What are the events at the different areas of a nephron and in the collecting duct?

Abstract

Nephron progenitors give rise to many parts of nephrons, including glomeruli and renal tubules, in vivo. Induction of nephron progenitors from human induced pluripotent stem cells (iPSCs) has enabled the generation of kidney organoids containing multiple nephrons that can be utilized for studies on human kidney development as well as modeling of kidney diseases in vitro. These nephron progenitors are expandable in vitro, and a selective induction method from nephron progenitors toward glomerular podocytes has been established. Another precursor of the kidney, the ureteric bud (UB), can also be induced from human iPSCs, and combinations of nephron progenitors and UBs, together with stromal progenitors, will lead to the generation of higher-order structures of the human kidney.

7.12 Filtration, reabsorption, and secretion

Nephrons first filter the blood and then modify the resulting filtrate into the urine. Many changes take place in the different parts of the nephron before the urine is created for disposal. The principal task of the nephrons is to balance the plasma to homeostatic set points and excrete potential toxins in the urine. They do this by accomplishing three key functions—filtration, reabsorption, and secretion. Filtration of blood is the movement of solutes and water from the glomerulus into the Bowman's capsule. The filtrate contains waste molecules as well as essential nutrients, ions, and water. These molecules must be claimed before the filtrate passes through the rest of the nephron. Those substances that are conserved from the tubular lumen back into the blood are referred to as reabsorption. Some substances such as water and salts are reabsorbed throughout the length of the tubule, whereas the reabsorption of others occurs specifically in one location. For example, 100% of the glucose is reabsorbed in the proximal tubule alone. Secretion is the release of substances into the filtrate from the blood in the peritubular capillaries or by the tubular epithelial cells (Fig. 7.6).

Maladaptive Tubular Responses in Surviving Nephrons

Remnant nephrons in the diseased kidney or following reduction of renal mass exhibit not only glomerular enlargement and hyperfiltration but also tubular hypertrophy, hyperplasia, and hyperfunction. These tubular processes are also implicated in CKD.3,25,42,43 Increased single nephron glomerular filtration rates in surviving nephrons necessitate increased sodium reabsorption to achieve glomerulotubular balance. Sodium reabsorption by the kidney is the principal determinant of renal oxygen consumption. Indeed oxygen consumption/nephron in the remnant kidney model44 and oxygen consumption/GFR in human CKD45 are both increased. However, oxygen consumption factored for sodium transport in the remnant kidney, compared with the intact kidney, is higher, thereby indicating augmented metabolic costs entailed by the hyperfunctioning and hypertrophied nephrons.44,45 Such metabolic alteration is implicated in CKD in at least two ways. First, such increased oxygen consumption may engender oxidative stress and attendant injury.44 Second, increased oxygen consumption by surviving nephrons may contribute to cortical hypoxia, a recognized pathway for CKD.46–48 Moreover, there may be a positive feedback between these pathways. Cortical hypoxia induced by increased oxygen consumption can itself promote mitochondrial oxidant generation, and oxidant stress can augment mitochondrial consumption.47

Another metabolic adaptation in surviving nephrons is enhanced ammoniagenesis, the latter needed to maintain net acid excretion in CKD.49 Such increased ammonia production leads to increased cortical partial pressure of ammonia with accompanying activation of the alternative complement pathway.49 Bicarbonate supplementation reduces renal ammonia production and tubulointerstitial injury. Other mechanisms underlying the beneficial effects of bicarbonate supplementation involve reduced generation of reactive oxygen species50 and decreased production of endothelin-1.51 A number of studies in humans demonstrate the beneficial effects of base supplementation in retarding the progression of CKD.51

