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Prenatal programming of nephron number and blood pressure

2007; Elsevier BV; Volume: 72; Issue: 3 Linguagem: Inglês

10.1038/sj.ki.5002307

1523-1755

Michiel F. Schreuder, J. Nauta,

Renal and related cancers

A low nephron number has been advocated to explain the association between intrauterine growth restriction (IUGR) and hypertension in later life. IUGR not only leads to a low birth weight but is also hypothesized to reprogram nephrogenesis, which results in a low nephron endowment. Several methods are used to estimate the total glomerular number, but only stereological techniques result in accurate (unbiased) and precise (reproducible) data. Several studies, both in humans and animal models that have used these methods indeed revealed that IUGR leads to a low nephron number. According to the hyperfiltration hypothesis, this reduction in renal mass is supposed to lead to glomerular hyperfiltration and hypertension in remnant nephrons with subsequent glomerular injury with proteinuria, systemic hypertension and glomerulosclerosis. Even though IUGR is associated with both a low nephron endowment and an increased risk of hypertension, only circumstantial evidence is available to support the hyperfiltration hypothesis after prenatal programming. A prerequisite for establishment of this association in long-term, prospective follow-up studies is the ability to estimate glomerular numbers in living human beings, for which a further advancement in radiological techniques is necessary. Only then can the association between nephron endowment and blood pressure in humans be studied more conclusively. A low nephron number has been advocated to explain the association between intrauterine growth restriction (IUGR) and hypertension in later life. IUGR not only leads to a low birth weight but is also hypothesized to reprogram nephrogenesis, which results in a low nephron endowment. Several methods are used to estimate the total glomerular number, but only stereological techniques result in accurate (unbiased) and precise (reproducible) data. Several studies, both in humans and animal models that have used these methods indeed revealed that IUGR leads to a low nephron number. According to the hyperfiltration hypothesis, this reduction in renal mass is supposed to lead to glomerular hyperfiltration and hypertension in remnant nephrons with subsequent glomerular injury with proteinuria, systemic hypertension and glomerulosclerosis. Even though IUGR is associated with both a low nephron endowment and an increased risk of hypertension, only circumstantial evidence is available to support the hyperfiltration hypothesis after prenatal programming. A prerequisite for establishment of this association in long-term, prospective follow-up studies is the ability to estimate glomerular numbers in living human beings, for which a further advancement in radiological techniques is necessary. Only then can the association between nephron endowment and blood pressure in humans be studied more conclusively. In the last decade, many researchers have focused on the effects of prenatal conditions on health and disease later in life (programming). Low birth weight as a marker for intrauterine growth retardation (IUGR), in combination with the growth trajectory during childhood, is associated with a higher blood pressure and an increased risk of diabetes and cardiovascular disease.1.Barker D.J. Adult consequences of fetal growth restriction.Clin Obstet Gynecol. 2006; 49: 270-283Crossref PubMed Scopus (594) Google Scholar Prenatal programming can be expected to be of importance to the kidney. Not only is nephrogenesis a process that takes place solely before birth in humans, that is until the 36th week of gestation,2.Nigam S.K. Aperia A.C. Brenner B.M. Development and maturation of the kidney.in: Brenner B.M. Brenner and Rector's the kidney. W.B. Saunders Company, Philadelphia1996: 72-98Google Scholar no new nephrons can be formed after that time as well. Impaired nephrogenesis would therefore lead to a low nephron endowment without the possibility of quantitative compensation. The resulting low nephron number is associated with glomerular and systemic hypertension according to the hyperfiltration-hypothesis from the group of Brenner,3.Brenner B.M. Garcia D.L. Anderson S. Glomeruli and blood pressure. Less of one, more the other?.Am J Hypertens. 