealing with small samples such as biopsies. Therefore, novel methods suitable for small tissue samples, and innovative approaches to facilitate high throughput measurements, such as MRI to estimate glomerular number and flow cytometry to estimate podocyte number, have recently been described.

SUMMARY

This review describes current gold-standard methods for estimating glomerular and podocyte number, as well as methods developed in the past 3 years. We are now better placed than ever before to accurately and precisely estimate glomerular and podocyte number, and examine relationships between these measurements and kidney health and disease.

Glomerular Density in Biopsy-Proven Hypertensive Nephrosclerosis

BACKGROUND

Previous autopsy studies suggested that a reduced nephron number is associated with increased risk of hypertension and chronic kidney disease. However, the significance of the nephron number estimated from a renal biopsy in patients with hypertensive nephrosclerosis (HNS) has not yet been elucidated.

METHODS

In this cross-sectional study, we examined the clinicopathological findings of biopsy-proven HNS patients with preserved renal function (estimated glomerular filtration rate ≥ 60 ml/min/1.73 m(2)). The glomerular density (GD; the number of glomeruli per total renal cortical area) in biopsy specimens was evaluated as a surrogate of the nephron number. Renal biopsies from kidney transplant donors were used as healthy controls.

RESULTS

A total of 58 HNS patients were enrolled. The GD value in the HNS patients was low compared with those in the kidney transplant donors (2.0 vs. 3.2 /mm(2)). These differences remained significant when globally sclerotic glomeruli were included in the calculation of the GD. Of note, the GD in HNS patients with overt proteinuria (≥1 g/day) was significantly lower than that of HNS patients with mild proteinuria (<1g/day; 1.8 vs. 2.2/mm(2), P = 0.014). In contrast, other histopathological parameters, including the severity of global glomerulosclerosis, interstitial fibrosis/tubular atrophy and arterial and arteriole lesions were comparable between the 2 HNS subgroups. In addition, the GD was identified as a factor that was associated with the amount of urinary protein excretion at biopsy, independent of other clinicopathological factors.

CONCLUSIONS

These results suggest that a low GD is a renal histological characteristic of HNS patients, especially those with overt proteinuria.

Factors associated with a vicious cycle involving a low nephron number, hypertension and chronic kidney disease

It has been reported that there is substantial variation in the nephron number between individuals. Previous studies using autopsy kidneys have demonstrated that a low nephron number, in relation to a low birth weight, may result in hypertension (HTN) and/or chronic kidney disease (CKD). However, recent studies have revealed that the association between a low nephron number and HTN is not a universal finding. This observation indicates that a low nephron number is unlikely to be the sole factor contributing to an elevated blood pressure. In addition to the nephron number, various genetic and congenital factors may contribute to increased susceptibility to HTN and/or CKD in a complex manner. Acquired factors, including aging, obesity and related metabolic abnormalities, and various causes of renal injury, may additionally promote further nephron loss. Such a vicious cycle may induce HTN and/or CKD via the common mechanisms of renal hemodynamic maladaptation.

Nephron hypertrophy and glomerulosclerosis and their association with kidney function and risk factors among living kidney donors

BACKGROUND AND OBJECTIVES

The relationship of kidney function and CKD risk factors to structural changes in the renal parenchyma of normal adults is unclear. This study assessed whether nephron hypertrophy and nephrosclerosis had similar or different associations with kidney function and risk factors.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

From 1999 to 2009, 1395 living kidney donors had a core needle biopsy of their donated kidney during transplant surgery. The mean nonsclerotic glomerular volume and glomerular density (globally sclerotic and nonsclerotic) were estimated using the Weibel and Gomez stereologic methods. All tubules were counted in 1 cm(2) of cortex to determine a mean profile tubular area. Nephron hypertrophy was identified by larger glomerular volume, larger profile tubular area, and lower nonsclerotic glomerular density. Nephrosclerosis was identified by higher globally sclerotic glomerular density.

RESULTS

The mean (± SD) age was 44 ± 12 years, 24-hour urine albumin excretion was 5 ± 7 mg, measured GFR was 103 ± 17 ml/min per 1.73 m(2), uric acid was 5.2 ± 1.4 mg/dl, and body mass index was 28 ± 5 kg/m(2). Of the study participants, 43% were men, 11% had hypertension, and 52% had a family history of ESRD. Larger glomerular volume, larger profile tubular area, and lower nonsclerotic glomerular density were correlated. Male sex, higher 24-hour urine albumin excretion, family history of ESRD, and higher body mass index were independently associated with each of these measures of nephron hypertrophy. Higher uric acid, higher GFR, and older age were also independently associated with some of these measures of nephron hypertrophy. Hypertension was not independently associated with measures of nephron hypertrophy. However, hypertension and older age were independently associated with higher globally sclerotic glomerular density.

CONCLUSIONS

Nephron hypertrophy and nephrosclerosis are structural characteristics in normal adults that relate differently to clinical characteristics and may reflect kidney function and risk factors via separate but inter-related pathways.

Clinicopathological assessment of the nephron number

Recent studies have demonstrated much larger variability in the total number of nephrons in normal populations than previously suspected. In addition, it has been suggested that individuals with a low nephron number may have an increased lifetime risk of hypertension or renal insufficiency, emphasizing the importance of evaluating the nephron number in each individual. In view of the fact that all previous reports of the nephron number were based on analyses of autopsy kidneys, the identification of surrogate markers detectable in living subjects is needed in order to enhance understanding of the clinical significance of this parameter. In this review, we summarize the clinicopathological factors and findings indicating a reduction in the nephron number, focusing particularly on those found at the time of a preserved renal function.

Obesity-related glomerulopathy and the nephron complement

Obesity-related glomerulopathy (ORG) is a secondary form of glomerular disease that can occur in individuals with obesity. However, the absolute risk for an obese individual to develop progressive renal deterioration is low. Therefore, obesity alone appears to be insufficient to develop such severe renal injury, and there are likely other factors that contribute to the development of this entity. The glomerular hyperfiltration found in patients with ORG has been postulated to lead to structural abnormalities in glomeruli, such as glomerulomegaly and focal segmental glomerular sclerosis, in a manner analogous to that described in patients with reduced renal mass. In fact, recent studies suggest that a reduction in nephron mass is implicated in patients with ORG and synergistically contributes to the development of this renal complication together with obesity-induced changes in renal hemodynamics.

Distribution of glomerular density in different cortical zones of the human kidney

Our studies have demonstrated that a low glomerular density in renal biopsies is a plausible predictor of a worse renal outcome in patients with primary glomerular diseases. However, there remains a concern regarding the diversity that may exist in the distribution of glomerular density within the same kidney. This study therefore aimed to determine the differences in the glomerular density between anatomically different cortical zones of the human kidney. A total of 89 autopsy kidneys were analyzed to accurately measure the glomerular density in different parts of the renal cortex. As a whole, compared to the glomerular density in the superficial cortex (3.0 ± 0.7/mm(2)), the average glomerular density in the juxtamedullary cortex (2.2 ± 0.6/mm(2)) was approximately two-thirds. The glomerular density showed maximal 3.5-fold variations between individuals and was inversely correlated with the mean glomerular volume in both cortical areas. A low glomerular density of the superficial cortex was predominantly associated with the increase of global glomerulosclerosis. On the other hand, a low glomerular density of the juxtamedullary cortex was predominantly associated with an increase in the kidney weight. Thus, there are significant zonal differences in the distribution of the glomerular density in human kidneys independent of the potential variations observed between individuals.