Pathobiology

The intact nephron hypothesis helps explain why glomerular filtration rate (GFR) is the most accurate estimate of the remaining kidney function in CKD patients. The hypothesis states that each individual nephron functions as an independent unit, so the combined functions of all remaining nephrons will determine the “whole kidney” GFR. This is a useful concept because individual nephrons participate in all physiologic and metabolic functions of the kidney, including regulation of blood pressure, several endocrine functions, the concentrations of ions in extracellular and intracellular fluids, and the excretion of waste products. Losses of these functions produce direct consequences of CKD, including abnormalities arising from the accumulation of unexcreted waste products synthesized during the metabolism of proteins and amino acids. For example, CKD reduces the ability to excrete acid leading to hyperventilation with a compensatory decrease in Pco2 and skeletal muscle metabolic process activation by a mechanism that activates the ubiquitin-proteasome enzymatic process to degrade muscle proteins, causing loss of muscle mass.7 Accumulated acid is also buffered in bone, releasing calcium and phosphates to aggravate bone demineralization.5

VII.A Anatomy and Pathophysiology

The functional unit of the kidney is the nephron, which consists of a tuft of capillaries, termed the glomerulus, found within the Bowman's capsule located in the cortex. The proximal and distal convoluted tubules are a continuation of the Bowman's capsule, connected through the loop of Henlé. The distal tubules join into collecting ducts, which empty into calyces. The kidneys perform multiple and complex tasks, primarily maintaining the environment through the removal of toxic products and, secondarily, controlling fluid and electrolyte balance, blood pressure, red blood cell formation, and regulation of active vitamin D production. The target cells for radiation damage are multiple, with a variable clinical picture and time of onset, although vascular damage again plays an important role. Irradiation of the cortex is more likely to be detrimental in view of the location of the nephrons. The classic lesion is described as glomerulosclerosis with an obliteration of glomeruli. Tubular cell damage includes atrophy and necrosis of tubules. In addition, interstitial fibrosis is seen as a late effect of radiation therapy.

Renal Tubule

The nephron is the functional unit of the kidney. There are 1 million nephrons per human kidney. Each nephron contains elements shown in Figure 10.6. Different types of nephrons contain the various elements in different proportions. The nephron consists of Bowman's capsule, the network or tuft of capillaries inside it, and the tubule leading from Bowman's capsule to ducts leading to the bladder. The tubule consists of the proximal tubule, the loop of Henle, and the distal tubule. The proximal tubule consists of the proximal convoluted tubule and the proximal straight tubule, as indicated in the figure. In this region, most of the filtered bicarbonate and potassium are reabsorbed into the bloodstream. The distal tubule — which consists of the distal convoluted tubule, the connecting tubule, and the initial collecting tubule — participates in regulation of Na and K balance. This region contains the principal cells and is sensitive to aldosterone. The distal ends of several tubules empty into the collecting duct. The macula densa is a specialized region of the tubule occurring between the loop of Henle and the distal convoluted tubules. It senses the Na or C1 level in the lumen of the tubule and relays the information to nearby granulated cells located in the wall of the arteriole entering Bowman's capsule.

Renin is released from granulated cells located in the arteriole entering the glomerulus. The release of renin is controlled at two points. One point is directly sensitive to the blood pressure; the other is sensitive to the concentration of NaCl. The arteriole entering the glomerulus contains stretch receptors, called baroreceptors. The cells that constitute these receptors are subjected to stretch when the blood pressure increases. The stretch is relaxed with a drop in blood pressure. A decrease in stretch triggers the granule cells to secrete renin into the bloodstream. Recent evidence suggests that the baroreceptors are identical to granulated cells. The release of renin is also controlled by a signal from the macula densa. (Macula densa is Latin for “dark spot.”) The macula densa is part of the renal tubule. It is in close contact with the arteriole entering the glomerulus. Apparently, the macula densa can sense the concentration of Na or Cl passing into the distal tubule and respond by sending a signal to the granulated cells. A drop in salt concentration results in a signal that induces secretion of renin. It is thought that the signal arising from the baroreceptors is more important than that from the macula densa.

