The most comprehensive descriptions of the process and timing of human kidney development are available from microdissection studies performed in the 1960s. ^, Briefly, the metanephric kidneys begin formation in embryos at approximately 5 weeks of gestation through the process of branching morphogenesis (forming the multipapillary collecting system), with the first nephrons (renal corpuscle and associated tubules) forming in a centrifugal manner from approximately 9 weeks (development of the kidneys is discussed in detail in Chapter 1 ). ^, Initially, nephrons are formed individually at the ampullae of ureteric bud branches and later in an arcade pattern with multiple nephrons forming per branch ^, ; the majority (∼60%) of nephrons develop in the third trimester of pregnancy. Nephrogenesis is normally completed in late gestation, with the timing of the cessation of nephrogenesis varying between individuals; it has been reported to have been completed as early as 32 weeks’ gestation in one case or still ongoing in other cases at 37 weeks’ gestation. ^, In infants born prematurely at a time when kidney development is ongoing, nephrogenesis may continue postnatally, but this process does not proceed normally (as discussed later). In both term and preterm infants, processes of glomerular and tubular growth and maturation continue after cessation of nephrogenesis into postnatal life. ^, The critical periods of development during which the immature kidneys are vulnerable to programming therefore extend through the majority of the gestation period for fetuses and into the postnatal period. Thus the time period during which the developmental and maturational processes of fetal and infant kidneys may be influenced by adverse exposures, and thereby program life-long renal health, is substantial.

Adverse exposures that impact kidney development are numerous and diverse and may be of maternal, placental, fetal, or environmental origin. ^, The type, timing, duration, and severity of exposure (or combination of exposures) all influence the way in which the kidneys are affected during development (e.g., with a reduction in nephron formation), as well as the long-term impact on blood pressure and kidney function.

Initial human studies focused on LBW, <2.5 kg as a marker for risk of developmental programming in the kidney. Over time, it has been recognized that preterm birth, being born small for gestational age (SGA), intrauterine growth restriction (IUGR), having a high birth weight (HBW), or being exposed to preeclampsia, a diabetic pregnancy or maternal obesity, are also important indicators of programmed risk. Table 20.1 outlines the definitions of birth weight and gestational age categories that are referred to throughout this chapter. The term Small and Vulnerable Newborns has been coined in a Lancet series to encompass all infants born too soon and/or too small, to improve awareness that these conditions are risk factors for neonatal death, as well as long-term chronic illness as illustrated in Fig. 20.1 . The term LBW has been deemed no longer granular enough, and the new classification of small and vulnerable newborns includes the three mutually exclusive categories of preterm SGA, preterm, and appropriate weight for gestational age (AGA) and term SGA. Globally, in 2020, 11.9 million live births were preterm AGA, 1.5 million were preterm SGA and 21.8 million were term SGA, comprising 35.2 million babies and 26.2% of all live births ( Fig. 20.1 A and B ). The global incidence of HBW ranges from 5% to 20% in high-income and 0.5% to 15% in lower-income countries. Maternal hypertension and diabetes during gestation occur with a frequency of around 8% to 10% and 15% of pregnancies, respectively. ^, By 2025 it is estimated that 21% of women will be obese. Overall, around one in five infants born each year are therefore at risk of adverse developmental programming and will thereby be at risk of chronic disease later in life.

(A) Global Burden of Small and Vulnerable Newborns in 2020. (B) Potential in utero growth trajectories reflecting the presence or absence of growth restriction and timing of birth, which impact weight for gestational age.

Low birth weight per se is no longer used as a risk threshold. Instead, appropriateness of weight for gestational age provides more granular identification of programming risk in infants born preterm or at term. AGA , Appropriate for gestational age; FGR , fetal (intrauterine) growth restriction; LGA , large for gestational age; SGA , small for gestational age.

Adapted from Terstappen F, Lely AT. Long-term renal disease after prematurity or fetal growth restriction: who is at risk? Nephrol Dial Transplant. 2020;35:1087–1090 and Lawn JE, Ohuma EO, Bradley E, et al. Small babies, big risks: global estimates of prevalence and mortality for vulnerable newborns to accelerate change and improve counting. Lancet. 2023;401[10389]:1707–1719. ^,

Despite the fact that birth weight is a widely accessible and easily obtained marker of infant morbidity and mortality risk, in 2020, global data on birth weight was only available from 81% of countries. ^, Data on preterm births and SGA babies were only reported by 53% and 4% of countries, respectively. Tracking of these markers is therefore incomplete, leading to missed opportunities to identify babies at risk for future kidney and cardiovascular disease. In addition, although LBW, SGA, and preterm birth are currently the best available clinical surrogates for an adverse early life environment, some programming stresses may not manifest as such, and therefore infants at risk may not be identified. For example, an infant affected by IUGR may still be delivered AGA if growth restriction occurred late in gestation (see Fig. 20.1B ); a large proportion of babies born SGA have a birth weight above 2.5 kg and would therefore not be classified as LBW. These infants may, however, still be at risk of long-term consequences. Conversely, some babies may be normally constitutionally small and may therefore not be at increased risk. Ongoing work is required to develop more sensitive measures of developmental stress for the identification and mitigation of risk at birth.

In 1988, Brenner and colleagues proposed that congenital (programmed) low nephron number may explain why some individuals are susceptible to hypertension and kidney injury, whereas others may seem relatively resistant under similar circumstances (e.g., sodium excess and diabetes mellitus). Low whole-kidney glomerular filtration surface area (a product of average glomerular capillary surface area and total nephron number) would result in reduced sodium excretory capacity, enhancing susceptibility to hypertension, and a relatively reduced renal reserve capacity, limiting compensation for renal injury ( Fig. 20.2 ). A kidney starting with a lower nephron number would conceivably reach a critical reduction of nephron mass, either through age or in response to additional renal insults (second hits), earlier than a kidney with a greater nephron complement, thereby predisposing to hypertension and/or kidney dysfunction at an earlier age. The Brenner hypothesis proposed that an association between low nephron number and LBW, for example, could explain high prevalences of hypertension and CKD observed in populations with coexistence of lower birth weights (e.g., as observed in minoritized populations in high-income settings such as African Americans). Nephron number therefore became a major focus of kidney developmental programming research.

Nephron endowment follows a normal distribution across the population.

Individuals with a low nephron number have a limited capacity to tolerate superimposed kidney insults or to compensate for nephron loss and are therefore at risk of developing (progressive) kidney disease. People with higher nephron numbers are likely more resistant to developing a critical loss of kidney function in the setting of kidney injury because they have an abundant nephron reserve. AKI , Acute kidney injury; CKD , chronic kidney disease; FSGS , focal segmental glomerulosclerosis; SCr , serum creatinine.

From Luyckx VA, Rule AD, Tuttle KR, et al. Nephron overload as a therapeutic target to maximize kidney lifespan. Nat Rev Nephrol. 2022;18(3):171–183.

An obstacle to investigation of the role of nephron number in programming of hypertension and kidney disease risk has been the difficulty in accurately counting or estimating the total number of nephrons in a kidney. Acid-maceration techniques, as well as early model-based or design-based stereologic techniques for analysis of histologic samples have been used, but these are prone to bias because of required assumptions, extrapolations, and operator sensitivity. Over the past 30 years, the design-based (often termed “unbiased”) disector/fractionator method has emerged as the gold-standard method for estimating total glomerular (and thereby nephron) number because it generates accurate (no bias) and precise (low-variance) estimates. With the disector/fractionator method, the disector principle is used to sample glomeruli in representative histologic samples of the three-dimensional renal cortex with equal probability (regardless of their size, shape, and location). Sampled glomeruli are counted in a “known” fraction of kidney tissue, allowing simple algebraic estimation of the total number of glomeruli in the whole kidney.

Most estimates of glomerular number have been obtained from whole kidneys obtained at autopsy. While useful, reliance on autopsy samples precludes studies in living animals and people, as well as longitudinal studies. In recent years, several new approaches for imaging, counting, and sizing glomeruli have emerged, some of which do not require autopsy tissue. For example, the methods of Denic and colleagues and Fulladosa and colleagues, which use computed tomography (CT) or magnetic resonance imaging (MRI) estimation of cortical volume, and an estimation of glomerular numerical density derived from biopsies, do not require autopsy tissue. The number of functional (nonsclerotic) glomeruli has also been estimated by dividing the whole-kidney ultrafiltration coefficient (K _f ; calculated from renal plasma flow from para-amino-hippurate clearance) by single-nephron K _f estimated from electron microscopic measurements of the glomerular filtration barrier using kidney biopsies. ^, These biopsy-based methodologies have allowed for a number of small- to large-scale studies of nephron number to be conducted in living people including healthy donors and people with kidney disease; although limited to sampling only a small number of glomeruli, the accuracy of these techniques continues to improve ( Table 20.2 ). ^{, ,} An intriguing short report also examined 24-hour urine uromodulin excretion as a potential correlate for nephron number before and after living donor nephrectomy. Urine uromodulin excretion fell by around half at 3 days after nephrectomy and rose to around two-thirds of the predonation value by 1 year, possibly reflecting the known adaptive increase in single kidney function.