1988; 1: 335-347Crossref PubMed Scopus (1004) Google Scholar,4.Brenner B.M. Lawler E.V. Mackenzie H.S. The hyperfiltration theory: a paradigm shift in nephrology.Kidney Int. 1996; 49: 1774-1777Abstract Full Text PDF PubMed Scopus (598) Google Scholar and may therefore be (partially) responsible for the association between low birth weight and hypertension (Figure 1). In recent years, the issue of renal programming with subsequent hypertension has been subject of thorough research. In this review, we will focus on the prenatal programming of nephron number, including the importance of methods used to determine this number, and the association between a low nephron number and hypertension. Quantitation of nephron number in the kidney can be performed using several techniques (for a review see Nyengaard5.Nyengaard J.R. Stereologic methods and their application in kidney research.J Am Soc Nephrol. 1999; 10: 1100-1123PubMed Google Scholar and Bertram6.Bertram J.F. Counting in the kidney.Kidney Int. 2001; 59: 792-796Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar). Even though the described techniques are unbiased, most of the methods used are based on approximations and assumptions that are unproven and therefore possibly biased. For instance, the acid-maceration technique7.Damadian R.V. Shwayri E. Bricker N.S. On the existence of non-urine forming nephrons in the diseased kidney of the dog.J Lab Clin Med. 1965; 65: 26-39PubMed Google Scholar relies on the resistance of glomeruli to acid, whereas the rest of the kidney dissolves. Glomerulosclerosis is a factor known to influence acid-resistance, and it is therefore only assumed that all glomeruli survive the maceration equally, which introduces bias. Another biased method is based on the number of glomerular profiles per section, resulting in a number per mm2. Examining one section introduces bias by size-dependency, which is based on the fact that larger particles have a higher chance of being present in a random section, and thereby being counted, than smaller particles (for an illustrative example see Bertram6.Bertram J.F. Counting in the kidney.Kidney Int. 2001; 59: 792-796Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar). The Weibel–Gomez method, another widely used technique, is used to determine the glomerular density, resulting in a number per mm3.8.Weibel E.R. Gomez D.M. A principle for counting tissue structures on random sections.J Appl Physiol. 1962; 17: 343-348Crossref PubMed Scopus (630) Google Scholar Basing conclusion solely on the number of profiles per section or the density provides no information on the total number of particles as long as the reference, that is the volume when working with densities, is unknown: this is referred to as the 'reference trap'. In order to try to compensate for this, researchers have measured the volume of the kidney, which, again based on the methods used, may be biased as well due to dimensional changes such as shrinkage. However, this still does not circumvent the problem of size-dependency in the number estimation. Yet, design-based, and thereby unbiased, techniques are available for number estimation.5.Nyengaard J.R. Stereologic methods and their application in kidney research.J Am Soc Nephrol. 1999; 10: 1100-1123PubMed Google Scholar,6.Bertram J.F. Counting in the kidney.Kidney Int. 2001; 59: 792-796Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar These stereological techniques are labor-intensive and time-consuming, but offer the opportunity to estimate (glomerular) number and size in an accurate (unbiased) and precise (reproducible) way that is based on firm statistical and mathematical principles. Estimation of the nephron number with stereological techniques is based on counting glomeruli as a marker for nephrons in a known sample of the kidney. To prevent size-dependency, particles (like glomeruli) are only counted when the first appearance of the particle is identified in a set of serial tissue sections. Each particle has only one such 'beginning', no matter what its size or shape. By examination of two adjacent sections (therefore called the disector technique), the beginning of the particle is identified by its presence in the first section but not the second section, and as a result it is counted. This counting can be performed in the whole kidney, but an estimation of the total number can be obtained by limiting the sample size without much influence on reliability. Using the fractionator technique with a predetermined sampling fraction, mostly based on a multistage sampling scheme, the reference trap can be avoided while reducing the workload with the advantage of preventing the effect of tissue deformation.5.Nyengaard J.R. Stereologic methods and their application in kidney research.J Am Soc Nephrol. 