Renal Endowment in Men and Women: Start from the Beginning

Renal diseases affect millions of people worldwide and can have significant implications for health and quality of life. Gender differences play a notable role in the prevalence, progression, and outcomes of renal diseases. For example, males with chronic kidney disease (CKD) have been reported to have a higher risk of developing end-stage renal disease than females, and they may also experience a higher incidence of cardiovascular complications associated with renal disease. These topics were at the core of the meeting titled “Gender differences in renal disease, focus on diabetes and obesity,” held in Garachico, Spain, and the present review is part of the proceedings of the meeting. This review highlights that the differences in renal development between males and females begin during embryonic stages and lead to a disparity in the final number of nephrons, with long-lasting effects on renal health later in life.

The nephron is the key structure that supports the function of the kidney, a complex organ composed of specialized cells that collectively contribute to filtering the blood and reabsorbing physiologically relevant molecules. Nephrogenesis in humans begins at 9 weeks of gestation and is complete by 36 weeks. However, there is variability among individuals and the cessation of nephrogenesis can occur between 32 and 37 weeks of gestation. By full-term birth, the number of nephrons, more appropriately termed nephron endowment, is definitively established and is the highest it will be for the individual; it cannot be increased after birth. In adult kidneys, the number of nephrons does not necessarily correspond to nephron endowment at birth, and both can be influenced by several factors (Fig. 1). The number of nephrons in healthy subjects is influenced by a substantial age-related decline, as shown in studies conducted on autoptic kidneys and biopsies on kidney donors [1, 2]. During adult life, a substantial loss of nephrons may also occur as a consequence of several pathological conditions, including hypertension, cardiovascular disease, diabetes, and kidney diseases [3]. The quantification of the number of nephrons, mainly from autoptic studies, has revealed high variability in individual nephron endowment, ranging from 210,000 to 2,700,000 nephrons per kidney, with an average of 600,000–800,000 [3‒5]. A recent publication highlights that climate change has severe consequences on the number of premature births or low birth weight (LBW), ultimately impacting nephron mass and postnatal kidney injury [6].

Measuring the number of nephrons in humans, however, remains a challenging issue. For decades, in ex vivo quantification methods, the gold-standard tool has been the dissector/fractionator method [7]. However, more recent alternatives are based on light sheet microscopy after optical cleaning [8] and X-ray CT [9]. Another approach for ex vivo quantification of nephrons is based on cationized ferritin-enhanced MRI. To facilitate CFE-MRI use as a clinical parameter, cationized ferritin has been adapted to form a PET radiotracer [10]. Currently, these techniques show high promise, but they are not applicable to humans since in vivo translation requires nondestructive and relatively noninvasive techniques.

The mammalian kidney arises from the metanephros, whose development involves the interaction of two specific structures – the metanephric mesenchyme, which originates from the intermediate mesoderm, and the ureteric bud, which branches and develops into the ureter and collecting ducts [11]. The complex processes that drive kidney development require inductive interactions between different cell types, including epithelial and mesenchymal cells that differentiate into highly specialized cell types [12]. The different phases of kidney development are coordinately controlled by key genes, whose mutations or defects have often been associated with congenital abnormalities in kidney and urinary tract development [13]. One of the regulatory factors that is indispensable for kidney development is the transcription factor PAX2, which is expressed in the nephric duct, cap mesenchyme, differentiating nephron, and collecting duct of the embryonic kidney [14]. In mice, PAX2 is required for the differentiation of the mesenchyme into the epithelium. It has been reported that Pax2 homozygous mutant newborn mice lack kidneys, ureters, and genital tracts, while heterozygous mutants had smaller kidneys [14]. We recently derived induced pluripotent stem cells (iPSCs) from a patient with focal segmental glomerulosclerosis with heterozygous mutation in the octapeptide domain of the transcription factor PAX2 [15]. iPSCs with the PAX2 mutation were differentiated into podocytes that exhibited altered motility, were more susceptible to apoptosis, and had reduced ureteric bud morphogenetic potential [15, 16]. The PAX2 mutation was edited in iPSCs using CRISPR-Cas9-based homology-directed repair, and edited iPSCs were differentiated into podocytes [15, 16]. The latter had altered motility, were more susceptible to apoptosis, and had reduced ureteric bud morphogenetic potential. When the PAX2 mutation was corrected in iPSCs using CRISPR-Cas9-based homology-directed repair, podocytes derived from edited iPSCs exhibited restored motility, increased viability, and rescue of ureteric bud-like tubule formation [15, 16].