An MRI technique using cationic ferritin to label glomeruli also shows promise. Cationic ferritin is injected intravenously into animals or into ex vivo human kidneys via the renal artery, and it binds to anionic charges on the glomerular basement membrane. With this approach, all glomeruli in whole rat, mouse, ^, and ex vivo human ^, kidneys have been imaged, counted, and sized, providing, for the first time, the glomerular volume distribution for whole kidneys. In addition, mapping of renal pyramids and the identification of areas of global sclerosis in whole human kidneys have shown this technique to have great potential for improving our understanding of regional renal structure and pathology. New tissue-clearing approaches also facilitate imaging (using light sheet microscopy), counting, and sizing of all glomeruli in ex vivo rodent kidneys, but whether modifications to this approach can be safely used in vivo remains questionable. Overall, with these advances, accurate quantitation of nephron mass in vivo may become possible in the not-too-distant future.

In early studies, humans were believed to have an average of approximately 1 million nephrons per kidney. This implied that there was little variability in human nephron number and therefore likely little relationship with adult disease risk. Such studies, however, were performed using techniques that are prone to bias. Moreover, many of the early studies analyzed small numbers of kidneys, and therefore population variability in nephron number was not observed.

The disector/fractionator method requires sampling from a whole kidney; therefore all studies using this gold-standard technique have been performed on autopsy samples. An early study in 1992 among 37 normal Danish adults reported an average glomerular (nephron) number of 617,000 per kidney (range: 331,000–1,424,000). A positive correlation was noted between nephron number and kidney weight, but this has not been the case in all studies. Nephron number tends to be lower in women than men and lower in people with shorter adult height. ^{, ,} A large variation in nephron number between individuals has subsequently been reported in several other populations (see Table 20.2 ), with the widest range being a 13-fold variation in the largest sample investigated (159 African American people; 210,332 to 2,702,079 per kidney).

A single study of nephron number in 15 American infants aged younger than 3 months (who died from sudden infant death syndrome or congenital abnormality) showed a range of 246,181 to 1,106,062 nephrons per kidney, confirming that nephron number variability is already evident in early life, likely reflecting an impact of developmental programming, rather than nephron loss, on nephron number. A range of 8 to 12 glomerular generations (number of glomeruli alongside a medullary ray, from inner cortex (earliest formed nephron) to outer cortex (most recently formed nephron), a proxy measure of nephron number in cases where a whole kidney is not available for analysis, has also been reported in fetuses and neonates with completed nephrogenesis ( Fig. 20.3 ). ^, Wide variation is also evident in the number of papillae/lobes formed in the human kidneys (a product of variation in early branching morphogenesis and remodeling processes), ranging from 3 to 22 ^, ; however, whether an association exists between lobe number and nephron number is currently unknown.

Representative images of medullary rays (dotted lines) and generations of glomeruli (circled) in hematoxylin-eosin–stained sections of renal cortex from fetuses at (A) 22 weeks’ gestation (4 glomerular generations formed) and (B) 33 weeks’ gestation (9 glomerular generations formed).

Scale bar = 100 μm. Linear regression analyses of the number of glomerular generations formed within the kidney versus gestational age (C), kidney weight (D), and body weight (E) in female (O) and male (▪) fetuses and newborn infants.

From Ryan D, Sutherland MR, Flores TJ, et al. Development of the human fetal kidney from mid to late gestation in male and female infants. eBioMedicine. 2018;27:275–283.

The number of nephrons formed by the completion of nephrogenesis is termed nephron endowment. Three primary processes that are considered to play critical roles in determining nephron endowment are branching of the ureteric tree (branch initiation, elongation, and remodeling); condensation of metanephric mesenchymal cells at the ureteric branch tips; and conversion of these mesenchymal condensates into nephron epithelium (cellular proliferation, migration, metabolism, differentiation, and survival). Even minor alterations in the nephrogenic process may have a major impact on nephron endowment, with a 2% decrease in ureteric tree branching efficiency estimated to result in a 50% reduction in final nephron complement after 20 generations of branching. Kidney development is a complex process involving the tightly controlled expression of many genes and constant remodeling. ^{, , , ,} The molecular regulation of kidney development is exhaustively reviewed elsewhere (see Chapter 1 ). The specific molecular mechanisms whereby nephron numbers may be affected and/or function altered, however, are not yet completely understood. Gene expression can be influenced by genotype, inherited (epigenetically programmed) changes in DNA methylation and histone modification, as well as their interaction with factors in the intrauterine and early postnatal environment. Environmental perturbations that result in reduced nephron endowment are discussed later.

Regardless of the cause, the previously underappreciated variability in total nephron number may influence susceptibility to hypertension and kidney disease. ^{, ,} Additionally, age-related nephron loss may compound this susceptibility. In general, numbers of viable glomeruli are reduced in kidneys from older people, owing to age-related glomerulosclerosis and obsolescence (see Chapter 21 ). ^, Rates of loss of glomeruli per year have been variously reported as approximately 6750 glomeruli per year after the age of 18 years, 4500 glomeruli per year, and 6200 glomeruli per year. Denic and colleagues estimated nonsclerotic and sclerotic glomerular number in 1638 living donor kidneys using a CT/biopsy method. A striking decrease of almost 50% in the numbers of nonsclerotic glomeruli was observed between 18- and 29-year-olds (an average of 990,661 glomeruli) and donors aged 70–75 years (520,410 glomeruli). Moreover, the total number of sclerotic plus nonsclerotic glomeruli was much lower in the older donors, suggesting that many glomeruli had disappeared without trace following global glomerulosclerosis (and/or older donors were born with fewer nephrons).

In support of the nephron number hypothesis, it is known that persons born with severe nephron deficits (e.g., unilateral renal agenesis, bilateral renal hypoplasia, and oligomeganephronia) develop progressive proteinuria, glomerulosclerosis, and renal dysfunction with time. ^, Analogously therefore people born with nephron numbers at or below the median level may be more susceptible to superimposed postnatal factors that act as subsequent “hits” (see Fig. 20.2 ). Indeed, with a refinement of their technique of in vivo nephron counting, Denic and colleagues found that a low nephron number (defined as below the 10th percentile adjusted for age and sex) predicted future CKD risk (hazard ratio [HR] 3.15, 95% confidence interval [CI] 2.10–4.74) and kidney failure (HR 7.12, 95% CI 3.47–14.6) in patients who underwent tumor nephrectomy( Fig. 20.4 ). This is the first study to directly prospectively correlate nephron number with kidney function over time and supports the hypothesis that in the setting of a second hit (nephrectomy, comorbid disease), kidneys with fewer nephrons may lose function more rapidly with age.

Risk of adverse outcomes after nephrectomy among kidney tumor patients with nephron number above or below the 10th percentile for age.

Low nephron number of retained kidney (<10th percentile for age and sex in healthy subset) predicted progressive chronic kidney disease (CKD) (A) and kidney failure (B) (Log-rank P < 0.0001 for both).

From Denic A, Mullan AF, Alexander MP, et al. An improved method for estimating nephron number and the association of resulting nephron number estimates with chronic kidney disease outcomes. J Am Soc Nephrol. 2023;34:1264–1278.

The counterargument to the nephron number hypothesis, however, is that in experimental animals and in humans, removal of one kidney (presumed reduction in nephron number of 50%) under varying circumstances may be associated with higher blood pressures or low-grade proteinuria but does not always lead to hypertension and kidney disease. ^{, ,} Of interest, uninephrectomy on postnatal day 1 in rats or fetal uninephrectomy in sheep, at a time when nephrogenesis is not yet completed, leads to adult hypertension before evidence of renal injury. Other experimental studies have shown greater compensatory glomerular hypertrophy and increased susceptibility to hypertension and injury, in response to neonatal nephrectomy compared with nephrectomy in later life. Unilateral in utero nephrectomy in sheep was associated with significant renal hypertrophy and a 45% increase in nephron number in the remaining kidney, suggesting that a congenital solitary kidney may have a higher than normal nephron endowment and therefore be relatively protected compared with an acquired single kidney. ^, The timing of kidney loss may impact adaptive changes in the contralateral kidney.

In humans, a study of 97 people with a radiologically normal single kidney, aged 2.5–25 years, found that renal function declined faster over time in those with acquired single kidneys (surgical removal of the other kidney) versus congenital single kidneys, although blood pressures and proteinuria were not different. These findings may be confounded by indication for nephrectomy, however, as approximately 25% of the nephrectomies were performed for obstruction, which may impact nephron development in the contralateral kidney, as discussed later. ^, In contrast, among 944 children with a solitary functioning kidney, at a median of 12.8 years of follow-up from diagnosis (age 18 years), major risk factors associated with reduced estimated glomerular filtration rate (eGFR)/CKD included congenital (vs. acquired) single kidney, anomalies in the single kidney, and an elevated body mass index (BMI). Birth weights and gestational ages tend to be lower in children with congenital kidney anomalies, likely further impacting their remaining nephron number. ^, Importantly, the proportion of children with kidney dysfunction increased with age in both the congenital and acquired solitary functioning kidney subgroups, suggesting accumulation of kidney injury through hyperfiltration and second hits over time in both groups.