1999; 10: 1100-1123PubMed Google Scholar In this review, only data from number estimations using appropriate stereological methods will be presented. Counting glomeruli in the kidney with these stereological techniques is currently only possible when the whole kidney is available, that is in an ex vivo situation. In order to approximate nephron endowment in living individuals, kidney size is used as a surrogate marker. Kidney weight has been shown to correlate with nephron number in humans9.Nyengaard J.R. Bendtsen T.F. Glomerular number and size in relation to age, kidney weight, and body surface in normal man.Anat Rec. 1992; 232: 194-201Crossref PubMed Scopus (663) Google Scholar and kidney size is associated with nephron number in several animals including primates.10.Gubhaju L. Black M.J. The baboon as a good model for studies of human kidney development.Pediatr Res. 2005; 58: 505-509Crossref PubMed Scopus (40) Google Scholar However, the (absence of) strength of this association does not allow for estimation of individual glomerular number, thereby reducing the value of kidney size to population studies. After completion of nephrogenesis, on average ±750 000 nephrons are present per kidney, with a wide interindividual range (250 000–2 000 000). 9.Nyengaard J.R. Bendtsen T.F. Glomerular number and size in relation to age, kidney weight, and body surface in normal man.Anat Rec. 1992; 232: 194-201Crossref PubMed Scopus (663) Google Scholar, 11.Keller G. Zimmer G. Mall G. et al.Nephron number in patients with primary hypertension.N Engl J Med. 2003; 348: 101-108Crossref PubMed Scopus (844) Google Scholar, 12.Hughson M. Farris A.B. Douglas-Denton R. et al.Glomerular number and size in autopsy kidneys: the relationship to birth weight.Kidney Int. 2003; 63: 2113-2122Abstract Full Text Full Text PDF PubMed Scopus (551) Google Scholar, 13.Hughson M.D. Douglas-Denton R. Bertram J.F. et al.Hypertension, glomerular number, and birth weight in African Americans and white subjects in the southeastern United States.Kidney Int. 2006; 69: 671-678Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar It has been suggested that this large interindividual variance may be an important determinant of health and disease. Many factors may be involved in this process. Differences in glomerular numbers between genders and races have been reported,13.Hughson M.D. Douglas-Denton R. Bertram J.F. et al.Hypertension, glomerular number, and birth weight in African Americans and white subjects in the southeastern United States.Kidney Int. 2006; 69: 671-678Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar,14.Hoy W.E. Hughson M.D. Singh G.R. et al.Reduced nephron number and glomerulomegaly in Australian Aborigines: a group at high risk for renal disease and hypertension.Kidney Int. 2006; 70: 104-110Abstract Full Text Full Text PDF PubMed Scopus (192) Google Scholar and more subtle individual differences in genetic make-up are also likely to play a role. Environmental factors during nephrogenesis are also known to influence total nephron number, as a reduction in nephron formation can be caused by any disturbance in the complicated interaction between numerous growth factors and hormones that are necessary for nephrogenesis. Specific situations, like maternal vitamin A deficiency or use of angiotensin converting enzyme inhibitors during pregnancy, have been described to hamper normal renal development.15.Quan A. Fetopathy associated with exposure to angiotensin converting enzyme inhibitors and angiotensin receptor antagonists.Early Hum Dev. 2006; 82: 23-28Abstract Full Text Full Text PDF PubMed Scopus (134) Google Scholar,16.Xu Q. Lucio-Cazana J. Kitamura M. et al.Retinoids in nephrology: promises and pitfalls.Kidney Int. 2004; 66: 2119-2131Abstract Full Text Full Text PDF PubMed Scopus (58) Google Scholar A general reduction in growth of the fetus during active nephrogenesis, for instance owing to IUGR or premature birth, can also impair nephron formation. Hinchliffe et al.17.Hinchliffe S.A. Lynch M.R. Sargent P.H. et al.The effect of intrauterine growth retardation on the development of renal nephrons.Br J Obstet Gynaecol. 1992; 99: 296-301Crossref PubMed Scopus (467) Google Scholar studied the influence of IUGR on nephron number, but based the definition of IUGR solely on a single measurement, that is, a percentile score of birth weight relative to gestational age. However, as growth retardation implies knowledge of the growth velocity and therefore requires more than one measurement, Hinchliffe's approach leads to inclusion of constitutionally small children that account for up to 22% of such a 'IUGR' group.18.Mamelle N. Cochet V. Claris O. Definition of fetal growth restriction according to constitutional growth potential.Biol Neonate. 