Classically, the renin/angiotensin system has been viewed as requiring the dispersal of these polypeptides in the general circulation, that is, systemically; however, recent work has revealed that angiotensinogen and renin are made in a variety of specialized regions of the body. The dot blot technique has revealed messenger RNAs coding for angiotensinogen and renin in these tissues. The dot blot technique has an advantage over techniques used for the direct detection of polypeptides or proteins. Techniques sensitive to polypeptides cannot distinguish whether the polypeptide found in a tissue originated in the tissue under scrutiny or arrived there from some other tissue via the bloodstream. Locally acting renin/angiotensin systems appear to be responsive to the diet. Ingelfinger et al. (1986) found that the amount of mRNA coding for angiotensinogen in the kidney was 3.5-fold greater with a low-salt diet than with a high-salt diet. This result, however, should not minimize the importance of the liver as the major source of the prohormone in the body.

4.3.2.6 Kidney

The functional unit of the kidney, the nephron, consists in a glomerulus and a long folded renal tubule (Fig. 4.3.13A). The glomerulus is composed of Bowman’s capsule and the glomerular tuft. Bowman’s capsule is covered by a thin layer of squamous epithelium known as the parietal epithelial cell. Tight junctions are made by the parietal epithelial cells to prevent the leakage of the glomerular filtrate into the perivascular space. The glomerular tuft is a microvascular bed that contains three types of cells, including the glomerular endothelial cell, the visceral epithelial cell, also known as podocyte, and the mesangial cell. During the glomerular development, the presumptive podocytes are connected by tight junctions, which are found near the apical membrane of the podocytes (Quaggin & Kreidberg, 2008). Mature podocytes are devoid of the tight junction but form a special cell junction known as the slit diaphragm (Grahammer, Schell, & Huber, 2013). The renal tubule can be divided into three major segments: the proximal tubule, including the proximal convoluted tubule and the proximal straight tubule (Fig. 4.3.13B), the loop of Henle, including the thin descending and ascending limb and the thick ascending limb (Fig. 4.3.13C), and the distal tubule, including the distal convoluted tubule and the collecting duct (Fig. 4.3.13D). Each tubular segment is responsible for reabsorbing a fraction of the glomerular filtrate. The epithelium lining the renal tubule is of simple cuboidal shape. Tight junctions are abundant in epithelia from all segments of the renal tubule (Hou, Rajagopal, & Yu, 2013). Illustrated are examples of the parietal epithelium from Bowman’s capsule (Fig. 4.3.14), the epithelium of the proximal tubule, the epithelium of the thick ascending limb of Henle’s loop, and the epithelium of the collecting duct (Fig. 4.3.15).

Histology

The functional unit of the kidney is the nephron. Each nephron is composed of a renal corpuscle (glomerulus within Bowman's capsule), a proximal tubule (convoluted and straight components), an intermediate tubule (loop of Henle), a distal convoluted tubule, a connecting tubule, and cortical, outer medullary, and inner medullary collecting ducts.1,2 A study of kidney organogenesis using unbiased stereological techniques found that the equine left kidney contains approximately 10 million glomeruli (for a total of 20 million in both kidneys) and, as in other species, the total number of glomeruli do not increase after birth despite continued growth of the kidney (with increasing glomerular size) until about 1 year of age.6 Histologically, equine nephrons are similar in most respects to those of other mammalian species; however, the diameter and epithelial height of the collecting duct segments are comparatively larger. In addition, the equine macula densa (segment of the ascending loop of Henle that lies in close association with the juxtaglomerular apparatus of the afferent arteriole) appears more prominent than that of other mammals.7 Whether these subtle histologic differences are accompanied by functional differences has not been investigated.