As noted earlier, total nephron number varies widely in the normal human population (see Table 20.2 ). ^, A significant proportion of the interindividual variability in nephron number appears to be already present perinatally, demonstrating a strong developmental effect. ^, Overall, the data support a direct relationship between nephron number and birth weight and an inverse relationship between nephron number and glomerular volume. ^, This relationship suggests that larger glomeruli may reflect compensatory hyperfiltration and hypertrophy in people with fewer nephrons and may therefore be a surrogate marker for a reduced nephron number. ^,

Hinchliffe and associates ^, studied nephron number in preterm or term stillborn infants or infants who died at 1 year of age and who were born AGA or SGA. At both time points, growth-restricted infants had significantly fewer nephrons than controls ( Fig. 20.5A ). Manalich and colleagues examined the kidney structure of neonates who died within 2 weeks of birth, in relation to their birth weights ( Fig. 20.5B ). A significant direct correlation was found between glomerular number and birth weight, as well as a strong inverse correlation between glomerular volume and glomerular number, independent of sex and race. LBW infants had on average a 12% reduction in nephron number compared with non-LBW infants. Similarly, a study utilizing the acid-maceration technique to count nephrons also showed a linear correlation between nephron number and birth weight percentile; an average of 30% fewer nephrons were counted in the kidneys of SGA infants compared with those that were AGA. Although only a small number of studies have evaluated nephron numbers in infants, each with a low sample size, they all reinforce that an adverse intrauterine environment, which may manifest as LBW, SGA and/or IUGR, and preterm birth, is associated with a congenital reduction in nephron endowment and an early, compensatory increase in glomerular volume.

Effect of intrauterine grown restriction on nephron number in humans.

(A) Nephron number at birth in relation to gestational age (upper panel) and lack of postnatal catch-up in nephron number (lower panel). (B) Relationship between birth weight and glomerular number (upper panel) and between glomerular number and glomerular volume (lower panel) in neonates.

A from Hinchliffe SA, Lynch MR, Sargent PH, et al. The effect of intrauterine growth retardation on the development of renal nephrons. Br J Obstet Gynaecol. 1992;99:296–301) and B from Manalich R, Reyes L, Herrera M, et al. Kidney Int. 2000;58:770–773.

An association among birth weight, nephron number, and glomerular size has also been observed in later life ( Table 20.3 ). Hughson and colleagues reported a linear relationship between glomerular number and birth weight and calculated a regression coefficient predicting an increase of 257,426 glomeruli per 1 kg increase in birth weight (Caucasian and African American population across a wide age range; four individuals were born LBW). The association was higher when excluding adults (likely affected by nephron loss), with a calculated increase of 518,038 glomeruli per 1 kg increase in birth weight in infants and children (although only 1 was born LBW). In 140 American adults aged 18 to 65 years who died of various causes, a significant correlation was also observed between birth weight and glomerular number. Glomerular volume was again inversely correlated with number. Total glomerular number did not differ statistically between African American and white Americans, although the distribution of glomerular numbers among African Americans appeared bimodal. The range of nephron number was also greatest in African Americans. Significantly, however, none of the people in this study had been born LBW.

In humans, approximately two-thirds of the nephrons develop during the last trimester, making this the window of greatest susceptibility to adverse effects, although earlier insults can also impact nephrogenesis. ^, In infants born preterm with developmentally immature kidneys, nephrogenesis can continue postnatally and is therefore susceptible to insults after birth. Rodriguez and colleagues studied kidneys at autopsy from 56 extremely preterm infants compared with 10 term-born infants as controls. Evidence of active glomerulogenesis was seen in preterm infants who died before postnatal day 40, but it was absent in older infants. The radial glomerular counts (glomerular generations; see Fig. 20.3 ) correlated with gestational age and were lower in the kidneys of preterm versus term infants, suggestive of a nephron deficit; however, it is to be noted that SGA/IUGR infants were included in this study, which may have confounded these results. Carpenter and colleagues studied 7 preterm infants who survived beyond 40 days and found evidence of ongoing nephrogenesis in 4 infants, including at day 62 in 1 (examined at 34 weeks post conception), suggesting that the duration of ongoing postnatal nephrogenesis is likely dependent on gestational age at birth. In this study, however, nephrogenesis appeared to cease 2 weeks earlier in kidneys from preterm infants compared with controls who had died at the matched postmenstrual age in utero or within 4 days of birth. This observation suggests additional postnatal nephron stresses in the surviving preterm infant kidneys. Another autopsy study by Sutherland and colleagues of 28 kidneys from preterm neonates (with 2–68 days postnatal survival) compared with 32 stillborn gestational age controls also confirmed that nephrogenesis is ongoing postnatally in infants born preterm. However, there was evidence of accelerated postnatal maturation with an increased number of glomerular generations and a concurrent decreased width of the nephrogenic zone in the kidneys of preterm neonates compared with stillborn infants at the comparative age post conception, consistent with the observed earlier cessation of nephrogenesis by Carpenter and colleagues.

In their earlier study, Rodriguez and colleagues also stratified the preterm infants by the presence or absence of kidney failure. Among infants surviving longer than 40 days, those with kidney failure (serum creatinine >2.0 mg/dL) had significantly fewer glomerular generations than those without. This cross-sectional observation may suggest that kidney failure inhibited glomerulogenesis or, conversely, that fewer glomeruli lowered the threshold to develop kidney failure in these infants. Preterm infants surviving longer than 40 days without kidney failure exhibited glomerulomegaly, which may reflect, at least in the short term, a compensatory renoprotective response.

The appearance of renal corpuscles with shrunken glomerular tufts and enlarged Bowman spaces (resembling atubular glomeruli) has also been noted in the outer renal cortex of kidneys from preterm infants, suggestive of a developmental defect and/or injury to those nephrons newly formed in the extrauterine environment; 0% to 13% of glomeruli were affected per kidney. It is probable that the early loss of these affected glomeruli, along with a hastened cessation of nephrogenesis after birth, contributes to a reduced nephron endowment in infants born preterm; however, this is yet to be confirmed. In support of this possibility, reduced glomerular density and higher glomerular volume were associated with lower gestational age and birth weights in kidney biopsies of Japanese children with proteinuric kidney diseases, suggesting reduced nephron numbers and compensatory glomerular hypertrophy.

An analysis of the association between kidney weight and nephron endowment in infants younger than 3 months of age (a time at which compensatory hypertrophy has likely not yet occurred) has revealed a direct relationship ( Fig. 20.6 ). The regression analysis predicted an increase of 23,459 nephrons per gram of kidney weight. Kidney mass is therefore proportional to nephron number in the early postnatal period, and kidney volume is proportional to kidney mass. Kidney volume has therefore been used as a surrogate for nephron endowment in infants in vivo.

Relationship between nephron number and mass of the right kidney in Caucasian infants aged ≤3 months who died within the first 3 months of life.

Nglom/1000, Number of glomeruli per kidney in 1000s.

From Zhang Z, Quinlan J, Hoy W, et al. A common RET variant is associated with reduced newborn kidney size and function. J Am Soc Nephrol. 2008;19:2027–2034.

Fetal ultrasound studies have consistently shown that fetuses exposed to IUGR or infants born SGA have significantly smaller kidney volumes than AGA infants. A strong correlation between fetal body size and kidney size is also reported. indicating that growth of the developing kidneys is highly vulnerable to suboptimal blood flow. Besides a reduction in kidney volume, ultrasound studies have also revealed a reduction in hourly urine volume, a higher prevalence of oligohydramnios, and reduced kidney perfusion in growth-restricted fetuses. ^{, , ,} These findings may represent reduced fetal perfusion in situations of intrauterine compromise, however, and do not necessarily reflect altered kidney development. Most (but not all ) studies among preterm infants also show a reduction in kidney size compared with term-born infants at equivalent ages, with reports of reduced kidney volume ^, and slowed postnatal growth.

Smaller kidney size can persist into postnatal life. An ultrasound analysis of 178 Danish children born preterm or SGA compared with 717 term AGA children at 0, 3, and 18 months found that weight for gestational age was positively associated with kidney volume at all three time points. Slight catch-up in kidney growth was observed in growth-restricted infants but not in preterm infants. Similarly, in a South Indian population, kidney volumes were lower in LBW and SGA neonates compared with those born AGA. Kidneys in the LBW and SGA infants remained small and grew more slowly than kidneys of AGA infants during the first 2 years of life. In a study of multiethnic children in the Netherlands, lower fetal weight gain and lower early infant weight gain led to smaller kidneys at 6 years of age. However, only lower fetal weight gain was associated with lower eGFR. These findings show that suboptimal early growth affects kidney structure and function in later life.

Given the strong association between body and kidney size, however, it is important to consider kidney volume relative to body size, rather than absolute kidney volumes alone. In Australian Aboriginal children, LBW was found to be associated with significantly lower kidney volumes corrected for body size compared with children born AGA. Comparison of kidney volume between Swedish children aged 9 to 12 years born preterm and either SGA or AGA, as well as term controls, also found that kidneys were smallest in those who had been preterm and SGA, but when adjusted for body surface area (BSA), there were no significant differences between the groups. Similarly, recent studies in 5-year-old Colombian children also found that the significant reduction in kidney volume in children born preterm compared with children born at term was not evident when corrected for BSA. While a smaller kidney size may be a surrogate marker for a reduced nephron endowment (particularly in fetuses), it is important to recognize that an assessment of kidney size by ultrasound cannot distinguish between normal growth with age (e.g., layering of generations of nephrons in the renal cortex) and kidney hypertrophy in response to the body’s metabolic demands. In adults in particular, kidney size is not a reliable marker of nephron number. Findings from a meta-analysis of six studies in adults, in which both kidney weight and nephron number (estimated using the disector/fractionator technique) were analyzed, showed that only 5% of the variation in nephron number was explained by differences in kidney weight.