Differential gene expression during kidney development is also regulated by epigenetic mechanisms, and disturbances of epigenetic regulation in key developmental genes may affect the nephron progenitor pool, proper kidney growth, and maturation [13]. Epigenetic modification includes DNA methylation, histone modification, and microRNA action. During kidney development, DNA methyltransferases (DNMT1 and DNMT3) are highly expressed in the nephrogenic mesenchyme and play important roles in the maturation of kidney structures [13]. Histone modification also regulates kidney development and may involve histone deacetylase activity and histone methyl transferase. We have recently identified a novel role that sirtuin 3 (SIRT3), a conserved protein that belongs to the sirtuin family, plays in controlling early renal development through the regulation of epigenetics [17]. We showed that nuclear SIRT3 exerted de-2-hydroxyisobutyrylation activity on lysine residues of histone proteins. SIRT3 deficiency resulted in impaired regulation of metabolic processes and translated into a permanent nephron deficit. Supplementation of nicotinamide riboside, a precursor of the SIRT3 co-substrate NAD+, improved the number of nephrons in a low-protein diet model [18], thus preventing shortage of nephron mass at birth.

The number of nephrons is an important factor in determining susceptibility to kidney disease and its progression. Dr. Barry Brenner was the first, in 1988, to hypothesize that low nephron endowment is a critical contributing factor to high blood pressure and an enhanced risk for CKD later in life [19‒21]. Experimental studies demonstrated that in the presence of a low number of nephrons, the remaining glomeruli undergo hyperfiltration and consequent maladaptive changes, which may ultimately lead to sclerotic glomeruli and kidney disease [22]. Several studies have highlighted that the number of nephrons correlates with LBW, and neonates born small for gestational age have fewer nephrons [23, 24]. In addition, intrauterine growth restriction due to a variety of unfavorable conditions, including exposure to drugs, diabetes, infections, and diet restriction (Fig. 1), leads to a congenital reduction in the number of nephrons [24]. Awareness on preterm and very LBW neonates that may undergo therapeutic interventions in intensive care units is extremely relevant. Indeed, they are particularly susceptible to acute kidney injury due to their congenital reduction in the number of nephrons and the exposure to nephrotoxic drugs, such as aminoglycoside antibiotics and nonsteroidal anti-inflammatory drugs [25]. This evidence suggests that these infants should be monitored closely during their childhood and adulthood [26].

Sexual dimorphism plays a significant role in regulating kidney development and in determining the number of nephrons in humans. Numerous studies have elucidated sex-related differences in the number of nephrons, with findings that indicate that there is a disparity in glomeruli count between males and females. For instance, data from 37 autopsies conducted in Denmark revealed that females had a 4.3-fold lower number of glomeruli than males [27]. Additionally, results from 81 autoptic kidneys in Senegal have shown that females have approximately 12% fewer nephrons than males, while the mean glomerular volume is similar in men and women [28]. There is also an age-related decline in the number of nephrons, with a predicted reduction of 3,676 glomeruli per kidney, per year after the age of 18 [27, 29]. Moreover, birth weight, which has emerged as a surrogate marker for nephron endowment, is lower in females [30]. Notably, in Australian Aboriginal people, birth weight also holds independent significance in predicting urinary albumin/creatinine ratio [31]. Higher levels of albuminuria and a higher incidence of renal failure have been found in this population, probably due to higher BMIs and rates of diabetes in females other than constitutive lower number of nephrons as demonstrated in other ethnic groups globally [23, 27].