2001; 80: 277-285Crossref PubMed Scopus (81) Google Scholar Using this birth weight-based definition of IUGR, Hinchliffe et al.17.Hinchliffe S.A. Lynch M.R. Sargent P.H. et al.The effect of intrauterine growth retardation on the development of renal nephrons.Br J Obstet Gynaecol. 1992; 99: 296-301Crossref PubMed Scopus (467) Google Scholar described a 35% lower glomerular number in the low birth weight group relative to controls. Without correction for gestational age, Hughson et al.12.Hughson M. Farris A.B. Douglas-Denton R. et al.Glomerular number and size in autopsy kidneys: the relationship to birth weight.Kidney Int. 2003; 63: 2113-2122Abstract Full Text Full Text PDF PubMed Scopus (551) Google Scholar found a significant correlation between birth weight and glomerular number (n=56, r=0.42, P=0.001) with a decrease of 250 000 glomeruli per kidney per kg decline in birth weight based on the regression coefficient. In an expanded study of 140 individuals, there was still a significant overall correlation between birth weight and glomerular number (r=0.33, P=0.003).13.Hughson M.D. Douglas-Denton R. Bertram J.F. et al.Hypertension, glomerular number, and birth weight in African Americans and white subjects in the southeastern United States.Kidney Int. 2006; 69: 671-678Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar Even though there was no significant difference in glomerular number between African American and white subjects, the correlation between birth weight and glomerular number was highly influenced by race (white subjects: r=0.57, P=0.002; African Americans: r=0.24, P=0.16; both groups limited to subjects 30–65 years of age). These data implicate that glomerular number in African Americans is not as clearly influenced by prenatal conditions. Whether this is due to a lack of statistical power or based on a true difference in mechanism is not clarified. Another possible explanation may be that an additional, yet unidentified factor is involved in the pathway that leads to a low nephron endowment, and that this factor differs among races, for instance, based on environmental differences between African Americans and whites. Using kidney size as a marker for nephron number, both IUGR and prematurity have been shown to hamper renal development.19.Schmidt I.M. Chellakooty M. Boisen K.A. et al.Impaired kidney growth in low-birth-weight children: distinct effects of maturity and weight for gestational age.Kidney Int. 2005; 68: 731-740Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar Premature birth implies being born during active nephrogenesis. The consequences are studied by Rodriguez et al.,20.Rodriguez M.M. Gomez A.H. Abitbol C.L. et al.Histomorphometric analysis of postnatal glomerulogenesis in extremely preterm infants.Pediatr Dev Pathol. 2004; 7: 17-25Crossref PubMed Scopus (341) Google Scholar who described a shorter period of active nephrogenesis and a reduced number of glomerular layers. Whether this is due to the premature birth in itself, or to a harsh postnatal environment, such as exposure to nephrotoxic medications and ischemia cannot be distinguished. Another factor may be postnatal growth restriction, which is an almost universal finding in premature neonates.21.De Curtis M. Rigo J. Extrauterine growth restriction in very-low-birthweight infants.Acta Paediatr. 2004; 93: 1563-1568Crossref PubMed Google Scholar Previously, we have shown in a rat model of postnatal food restriction that this is associated with a reduction in nephron number.22.Schreuder M.F. Nyengaard J.R. Remmers F. et al.Postnatal food restriction in the rat as a model for a low nephron endowment.Am J Physiol Renal Physiol. 2006; 291: F1104-F1107Crossref PubMed Scopus (59) Google Scholar Whether nephrotoxic medications, like aminoglycosides, diuretics, or nonsteroidal anti-inflammatory drugs, affect nephrogenesis in premature babies has not yet been clarified. In 1988, Brenner et al.3.Brenner B.M. Garcia D.L. Anderson S. Glomeruli and blood pressure. Less of one, more the other?.Am J Hypertens. 