Rare genetic and congenital anomalies of the kidney and urinary tract (CAKUT) contribute to about 40% of all childhood ESKF (discussed in more detail in Chapter 45 , Chapter 72 ). ^, Full or partial deletion of more than 25 genes has been shown to result in kidney hypoplasia, and deletion of several of these genes results in low nephron endowment; not all have been studied in humans. ^{, ,} Of the mutant genes associated with monogenic forms of CAKUT, most encode transcription factors (e.g., PAX2 and GATA3 ) and growth factor receptors (e.g., RET ) expressed in ureteric bud/tree cells during branching morphogenesis or in intermediate mesoderm (e.g., SIX2, EYA1, and ROBO ) where they set the fate of renal progenitor cells and regulate interactions with the ureteric bud. ^,

There is some evidence that mild mutations of these same genes exert subtler effects on kidney mesenchyme/ureteric bud interactions and may be commonplace in the normal population ( Table 20.4 for clinical studies; experimental studies shown in eTable 20.1 ). PAX2 is highly expressed in the ureteric bud, where it suppresses apoptosis and optimizes the extent of branching. An intronic PAX2 polymorphism, which reduces PAX transcript levels from the mutant allele by only 50%, is found in 18.5% of Canadians and is associated with a subtle (10%) reduction in newborn kidney size. Ureteric branching morphogenesis is also highly dependent on GDNF signaling from the metanephric mesenchyme to the ureteric bud via the RET tyrosine kinase receptor on ureteric tip cells. A polymorphic variant of the GDNF receptor, RET(1476A), was associated with a 10% reduction in kidney volume at birth compared with the wild-type ( RET 1476G ) allele. In this study, newborn kidney volume was shown to be proportional to nephron number, suggesting that the modest kidney hypoplasia seen with the PAX2 and RET polymorphisms represents a reduction in nephron endowment. Newborn kidney size was reduced by 13% among Polish babies with a common variant of the BMPR1A gene, encoding a bone morphogenetic protein receptor on ureteric epithelial cells. Conversely, 22% of Canadian newborns inherit a variant of the ALHD1A2 gene (rs7169289) associated with increased production of all- trans retinoic acid metabolism in fetal tissues; newborns with the G allele were shown to have a 22% increase in newborn kidney size compared with the wild-type allele. One of the earliest transcription factors marking the nephron progenitor cells of intermediate mesoderm is OSR1; a variant of the human OSR1 gene that interferes with mRNA splicing was identified in about 6% of Caucasians. This OSR1rs12329305(T) variant was associated with a 12% reduction in newborn kidney size.

Taken together, these observations suggest that final nephron endowment may represent a complex polygenic trait determined by the additive effects of multiple genes regulating either the extent of ureteric branching or the renal progenitor cell pool during fetal life. The impact of genetic variation on nephron endowment alone, as well as in the context of developmental stresses and the risk of later-life hypertension and kidney disease, however, requires further study.

Developmental programming of nephron number has been the most rigorously studied link to later-life hypertension and kidney disease so far. Numerous animal models have demonstrated the association between in utero growth restriction (e.g., induced by gestational exposure to low-protein or low-calorie diets, uterine ischemia, dexamethasone, vitamin A deprivation, and alcohol) with subsequent hypertension. The link between adult hypertension and growth restriction in these animal models appears to be mediated, at least in part, by an associated congenital nephron deficit. ^{, ,} Corresponding blood pressures and nephron numbers in various programming models are outlined in eTable 20.1 . The association among birth weight, nephron numbers, and blood pressure varies between models, as discussed in detail later, underscoring the complexity of developmental programming and the need for better markers than birth weight.

Vehaskari and colleagues demonstrated an almost 30% lower glomerular number in offspring of pregnant rats fed a low-protein diet, along with a reduction in their birth weight, compared with offspring with a normal-protein diet during gestation ( Fig. 20.7A ). As shown in Fig. 20.7B , the low-protein offspring had tail-cuff systolic blood pressures that were around 40 mm Hg higher by 8 weeks of age, and these rats died prematurely. Similarly, prenatal administration of dexamethasone was associated with reduced offspring body weight and fewer glomeruli than controls. In these nephron-deficient rats, GFR and sodium excretion were reduced and albuminuria was increased. Uteroplacental insufficiency, induced by maternal uterine artery ligation late in gestation, also results in low nephron number and was associated with increased profibrotic renal gene expression with age, although hypertension developed only in males. ^, Conversely, adequate postnatal nutrition, by cross-fostering onto normal lactating females at birth, restored nephron number and prevented subsequent hypertension in growth-restricted male rats. Mice delivered preterm (i.e., 1–2 days early [normal mouse gestation, 21 days]) had lower nephron numbers, lower GFR, higher blood pressures, and more albuminuria than mice born at term. Nephron numbers were lower in mice delivered 2 days compared with 1 day early, suggesting the degree of preterm birth is important in determining final nephron endowment, even though nephrogenesis normally continues for about 5 days after birth in mice. Not surprisingly, the timing of the gestational insult has been found to be crucial in renal programming, with the greatest impact on nephron number occurring during periods of most active nephrogenesis.

Fetal programming of hypertension in growth-restricted rats (induced by maternal low-protein diet).

(A) Total number of glomeruli per kidney at 8 weeks of age and (B) systolic blood pressure throughout life, in male ( n = 7) and female ( n = 6) offspring from low-protein pregnancies and in male ( n = 7) and female ( n = 7) control rats. CTR, Control diet; LBW , low birth weight (growth restricted); LP , low-protein diet.

Adapted from Vehaskari VM, Aviles DH, Manning J, et al. Prenatal programming of adult hypertension in the rat. Kidney Int. 2001;59[1]:238–245.

The complexity of the effects of perturbations to the fetomaternal environment and genetic variants on kidney development has been highlighted in two studies. Sampogna and colleagues compared the effects of vitamin A deficiency, protein deficiency, and fibroblast growth factor 7 (FGF7) deletion on kidney development and nephron endowment in mice ( Fig. 20.8 ). Perturbations to kidney development, which varied according to condition, included developmental delay, defects in nephron induction, changes in the growth axis, and alterations in ureteric branching morphogenesis. These produced up to a threefold decrease in glomerular number and a twofold decrease in total branching events. Boubred and colleagues carefully compared the effects of two different perturbations to the fetomaternal environment on nephron number and adult renal physiology in rats. Maternal gestational low-protein diet and betamethasone both led to a similar level of fetal growth restriction, but nephron numbers were lower in betamethasone-exposed than low-protein diet offspring. Betamethasone-exposed offspring had impaired GFR, increased blood pressure, and glomerulosclerosis, whereas kidney function and structure were unaltered in low-protein diet offspring at 22 months of age. These findings suggest that the degree of reduction in nephron number is a risk factor for cardiovascular and renal disease.

Architecture of wild type (WT), fibroblast growth factor 7 (FGF7) null mutant, vitamin A–deficient, and protein-deficient mouse kidneys.

Embryonic day 15.5 ureteric tree tracings for each condition (to scale). N is the maximum branching generation number, and Glom represents the mean glomerular count (minimum of three kidneys studied per condition). In each case, glomerular count is reduced compared with the wild type. The number of branching generations is specific to each condition.

From Sampogna RV, Schneider L, Al-Awqati Q. Developmental programming of branching morphogenesis in the kidney. J Am Soc Nephrol. 2015;26[10]:2414–2422.

The definitive pathophysiologic impact of a reduction in nephron number on the development of kidney dysfunction is difficult to elucidate from the existing literature comprising varied experimental conditions. Compensatory mechanisms, such as glomerular hypertrophy and increased single-nephron GFR (SNGFR), may maintain whole-kidney GFR when nephron mass is low. Often there are findings of normalized total kidney filtration surface area through glomerular hypertrophy at the time of assessment (see Table 20.3 ), meaning that the impact of nephron number per se on kidney function cannot be confirmed or refuted. A secondary insult is often required for dysfunction to emerge. Overall, the available evidence suggests that the effect of a suboptimal fetomaternal environment on offspring nephron endowment and adult health may be influenced by species; the nature, timing, duration and severity of the perturbation; sex and age of the offspring; the postnatal nutritional environment (including lactation, infant nutrition, and subsequent growth); and superimposed kidney stresses.

The podocyte depletion hypothesis has emerged as a potentially unifying concept in glomerular pathology in recent years, with podocyte depletion defined as a loss of podocytes, decreased podocyte density due to glomerular hypertrophy, and/or a change in podocyte phenotype. In each situation, the ability of podocytes to fully envelop the glomerular basement membrane to form an effective filtration barrier is impaired, resulting in proteinuria and glomerulosclerosis. ^, In rats, a reduction of 20%, or more, in podocyte number is required to initiate these pathologic changes. ^, Podocyte loss has been associated with kidney disease, hypertension, diabetes, and aging, as well as preterm birth. ^, Offspring podocyte deficits have been observed to be programmed by maternal hypoxia and low-protein diet. ^, Humans and animals born with both a low nephron and low podocyte endowment could be expected to be at additional risk of developing CKD; however, whether an early life podocyte deficit increases susceptibility to glomerular injury is unclear. Experimental studies examining the impact of postnatal stressors on kidney function and podocyte structure have yielded conflicting results, with either the growth-restricted offspring or normally grown offspring found to be at greater risk. ^, Furthermore, podocyte number has the potential to increase in postnatal life, limiting the initial deficit.