19.3.1.2 Nephron types and numbers

The structure of avian nephrons is highly heterogeneous. The smallest nephrons are located most superficially (Figure 19.1) and have simple glomeruli (Figure 19.2A). Because they resemble nephrons of reptilian kidneys, particularly in that they lack loops of Henle, these nephrons have been termed reptilian-type (RT) nephrons (Huber, 1917). Nephron size increases progressively with depth from the kidney surface. Those located most deeply have larger, more complex glomeruli (Figure 19.2B) and do possess a loop of Henle, leading to the name mammalian-type (MT) nephron. Between definitive RT and MT nephrons is a continuous gradation of nephrons (Goldstein and Braun, 1989; Roush and Spotts, 1988; Mobini and Abdollahi, 2016), including those with elongated, looping intermediate segments that are not bound into the medullary cones (Boykin and Braun, 1993). The RT/MT terminology has largely been replaced with the terms loopless (LLN), transitional (TN), and looped (LN) nephrons (Braun, 1993; Boykin and Braun, 1993). Avian glomeruli may have filtration barriers that are less restrictive than those of mammalian kidneys, potentially permitting larger and more highly charged molecules to enter the urine (Casotti and Braun, 1996).

Both nephrons with and those without loops of Henle empty in a highly regular pattern into common collecting ducts (Boykin and Braun, 1993). Collecting ducts conjoin as they descend through the medullary cone, and the terminus of the medullary cone is a single large collecting duct that empties directly into the ureter. The avian kidney has no renal pelvis.

Of the total nephron population, between 10 and 30% possess loops of Henle in most species examined (Goldstein and Braun, 1989). An exception is found in Anna's hummingbird, Calypte anna, whose kidneys, with more than 99% loopless nephrons, are capable of producing copious dilute urine in response to dilute nectar diets (Casotti et al., 1998). Several studies suggest habitat-related patterns in kidney structure, such as smaller kidneys, larger volume of medulla, or smaller volume of cortex in desert species (Thomas and Robin, 1977; Warui, 1989; Casotti and Richardson, 1992). Likewise, birds with greater osmoregulatory challenge (granivorous birds eating dry diets; passerines encountering high salt intake) may have greater percentages of medullary mass (Goldstein et al., 1990; Barceló et al., 2012; Peña-Villalobos et al., 2013). It is not yet clear how these variations relate to the different proportions of looped, transitional, and loopless nephrons.

4.1 Epithelial architecture of the kidney and location of primary cilia

The basic functional unit of the kidney is the nephron, consisting of a finely vascularized glomerulus connected to a tubular portion which has a number of distinct segments. The human kidney has in the order of a million nephrons (Bertram et al., 2011), each of which connects to an extensive collecting duct system and ultimately to the ureter. The vascular network of the glomerulus generates a filtrate from the blood which passes into the urinary space bounded by Bowman's capsule and into the tubule and duct where the final composition of urine is determined by the reabsorption of ions, proteins, and water back into the bloodstream. A single centrally located primary cilium 2–4 μm long is typically found on the apical surface of the epithelial cells lining the nephron (Bowman's capsule and the tubule) and collecting duct of the adult kidney in mammals (Webber and Lee, 1975; Fig. 6.2). An exception to this arrangement is a cell type known as the intercalated cell which is interspersed along the collecting duct and does not produce a cilium. The apical location of primary cilia on epithelial cells in the kidney means that they are in constant contact with the contents of Bowman's capsule, the tubule, and collecting duct. Structurally, renal primary cilia are unremarkable in that they are based on the standard 9 + 0 pattern of microtubules, are nonmotile, and do not possess any obvious structural modifications that provide clues to their function (Webber and Lee, 1975). Fish also have epithelial cells that bear cilia in their kidney, but these cilia have a 9 + 2 arrangement of microtubules and are highly motile (Kramer-Zucker et al., 2005). The significance of primary cilia in the kidney of mammals was unclear for many years. However, they have been the subject of intense investigation following the discovery that defects of this organelle can cause PKD (reviewed in Deane and Ricardo, 2007).