1988; 1: 335-347Crossref PubMed Scopus (1004) Google Scholar suggested a link between a low nephron number and the development of hypertension later in life. The basis of their hyperfiltration hypothesis is that a reduction in renal mass, either due to a low endowment or to renal mass reduction based on renal disease or surgical removal, leads to glomerular hyperfiltration and hypertension in remnant nephrons with subsequent glomerular enlargement in order to sustain adequate renal clearance. However, the underlying glomerular hypertension and/or hyperfiltration leads to glomerular injury with proteinuria, systemic hypertension and glomerulosclerosis, which starts a vicious cycle of further loss of functioning nephrons and ultimately renal failure (Figure 1). Keller et al.11.Keller G. Zimmer G. Mall G. et al.Nephron number in patients with primary hypertension.N Engl J Med. 2003; 348: 101-108Crossref PubMed Scopus (844) Google Scholar provided evidence in humans for an association between a low nephron number and hypertension. They examined kidneys of patients that died in accidents, and found significantly fewer (on average 50% less) and larger (mean glomerular volume more than doubled) nephrons in patients previously diagnosed with hypertension than in matched controls with a normal blood pressure. In another post-mortem study of white adults, Hughson et al.13.Hughson M.D. Douglas-Denton R. Bertram J.F. et al.Hypertension, glomerular number, and birth weight in African Americans and white subjects in the southeastern United States.Kidney Int. 2006; 69: 671-678Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar found a significant reverse relationship between glomerular number and blood pressure, indicating a higher blood pressure in individuals with a low nephron endowment. Again, glomerular enlargement was found to accompany low nephron numbers. However, a similar relationship between glomerular number and blood pressure was not found in a comparable group of African Americans, even though mean glomerular number was similar in both groups. These differences may indicate that the role of nephron number is not so important in African Americans as it is in Caucasians. This seems to contradict the hyperfiltration hypothesis, as a low nephron number, irrespective of the cause, is considered to be associated with glomerular hyperfiltration and subsequent systemic hypertension. Blood pressure was significantly higher in the group of African Americans than in white subjects,13.Hughson M.D. Douglas-Denton R. Bertram J.F. et al.Hypertension, glomerular number, and birth weight in African Americans and white subjects in the southeastern United States.Kidney Int. 2006; 69: 671-678Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar as is a general finding. The question may therefore be why African American individuals with a normal nephron number do not reach a normal blood pressure, instead of the question why individuals with a lower number do not show an increase in blood pressure. These results indicate that there is more involved in blood pressure regulation than simply the nephron number. In order to study these issues prospectively, various animal models have been used. We have previously shown that rats born after spontaneous IUGR or IUGR owing to placental insufficiency based on maternal uterine artery ligation have a low nephron endowment with an increase in glomerular size.23.Schreuder M.F. Nyengaard J.R. Fodor M. et al.Glomerular number and function are influenced by spontaneous and induced low birth weight in rats.J Am Soc Nephrol. 2005; 16: 2913-2919Crossref PubMed Scopus (85) Google Scholar In addition, we found that adult male rats born after induced IUGR are hypertensive,24.Schreuder M.F. van Wijk J.A. Delemarre-van de Waal H.A. Intrauterine growth restriction increases blood pressure and central pulse pressure measured with telemetry in aging rats.J Hypertens. 2006; 24: 1337-1343Crossref PubMed Scopus (43) Google Scholar which indirectly suggests an association between glomerular number and blood pressure. Also, spontaneous hypertensive rat strains have fewer glomeruli than their normotensive counter parts,25.Skov K. Nyengaard J.R. Korsgaard N. et al.Number and size of renal glomeruli in spontaneously hypertensive rats.J Hypertens. 1994; 12: 1373-1376Crossref PubMed Scopus (75) Google Scholar even though not all studies of nephron number in spontaneous hypertensive rat rats have reported a reduced nephron count.26.Kett M.M. Alcorn D. Bertram J.F. Anderson W.P. Glomerular dimensions in spontaneously hypertensive rats: effects of AT1 antagonism.J Hypertens. 