In humans, nephrogenesis continues until around 36 weeks in normotrophic fetuses but may continue for several weeks postnatally in preterm infants, as discussed earlier. ^, In rodents, nephrogenesis continues for up to 10 days after birth but is most active mid to late gestation when studies show the most impact from manipulation of environmental factors. It is important to note that many differences in the timing of gestation, timing of nephrogenesis, gene expression profiles, patterning of kidney development, and structure of the kidneys exist between different species, which must be taken into account when interpreting the findings from experimental studies.

Many experimental models of programming, as outlined in eTable 20.1 , have been shown to result in a reduced nephron number in affected offspring, and these have been used to gain insight into potential mechanisms contributing to disease outcomes. Clinical associations with nephron number and kidney size in humans are also shown in Table 20.4 . Maternal and environmental factors that affect birth weight and gestation length in humans can directly affect nephrogenesis, as illustrated in eTable 20.2 , and some of these factors may compound the effects of LBW or preterm birth. ^, Given the wide variety of factors that can influence fetal/neonatal kidney development, occurring alone or in combination, however, the specific mechanisms underlying nephron deficits are not fully understood ( eTable 20.3 ). In addition, other more subtle structural changes within the affected kidneys may occur, which also have the potential to adversely impact later-life kidney function and disease susceptibility, even in the setting of a normal nephron endowment.

Principles of evolutionary theory and bioenergetics may explain the process by which a fetal nephron deficit can occur in the situation of a suboptimal fetomaternal environment, as detailed in recent papers by Chevalier. ^, From an evolutionary biology perspective, the processes of structural and functional development of an organism prioritize what is required for survival into the years of reproductive fitness. When the growth of a developing fetus is constrained by the poor availability of necessary energy and substrates, adaptations occur in resource allocation, which favor vital organs to the detriment of other body systems. With respect to the kidneys, this potentially means that only the minimum number of nephrons necessary for adequate renal filtration to support body growth, maintenance, and reproduction into early adulthood may be formed; nephron hypertrophy compensates for the nephron deficit where needed, and CKD may follow especially in the setting of further stresses. ^, The adjustment of nephron endowment to match available energy during development appears to be a highly conserved adaptive evolutionary trait, given that it is evident across disparate branches of the animal kingdom (e.g., birds, rodents, and primates) (see eTable 20.1 ).

The process of resource (energy) allocation is certainly evident in IUGR pregnancies, where fetal hypoxia triggers peripheral vasoconstriction resulting in a redistribution of blood flow toward the fetal brain, heart, and adrenal glands, with a consequent reduction in blood flow to the kidneys. ^, IUGR pregnancies are also associated with changes in fetoplacental endocrine and metabolic function, placental structure and growth, reduced protein accretion, and slowed somatic growth of the fetus. ^,

The reduction in nephron formation within the developing kidneys of affected fetuses likely results from alterations in cellular metabolism. Metabolic plasticity of nephron progenitor cells enables them to coordinate the high metabolic demand of balancing cell survival and self-renewal (to maintain the mesenchymal population) and differentiation (aggregation and epithelialization) during nephrogenesis, as well as to support tissue homeostasis in a dynamic fetomaternal environment. Activity of the 5′ adenosine monophosphate-activated protein kinase (AMPK), mechanistic target of rapamycin (mTOR), and hypoxia-inducible factor (HIF)-related metabolic pathways all directly respond to changes in substrate availability (such as variations in oxygen, glucose, fat, and amino acid levels, and enzyme cofactors). ^, A stressed environment can induce metabolic reprogramming, whereby the balance between the modes of energy production, oxidative phosphorylation (promotes cell differentiation and maturation), and glycolysis (promotes cell proliferation), is disturbed. ^, Experimental and cell culture studies suggest that prolonged glycolytic metabolism induced in nephron progenitor cells is associated with significant impairments in nephron formation, whereas inhibiting glycolysis increases the number of induced nephrons (without affecting ureteric branching). ^, Fetal baboons, exposed to a global nutrient-restricted maternal diet, exhibited a significant downregulation in genes involved in mitochondrial oxidative phosphorylation pathways. ^, In a mouse model, activating the mTOR complex 1 (mTORC1) pathway was shown to partially abrogate the adverse effect of a maternal calorie-restricted diet on offspring nephrogenesis. Although cellular metabolism–related mechanisms of nephron deficit have not yet been explored in detail, it is expected that there is a multitude of circumstances (with or without evident SGA, LBW, or IUGR) under which fetal bioenergetics may be affected, including maternal nutrient deficiencies, maternal/placental health conditions, maternal smoking/hypoxia, and postnatal hyperoxia and nutrient restrictions in preterm infants.

Elevated maternal glucocorticoids due to stress or glucocorticoid therapy may impact fetal growth and kidney development (see eTable 20.3 ). Under normal circumstances, the fetus is protected, at least in part, from exposure to excess endogenous corticosteroids by the placental enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), which metabolizes corticosterone to the inert 11-dehydrocorticosterone. Rats and humans with mutations in the 11β-HSD2 gene have low levels of placental 11β-HSD2 and give birth to LBW offspring in whom hypertension develops prematurely. ^, The administration of high concentrations of cortisol (in sheep) or corticosterone (in rats and mice) may also overwhelm this enzyme and affect the fetus, as evidenced by offspring with low nephron number, and later hypertension and impaired kidney function. These outcomes occurred even after short-term exposure that did not affect birth weight. ^, Prenatal administration of dexamethasone, a steroid not metabolized by 11β-HSD2, led to fetal growth restriction, a 20% to 60% reduction in nephron number, glomerulomegaly, and subsequent hypertension in rats and sheep. ^{, , ,} Metanephric organ culture experiments have demonstrated that dexamethasone can cause a dose-dependent inhibition of branching morphogenesis, in part through regulation of GDNF gene expression. Excessive fetal steroid exposure may drive inappropriate gene expression and affect growth and nephrogenesis, potentially through more rapid maturation of tissues.

Studies also implicate glucocorticoids as modulators of nephrogenesis in the setting of maternal low-protein diet, with increased nephron number in offspring of dams treated with an inhibitor of steroid synthesis. ^{, ,} Expression of steroid-responsive receptors including renal Na/K-ATPase α ₁ – and β ₁ -subunits were also found to be significantly increased in offspring of rats fed a low-protein diet during gestation.

In humans, antenatal glucocorticoid administration for women at risk of preterm delivery has markedly improved neonatal survival and outcomes, likely through mechanisms that accelerate infant lung maturation. Findings from sheep and baboon models suggest that this also increases the structural and functional maturation of the kidneys. ^, Follow-up at age 30 years, of people whose mothers participated in a randomized placebo-controlled study of antenatal betamethasone, did not find any effect on blood pressure or other cardiovascular indicators. No difference in blood pressure or kidney function was noted among preterm infants who had received antenatal steroids within the first 2 years of life compared with those who did not. ^, A Cochrane review found that there are, to date, insufficient follow-up data to determine unequivocally whether or not there are any long-term adverse effects of antenatal glucocorticoid treatment, but the substantial benefits for neonates support its use. However, a study of 23,363 singletons in Taiwan followed for 10 years, in which CKD was identified using ICD-9/10/CM codes (including CAKUT), found that maternal steroid use during pregnancy was associated with increased risk of childhood CKD (HR 1.69; 95% CI 1.01–2.84). The association was stronger in children who were born preterm, male, had exposure during the second trimester, and were exposed to a cumulative dose equivalent above 24 mg hydrocortisone. Longitudinal examination of the consequences of sustained glucocorticoid therapy (e.g., throughout pregnancy) has not been reported.

In addition to maternal iron deficiency (resulting in anemia) and placental insufficiency discussed earlier, a range of conditions, including pulmonary and cardiovascular disease, living at high altitude, maternal smoking, preeclampsia, and sleep apnea, can result in fetal hypoxia (see eTable 20.3 ). These conditions have been associated with LBW, SGA, IUGR, and/or preterm births in humans.

Offspring of pregnant sheep maintained in a high-altitude environment from 30 to 140 days’ gestation exhibited significantly reduced relative kidney weight, tubule injury, and increased serum creatinine levels. In mice, a short-term severe hypoxic insult (5.5%–7.5% maternal oxygen) from embryonic day (E)9.5 to 10.5 caused a CAKUT phenotype, whereas mild hypoxia in midpregnancy (12% from E12.5–14.5) resulted in low nephron endowment, mediated in part through suppression of β-catenin signaling. A similar mild hypoxia in late gestation resulted in fetal growth restriction, altered renal tubular development, low nephron number in male but not female offspring, and hypertension in both sexes. ^, Importantly, in the first reported evidence of podocyte programming, podocyte number per glomerulus was found to be significantly reduced by 15% in the male hypoxia-exposed mice offspring (but not in the females).

Maternal smoking is a major modifiable risk factor for IUGR and preterm births and has been associated with infant kidney malformations, as well as altered fetal kidney volume in a dose- and time-dependent manner. In a mouse model of cigarette smoke exposure, male offspring had fewer nephrons per area of tissue assessed and increased urine albumin/creatinine ratio. These adverse effects on kidney development relate to the fetoplacental response to reduced oxygen availability and reprogramming of cellular metabolism, altered activity of the HIF signaling pathway, and the associated rise in oxidative stress. ^, The origin of distinct sex differences requires further investigation. Oxidative stress is linked to tissue injury and inflammation, DNA damage, cell cycle arrest, accelerated cellular senescence, and telomere shortening. It occurs due to suboptimal intrauterine (hypoxic) environments but may also affect neonates with adverse postnatal exposures.