A recent study has explored sex disparities in podocyte endowment [32]. An autoptic analysis of kidneys from Japanese adults evidenced a direct correlation between the number of nephrons and podocyte count. Indeed, kidneys with a lower number of nephrons exhibited lower numbers of podocytes, both of which create an incremental risk of further nephron loss during kidney disease. The analysis of sex-related differences did not show any changes in podocyte number per tuft. Significantly, male subjects exhibited a 36% greater glomerular volume and 37% larger glomerular podocytes compared to females [32].

Age-related loss of kidney function also manifests differently depending on sex [33]. Among middle-aged and elderly individuals in the general population, at baseline, women had a lower total glomerular filtration rate (GFR), probably as a consequence of the lower number of nephrons and, consequently, lower metabolic demand [33]. Despite this, the decline in GFR is slower in women than in men, independently of health status [33]. Male sex has been associated with a negative impact on CKD progression and advancing to renal replacement therapy [34]. The proposed explanations for sex-related differences in GFR and CKD progression include differences between men and women regarding oxidative stress and renal hemodynamics, as well as the influence of sex hormones [35].

Investigating the complexities of nephron endowment variability involves recognizing the range of factors that contribute to the establishment of individual nephron endowment, including genetic predisposition and environmental influences during embryonic development, potentially leading to increased susceptibility to kidney diseases later in life. Given the close correlation between birth weight and the number of nephrons, it is advisable to record gestational age and birth weight in all subjects in order to identify subjects at high risk of developing noncommunicable diseases (NCDs) in adult life. Indeed, there is evidence that LBW is associated with an increased risk of developing various NCDs later in life, including hypertension, cardiovascular disease, type 2 diabetes, obesity, metabolic syndrome, and CKD [36‒38]. Although the pathophysiological etiology of these NCDs is not clear, Dr. Brenner’s hypothesis suggests that in individuals with LBW, which is indicative of a low nephron count, compensatory hyperfiltration by the remaining nephrons results in hypertension and consequent accelerated renal decline [19, 22]. Therefore, an individual’s nephron endowment significantly impacts their lifetime risk for various health issues, including renal diseases. Tailoring interventions based on an individual’s genetic profile and developmental history holds promise for improving outcomes and reducing the burden of kidney diseases on affected individuals and healthcare systems. Children and adults born preterm require long-term follow-up and preventive measures to preserve renal function and identify early signs of kidney disease with periodic monitoring based on individualized risk assessments, including degree of prematurity. As part of a preventive approach, they would need personalized nutrition and dietary adjustments, as they may be more susceptible, for example, to the deleterious effect of a high-protein diet or high salt intake. Patients should be advised to avoid potentially nephrotoxic drugs (e.g., nonsteroidal anti-inflammatory drugs) and made aware of other risk factors for CKD, including obesity, hypertension, and diabetes. Additionally, differences in kidney development have been highlighted between males and females in terms of number of nephrons and function. These differences call for the integration of gender-specific approaches into prevention and treatment strategies, involving the design of inclusive clinical trials, investigation of hormonal and genetic impacts, and the development of gender-sensitive guidelines and policies. Otherwise, primary preventive intervention should focus on supporting a healthy pregnancy to ensure an adequate number of nephrons for newborns. In pregnant women, especially in low-income regions, malnutrition, limited access to healthcare, and environmental factors can adversely impact fetal kidney development. Ensuring proper maternal nutrition, adequate prenatal care, and early intervention strategies can significantly improve nephron endowment in newborns. In addition, more research efforts should be directed toward developing targeted therapies or interventions to boost the number of nephrons during pregnancy or during early postnatal life. By prioritizing maternal health, we can promote better outcomes in infant kidney development and aspire to reduce the global burden of CKD.

The authors thank Antonella Piccinelli for figure preparation and Kerstin Mierke for English language editing.

The authors have declared that they have no conflicts of interest.

None.

Barbara Imberti and Ariela Benigni conceived and wrote the manuscript.