1996; 14: 107-113PubMed Google Scholar However, Black et al.27.Black M.J. Briscoe T.A. Constantinou M. et al.Is there an association between level of adult blood pressure and nephron number or renal filtration surface area?.Kidney Int. 2004; 65: 582-588Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar described a lower blood pressure but the same (significantly lower) number of glomeruli as the spontaneous hypertensive rats in the F1 population after crossbreeding of spontaneous hypertensive rat with a normotensive rat strain. The F2 population showed a wide range of blood pressures and nephron numbers, with no significant correlation between nephron number and blood pressure. Based on these findings, the authors concluded that there is no direct relationship between nephron number and blood pressure. This conclusion has to be interpreted with care, as the data of Black et al.27.Black M.J. Briscoe T.A. Constantinou M. et al.Is there an association between level of adult blood pressure and nephron number or renal filtration surface area?.Kidney Int. 2004; 65: 582-588Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar show a nonsignificant correlation, which in fact may be due to the limited number of animals (n=17) over such a wide range of glomerular number. Also, blood pressure was measured by the tail–cuff method that has been shown to lead to stress, and prenatal conditions have been shown to influence the cardiovascular response to stress.24.Schreuder M.F. van Wijk J.A. Delemarre-van de Waal H.A. Intrauterine growth restriction increases blood pressure and central pulse pressure measured with telemetry in aging rats.J Hypertens. 2006; 24: 1337-1343Crossref PubMed Scopus (43) Google Scholar Still, these results raise questions concerning the association between blood pressure and nephron number. Rettig and Grisk28.Rettig R. Grisk O. The kidney as a determinant of genetic hypertension: evidence from renal transplantation studies.Hypertension. 2005; 46: 463-468Crossref PubMed Scopus (48) Google Scholar have performed several transplantation studies, and described that blood pressure travels with the kidney. No transplantation study with kidneys from IUGR animals has been carried out yet. If such a transplantation study would be performed with two kidneys with low nephron numbers into one recipient, in order to prevent a reduction in renal mass due to the presence of just one kidney, and these animals would show increased blood pressures, this would further support the association between a low nephron endowment and hypertension. Patients with a single (functioning) kidney form another population with a congenital reduction in renal mass. The solitary kidney is based on non-formation of one kidney (unilateral renal agenesis) or formation of a unilateral non-functioning kidney (multicystic dysplastic kidney). Long-term follow-up of these patients also revealed an increased risk of hypertension, proteinuria, and renal insufficiency,29.Argueso L.R. Ritchey M.L. Boyle Jr, E.T. et al.Prognosis of patients with unilateral renal agenesis.Pediatr Nephrol. 1992; 6: 412-416Crossref PubMed Scopus (145) Google Scholar which is in line with the hyperfiltration hypothesis. As both unilateral renal agenesis and multicystic dysplastic kidneys are frequently associated with contralateral malformations, it cannot be ruled out that the long-term consequences are based on congenital problems in the remaining kidney rather than consequences of a low nephron endowment. This problem is circumvented by performing unilateral nephrectomy in neonatal animals, which has also been shown to lead to glomerular enlargement, hypertension, and proteinuria.30.Woods L.L. Weeks D.A. Rasch R. Hypertension after neonatal uninephrectomy in rats precedes glomerular damage.Hypertension. 2001; 38: 337-342Crossref PubMed Scopus (83) Google Scholar In this study, Woods et al. aimed to determine whether glomerular damage was present before the development of hypertension, or whether the hypertension preceded the glomerular damage. As nephrogenesis in rats continues until approximately 8 days after birth, the timing of nephrectomy (within 24 h after birth) was during active nephrogenesis. Removal of one kidney at that time did not lead to the formation of extra nephrons in the remaining kidney. The 50% renal mass reduction was associated with salt-sensitive hypertension before signs of glomerular damage evolved, thereby disproving that structural glomerular pathology was responsible for the hypertension. Both IUGR and prematurity hamper nephrogenesis, and result in a low nephron endowment. Hypothetically, this will lead to glomerular and systemic hypertension, and glomerular damage owing to hyperfiltration. Even though low glomerular numbers are found in some hypertensive groups, and animal models with reduced nephron numbers show hypertension, the association between the two characteristics is not present in all studies. The presumed association between low glomerular numbers and hypertension is mainly based on circumstantial evidence, which is largely derived from animal studies. In order to establish this challenging association, a large and prospective study in humans will be necessary.