Hyperoxia can occur in the situation of preterm birth, whereby infants are exposed to a much higher environmental oxygen environment after birth compared with in utero and, in addition, may receive supplemental oxygen therapy (21%–100% O ₂ ). This is known to impair neonatal vascular growth via alterations in HIF signaling, VEGF expression, and increased oxidative stress (with low levels of endogenous antioxidants) and contributes to conditions such as bronchopulmonary dysplasia and retinopathy of prematurity. ^, Circulating endothelial progenitor cells from hyperoxia-exposed preterm infants exhibit impaired VEGF-nitric oxide signaling and reduced proliferative growth. In the developing kidneys, a study of rat pups exposed to 80% O ₂ in the last days of ongoing postnatal nephrogenesis exhibited reduced nephrogenic zone width and smaller glomerular diameter ; these changes were completely abrogated through treatment with a HIF-1α stabilizing drug. Nephron number was not affected in pups, but in adulthood, there was a significant reduction in nephron number and evidence of high blood pressure, kidney injury (in males), and reduced creatinine clearance in hyperoxia-exposed rats. ^, In a similar model, where hyperoxia exposure was commenced 2 days earlier, there was proximal tubule dilation and injury, increased apoptosis, a thinning of the nephrogenic zone, and an associated downregulation of HIF, catalase, and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling. In preterm and term lambs (both with completed nephrogenesis) mechanically ventilated with supplemental oxygen after birth, there were significant reductions in glomerular capillary length, surface area, and total renal filtration surface area compared with unventilated term controls. It is not known whether capillary growth may be affected in other regions of the kidneys or what impact this may have in the long term.

Several medications commonly used during pregnancy or in the early postnatal period have been studied for their effects on birth weight and nephrogenesis (see eTables 20.1 and 20.3 ). Among 397 pregnant women, antibiotic use was found to be associated with 138 g lower infant birth weight and altered methylation of some growth-related imprinted genes. Of concern, in some regions antibiotic exposure may occur unavoidably through maternal food and drinking water intake, and ciprofloxacin, norfloxacin, and sulfamethoxazole exposures in particular have been associated with decreased fetal growth and/or preterm birth.

The aminoglycoside antibiotic gentamicin administered to pregnant rats results in a permanent nephron deficit in offspring. Significantly lower numbers of ureteric branch points and nephrons were observed in metanephric explants cultured in the presence of gentamicin. ^, In contrast, other investigators did not find a reduction in nephron number in rat pups administered gentamicin intraperitoneally in the latter stages of nephrogenesis (birth to 14 days of age). Administration of the β-lactam antibiotic ampicillin to pregnant rats leads to an 11% reduction in offspring nephron endowment, as well as evidence of focal cystic tubule dilatation and interstitial inflammation. Administration of ceftriaxone in vivo did not result in a nephron deficit, but histologically, there was evidence of renal interstitial inflammation. The penicillins were also found to inhibit nephrogenesis in cultured metanephroi in a dose-dependent fashion, an effect that was less evident with ceftriaxone. Importantly, nephrogenesis was affected even at therapeutic doses of penicillins in the rat. The mechanism whereby the β-lactams reduce nephron endowment is likely through an increase in apoptosis in the induced mesenchyme. ^, Further research on antibiotics that are frequently used in human pregnancy and preterm infants is warranted, especially given that treatment of bacteriuria during pregnancy reduces the risk of small and vulnerable neonate births (see Fig. 20.9 ).

The immunosuppressive medication cyclosporine is a known nephrotoxin in humans that crosses the placenta. Women treated with cyclosporine may have successful pregnancies, although infants tend to have birth weights in the low range, and its effect on the fetal kidneys is not well described. In a rabbit model, cyclosporine administration in the later, but not the earlier, period of gestation resulted in smaller litters and growth-restricted pups. All pups exposed to cyclosporine in utero had a 25% to 33% lower nephron number than controls. ^, There were also findings of glomerulomegaly, glomerulosclerosis, hypertension, proteinuria, and decreased GFR. Organ culture studies suggest that the presence of cyclosporine suppresses Wnt signaling pathways and cellular proliferation in the nephrogenic zone and inhibits the conversion of metanephric mesenchyme to epithelium, as well as increasing apoptosis. ^,

Nonsteroidal antiinflammatory drugs (NSAIDs) are sometimes used postnatally in children born preterm. Postnatal administration of a cyclooxygenase 2 inhibitor, but not a cyclooxygenase 1 inhibitor, in rat and mice pups resulted in reduced cortical volume, impairment of nephrogenesis, and reduced glomerular diameter. Administration of indomethacin or ibuprofen postnatally did not affect nephron number in rats. In the preterm baboon kidney, early postnatal administration of five doses of ibuprofen (consistent with recommended dosing in preterm infants with patent ductus arteriosus) was associated with a reduction in width of the nephrogenic zone, suggesting premature termination of nephrogenesis. The impact of these medications on human nephrogenesis is not known.

Regarding maternal alcohol consumption, kidney malformations have been reported in children with fetal alcohol syndrome. In fetal sheep, repeated ethanol exposure during the second half of pregnancy resulted in an 11% reduction in nephron number and impaired vascular function in late pregnancy. ^, In rats, 2 days of exposure to a high dose of alcohol in mid–late pregnancy resulted in a low nephron number and offspring with hypertension and sex-dependent impairments in GFR. Using metanephric organ culture, alcohol was found to cause dose-dependent decreases in branching morphogenesis, which were prevented by retinoic acid supplementation. Among Australian individuals at age 30 years whose mothers had consumed alcohol during pregnancy, investigators reported a 7% prevalence of CKD. The risk of CKD was doubled among those with exposure to moderate-heavy drinking in late pregnancy and was greater among females.

The impact of nicotine exposure on kidney development has been analyzed independently of the other toxins and hypoxia associated with maternal smoking. Nicotine, which crosses the placenta and is secreted in breast milk, was administered during pregnancy (beginning day 4 or day 7) and throughout lactation in rat dams. With the earlier commencement of nicotine exposure, pups had a significantly reduction in body weight and kidney weight at postnatal day 14; the later-onset exposure, however, did not affect kidney or body weights at postnatal day 7. The nicotine-exposed offspring exhibited decreased glomerular diameter, tubular injury, and increased collagen deposition, with a significant reduction in angiotensin II receptor type 2 (AT2R) expression and an upregulation in connective tissue growth factor (CTGF); nephron number has not been assessed. ^, Exposure to marijuana during pregnancy has also been associated with greater risks of SGA, LBW, and preterm birth, but the impact on fetal kidney development is not known.

The developmental programming research field is also alert to the potential impact of environmental toxins, with more than 60 different chemicals assessed to date. Few studies, however, have examined kidney outcomes (see eTable 20.1 ). High concentrations of bisphenol A (BPA), a common endocrine-disrupting chemical used in the manufacture of plastics and epoxy resins, have been associated with hypertension and albuminuria in adults. ^, In pregnant women, BPA is associated with a significant reduction in fetal growth rate. Experimental studies of BPA administration to pregnant mice showed evidence of disrupted glucose metabolism, oxidative stress, and decreased nephron number in female offspring. Similarly, another endocrine-disrupting class of chemicals, phthalates (commonly found in shampoo and cosmetics) have been shown to significantly reduce birth weight and nephron number in male and female offspring of exposed rat dams, with no effect on kidney weight; these findings were associated with a dose-dependent reduction in renin, Wnt11 and GDNF expression in the kidneys. Many other potentially harmful chemicals have been identified, in air pollution, soil, food, and items in everyday use; the specific effects these may have on kidney development and their mechanisms of action, however, are not fully understood.

Pregnant women are highly susceptible to the effects of climate change given that they are less able to thermoregulate, are more sensitive to dehydration, and are at greater risk of infections. ^, Climate change is leading to more frequent and more widespread exposure of pregnant women to extreme weather events including drought, floods, and hurricanes, as well as heat and wildfire smoke. Climate change is also leading to an increase in vector-borne and water-borne infections. These varied exposures have been associated with increased risks of LBW and preterm births, preeclampsia, and gestational diabetes, which in turn may impact fetal kidney development and long-term risks for hypertension and CKD ( Fig. 20.11 ). In children, exposure to air pollution has been associated with higher blood pressure and with variable effects on eGFR.

Schematic illustrating the potential far-reaching impact of climate change on maternal health and kidney development during pregnancy and superimposed kidney injury (second hits) in later life.

DM, Diabetes mellitus; LBW, low birth weight; RAAS, renin–angiotensin–aldosterone system; SGA, small for gestational age; SNS, sympathetic nervous system.

From Alvarez-Elias AC, Brenner BM, Luyckx VA, et al. Climate change and its influence in nephron mass. Curr Opin Nephrol Hypertens. 2024;33[1]:102–109.

Many developmental abnormalities of the urinary tract are associated with impaired nephrogenesis, which, in turn, is compounded by obstructive injury. ^, From animal studies, it has become clear that perinatal urinary obstruction may lead to reduced nephron numbers, which may exacerbate the impact of other programming factors. ^, In a rhesus monkey model, histologic changes evident in the obstructed kidney include cystic enlargement of the tubules and Bowman spaces, abnormalities in glomerular tuft development, podocyte apoptosis, and a significant reduction in glomerular density; the dysplasia worsened when the obstruction was initiated earlier in gestation. In rats, unilateral ureteral obstruction on postnatal day 1 and relieved on day 5, or on postnatal day 14 and relieved on day 19, reduced nephron number by 50% in both groups. Additionally, renin expression was decreased and glomerulotubular maturation was delayed. Importantly, temporary neonatal urinary obstruction was also associated with histologic scarring and loss of function of the contralateral kidney in rats. Developing and neonatal kidneys therefore appear to be highly susceptible to obstructive injury, suggesting that early relief of urinary tract obstruction may be important to preserve nephron number.

The molecular regulation of kidney development, particularly in the mouse, has been comprehensively described elsewhere. In animal models of developmental programming, changes in expression levels of genes regulating branching morphogenesis, apoptosis, and kidney growth have been described (for a review, see Wang and Garrett ), with some studies determining the effects of maternal perturbation on the whole genome. ^{, ,}

Affected molecular pathways may be common across different fetomaternal environments or, in other cases, may be specific to individual insults, as described earlier (see eTable 20.3 ). GDNF signaling through its receptor-tyrosine kinase RET is a key ligand–receptor interaction driving ureteric budding and branching, and this has been shown to be disrupted in models of maternal vitamin A deficiency (see Fig. 20.8 ), ^, as well as maternal glucocorticoid, phthalate, and alcohol exposure. Ureteric branching is also impaired in maternal diabetes models, which is associated with changes in TGF-β1 signaling due to increased expression of a hedgehog interacting protein. ^, Suppressed Wnt/β‐catenin signaling has been observed in the kidneys of offspring exposed to fetal hypoxia ^, and phthalates, as well as in vitro exposure to cyclosporin. ^,

Increased apoptosis in the developing kidneys is a common finding in programming studies and has been observed in experimental models of maternal low-protein diet, diabetes, ^, iron deficiency, antibiotic and cyclosporin exposure, ^, ureteral obstruction, and postnatal hyperoxia. Several studies have suggested that altered regulation of apoptosis in the developing kidneys may be due to downregulation of antiapoptotic factors (e.g., Pax-2 or Bcl-2) and/or upregulation of proapoptotic factors in response to environmental or other stimuli (e.g., Bax, p53, Fas receptor, caspase 3 and 9). ^, Inappropriate apoptosis may occur in offspring affected by an adverse fetomaternal environment in successive waves throughout nephrogenesis, even after birth. ^{, , ,}

More than 40 microRNAs, involved in posttranscriptional control of mRNA expression, were shown to be deregulated in the kidneys of rat fetuses exposed to maternal low-protein diet, influencing multiple developmental pathways including cellular proliferation, differentiation, and apoptosis. Epigenetics is thought to play an important role in the programming of health and disease across generations, and there is emerging evidence of its influence on kidney development and disease. ^,

A meta-analysis demonstrated that adult human kidneys show sex-specific expression of more than 200 genes. In utero, placentation processes and fetal growth patterns are affected by sex, ^, and sex differences in fetal glomerular growth through mid to late gestation have also been observed. Importantly, fetal sex-specific placental responses to maternal perturbations also occur (reviewed in Kalisch-Smith and colleagues ). The placentas of female fetuses are known to have higher levels of the 11β-HSD2 enzyme compared with males; therefore males are likely to be exposed to more glucocorticoids (endogenous and administered). ^, As described earlier, sexually dimorphic outcomes in kidney structure have been observed in offspring of experimental models of maternal hypoxia ^, and BPA exposure. ^{, ,} Epigenetic reprogramming, such as the differential DNA methylation of genes such as IGF2, has been proposed as a modulator of these sex differences. ^,

In some clinical studies, programming effects appear more pronounced in males, and in others, the differential effects of gender are modified by age, ethnicity, and BMI. In female rats with similar programmed reductions in nephron numbers, blood pressures are often not as high or increase much later than in males. ^, Often a secondary challenge such as pregnancy, however, will “unmask” a disease phenotype. Sex hormones may in part explain these differences, as well as sexual dimorphism in the relative expression of components of the RAAS, potentially differentially altering the balance between vasoconstriction, vasodilation, and sodium handling. ^, Sex differences in the programming of blood pressure, kidney function, kidney disease, and their outcomes are reviewed in detail elsewhere.

The relationship between renal sodium handling, intravascular volume homeostasis, and hypertension is well accepted. ^, That factors intrinsic to the kidney itself affect blood pressure has been demonstrated clinically in kidney transplantation, in which the blood pressure of the recipient after transplantation has been shown to be related to the blood pressure or hypertension risk factors of the donor: that is, hypertension “follows” the kidney. Nephron endowment is likely an independent factor determining susceptibility to essential hypertension and subsequent renal injury. However, low nephron number alone does not account for all observed programmed hypertension (see eTable 20.1 ). ^{, ,} Findings suggest that additional factors also contribute to the developmental programming of hypertension and kidney function, as outlined in Table 20.5 , such as alterations in tubular sodium handling and vascular function. ^,

Sodium Handling

Both birth weight and kidney size have been shown to be inversely associated with salt sensitivity in children and adults ( eFig. 20.1 ). ^, In each of these studies, salt-induced changes were independent of GFR, excluding confounding by kidney function. In a study of 1512 adults (mean age 62 years), only those with a birth weight <3.05 kg exhibited a progressive 2.48 mm Hg rise in systolic blood pressure per 1 g increase in daily salt intake. Those with a higher birth weight did not show an association between blood pressure and dietary salt, potentially suggesting protection against salt sensitivity. This, along with the findings of other smaller studies, suggests LBW (and likely low nephron number) are associated with increased salt sensitivity. Consistent with this, salt sensitivity has been reported in several animal models of low nephron number including maternal uterine artery ligation and gestational diabetes, ^, although timing of dietary intervention and age at study appear to play a role.

Correlation between birth weight and salt sensitivity in 27 normotensive adults.

Salt sensitivity of blood pressure was defined as the difference in mean arterial pressure on high-salt diet (200 mmol/day) compared with low-salt diet (60 mmol/day). R = −0.06; P =.002.

From de Boer et al. Hypertension. 2008;51[4]:928–932.

The structural development of the kidney tubules likely plays an important role in the programming of sodium handling, but little research has been done in this area. In prehypertensive offspring of rats fed a protein-restricted diet during gestation, expression of the Na-K-2Cl (NKCC2) and Na-Cl (NCC) transporters was significantly increased compared with controls. Furosemide administration reduced blood pressure in offspring of protein-restricted rats, supporting increased NKCC2 activity as a mediator of hypertension in the model. Maternal high-fructose diet also increased kidney expression of NKCC2, Na-H exchanger 3 (NHE3), Na-Cl cotransporter (NCC), and epithelial Na channels (ENaC) in rat offspring. Other programming models have observed significant increases in tubular Na-K-ATPase expression. ^{, , ,} Despite differences among models, the data suggest increased sodium transport in all segments of the renal tubule. Whether glomerulotubular balance in the setting of a reduced nephron number indirectly increases sodium transport or sodium transporter activity is independently programmed has not yet been elucidated.

Two meta-analyses and systematic reviews published in 2012 showed consistent associations among lower birth weight, preterm birth, and higher blood pressures in later life ( Table 20.7 ). ^, A meta-analysis of 27 studies found that systolic blood pressures were 2.28 mm Hg (95% CI 1.24–3.33 mm Hg) higher in people born with LBW, compared with those born >2.5 kg ( eFig. 20.2A ). Many studies do not discriminate between LBW occurring as a result of SGA/IUGR or as a result of preterm birth with an appropriate (low) weight for gestational age (AGA). Therefore their relative impact is not always clear. To investigate this, a study of 50-year-old adults born at term reported an odds ratio (OR) of 1.9 (95% CI: 1.1–3.3) for hypertension among those who were SGA compared with AGA. Growth restriction before birth per se therefore was associated with subsequent higher blood pressure. Regarding the risk among those born preterm, a systematic review of 10 studies found that people born preterm, with a mean gestational age of 30.2 weeks and a mean birth weight of 1280 g, had 2.5 mm Hg higher (95% CI: 1.7–3.3 mm Hg) systolic blood pressures in later life compared with those born at term ( eFig. 20.2B ). Similar findings have been reported in a subsequent meta-analysis. Preterm birth therefore is also independently associated with higher blood pressure, which in some studies is manifest in neonates and already meets the definition of hypertension by 1 to 2 years of age. ^,

Relationship among birth weight, preterm birth, and blood pressure.

(A) Meta-analysis of odds for hypertension (HTN) in individuals with birth weights below 2500 g (low birth weight , LBW ) compared with birth weights above 2500 g. The pooled odds ratio is shown as a diamond . (B) Meta-analysis of difference in systolic blood pressure (SBP) between individuals born preterm or very low birth weight (VLBW) compared with full term. Pooled SBP difference is indicated by the diamond and dashed vertical line .

A from Mu et al. Arch Cardiovasc Dis. 2012;105:99–113; see original paper for full references. B from de Jong F et al. Hypertension. 2012;59:226–234.

One systematic review and meta-analysis evaluated blood pressures in individuals aged 2 to 40 years who had been born preterm and either SGA or AGA. Among 25 studies, blood pressures were not different between the two groups, suggesting that preterm birth per se may be the overriding risk factor in these individuals. Additional factors, such as maternal preeclampsia, female sex, and current BMI, may impact risk. ^, In contrast, however, a study from Brazil followed 3585 individuals from birth to 22 years of age and found lower blood pressures among females who had been born preterm. The authors suggest that in lower-resource settings, individuals born preterm may remain small, protecting against blood pressure elevations. More data are required from diverse lower-resource settings.

Aging also has an important role in hypertension risk. Blood pressures of people born with LBW and normal birth weight may both still be within the normal range in childhood, but differences become amplified with age; adults who had been born with LBW often develop overt hypertension, with risk increasing with age. ^, Most studies have been conducted in Caucasian populations, but consistent data are also accumulating in other regions. Results of a systematic review of African studies, however, suggest that additional factors may contribute to the greater severity of blood pressure in those of African origin. ^, An important effect modifier of the association between LBW or preterm birth and blood pressure, noted in diverse populations, is BMI. Current BMI may override an effect of birth weight, especially in children at different stages of growth. ^, Furthermore, blood pressures are often highest in those who “catch up” fastest in postnatal weight (i.e., rapid upward crossing of weight centiles), highlighting the importance of early postnatal nutrition in developmental programming. ^{, ,} Among 513,802 Israeli adolescents eligible for military service, blood pressures were higher at age 17 years in those born with ELBW or VLBW, with the highest risk observed among those who developed subsequent overweight or obesity.

Associations between increased blood pressure and other markers of potential developmental stresses have also been reported, such as HBW in children (but not adults), ^{, ,} diabetic pregnancy in children and young adults (especially males), ^, and maternal smoking in children ( Table 20.7 ). Another potential risk factor is maternal gestational hypertension or preeclampsia. ^, In a systematic review of 18 studies among children and young adults, systolic blood pressures were 2.39 mm Hg (95% CI: 1.74–3.05) higher in those exposed to preeclampsia compared with those unexposed (see Table 20.7 ). Whether the effect is mediated by the often-accompanying IUGR or preterm birth and/or may be associated with humoral changes with preeclampsia requires further investigation. In a U.S. birth cohort, the association of offspring blood pressure with maternal preeclampsia was attenuated with higher cord-blood vitamin D levels, which may suggest a role for vitamin D in modulation of long-term cardiovascular risk. In a study of young adults born preterm, soluble endoglin and soluble fms-like tyrosine kinase-1 (sflt-1) levels were significantly elevated compared with controls born at term and proportional to systolic blood pressures, suggesting a programming effect of preterm birth on angiogenesis and blood pressure (see further discussion on preeclampsia in Chapter 58 ). Sflt-1 levels were even further elevated in those who had been exposed to a hypertensive pregnancy, suggesting an additional impact of preeclampsia. Interestingly, elevated blood pressures in young adults born preterm have also been correlated with lower urinary levels of the protein α-Klotho, which has antiaging and organ protective effects, along with decreased urine angiotensin-(1-7).

Clinical studies showing an association between nephron number and hypertension are shown in Table 20.4 . A study of Caucasians aged 35 to 59 years who died in accidents found that in 10 people with a history of essential hypertension, the number of glomeruli per kidney was significantly lower and glomerular volume significantly higher than in 10 matched normotensive controls ( Fig. 20.12 ). Birth weights were not reported in this study, but the authors concluded that a reduced nephron number is associated with susceptibility to essential hypertension. Kidneys from Indigenous Australian people with a history of hypertension contained approximately 30% fewer nephrons than those with no history of hypertension. Although the sample size was small and birth weights were not available, this is a population with high rates of socioeconomic disadvantage and LBW. Kanzaki and colleagues also reported that kidneys from Japanese men with a history of hypertension contained approximately 40% fewer nephrons than age-matched normotensive men. Similarly, among living donors in the United States, those with hypertension had significantly fewer nephrons per kidney than those without hypertension (determined using a biopsy/imaging technique). These studies attempted to exclude loss of nephrons due to hypertension as a potential confounder of the association, in the absence of birth weight data. The data therefore seem consistent across several populations that higher blood pressures are associated with lower nephron numbers.

(A) Nephron number, and (B) glomerular volume in Caucasian people with primary hypertension compared with normotensive controls.

From Keller G, Zimmer G, Mall G, et al. Nephron number in patients with primary hypertension. N Engl J Med. [2003] 348:101–108.

Hughson and coworkers reported significant correlations between birth weight, mean arterial pressure, and glomerular number, as well as between mean arterial pressure and birth weight, among Caucasian (but not African American) people. Among African Americans having nephron numbers below the mean, however, twice as many were hypertensive as normotensive, suggesting a possible contribution of lower nephron number in this group as well. The relationship between LBW and nephron number was also similar in a cohort of Black and White Cuban neonates ( Fig. 20.5B ). Glomerular volumes were found to be higher among hypertensive African American people than hypertensive White Americans. The consistent finding of larger glomeruli among African Americans may suggest a greater prevalence of low nephron number in this population as a result of the known higher prevalence of LBW or may reflect independent or additional programming of glomerular size. This topic warrants further research.

The pressure-natriuresis curve is shifted to the right in most forms of hypertension. A low total renal filtration surface area associated with a low nephron number is one plausible hypothesis to explain the associated higher blood pressures. Consistent with an association between nephron number and blood pressure, the salt sensitivity of blood pressure has been found to correlate inversely with birth weight and kidney size in adults and children ( eFig. 20.1 ). ^, Similarly, one study found that blood pressure–associated sodium excretion was reduced among preterm compared with term individuals, possibly reflecting a reduced excretory surface area and/or programming of tubular transporters.

As with blood pressure, programmed changes in kidney function that occur, at least in the early stages, may not be outside of normal limits. With time or exposure to additional insults, however, these changes may manifest as kidney disease (see Table 20.7 ).

Proteinuria

One of the earliest signs of hyperfiltration, which would be expected in the setting of a reduced nephron number and filtration surface area, is microalbuminuria, which may progress to overt proteinuria with ongoing renal injury and worsening hyperfiltration. Indigenous Australians who had LBW evidenced an OR for macroalbuminuria of 2.8 (95% CI: 1.26–6.31) compared with those who had normal birth weights; risk increased with age. Importantly, proteinuria was also associated with a higher rate of cardiovascular and renal deaths in this population, underscoring its clinical relevance. A meta-analysis including eight additional studies reported an OR of 1.81 (95% CI: 1.19–2.77) for albuminuria with LBW (see Table 20.7 ). Hoy and colleagues ^, subsequently reported that LBW, childhood poststreptococcal glomerulonephritis, and current body mass were all independent predictors of albumin to creatinine levels in Indigenous Australian adults. These findings are compatible with the “multihit” model of CKD, of which nephron endowment at birth may be the first “hit” increasing susceptibility to kidney disease throughout the life course ( Fig. 20.11 ).

Preterm birth has also been associated with albuminuria, with a meta-analysis of a small number of studies in children showing increased urine albumin/creatine levels in those born preterm compared with term controls (SMD 0.25, 95% CI 0.07–0.43). Similarly, a subsequent study reported a 36% prevalence of albuminuria in extremely low-gestational age neonates evaluated at 2 years corrected gestational age. Besides albumin, there is additionally some evidence of elevated low-molecular-weight protein excretion (a marker of proximal tubule dysfunction) in infants born preterm, which can also persist into later life. ^, Among children aged 4 years who had been born preterm, albuminuria was higher in both boys and girls who had reached normal height (presumably caught up in growth), and among 19-year-olds who had been born very preterm, albuminuria was higher among those who were SGA, highlighting the interplay among preterm birth, growth restriction, and catch-up growth on later risk of disease. ^, Similarly, among 14-year-old children, current overweight and obesity further increased the association between preterm birth and albuminuria. Not all studies have shown these associations, however. For example, in NHANES, despite there being significant differences in blood pressure and eGFR among participants, there was no difference in the urine albumin/creatinine ratio between adolescents born with LBW, VLBW, or normal birth weight.

An analysis of 724 people, aged 48 to 53 years, who had been exposed to malnutrition in midgestation during the Dutch famine (1944–1945) revealed an increased prevalence of microalbuminuria (12%) when compared with those exposed during early gestation (9%), late gestation (7%), or not exposed to famine (4%–8%) (see Table 20.7 ). Size at birth was not associated with microalbuminuria, emphasizing the need for surrogate markers in addition to birth weight to better identify individuals at risk for renal programming. In the Aguascalientes region of Mexico, an area with one of the highest global rates of early-onset CKD of unknown cause, persistent proteinuria in adolescents is associated with decreased kidney volume, suggestive of a nephron deficit ^, ; any other potential perinatal associations are as yet unknown.

A U-shaped association between birth weight and proteinuria was described among Pima people, with increased risk evident for birth weights below 2.5 kg and above 4.5 kg. The strongest predictor of proteinuria among HBW individuals in this study was exposure to gestational diabetes. In a Canadian study, urine albumin/creatinine ratios were lower in infants of mothers with diabetes compared with nondiabetic mothers at 1 year of age but were higher at 3 years, although independent of birth weight. The effects of gestational diabetes and HBW on renal programming require much more study.

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