Kidney diseases

Glomerular structure and function

The formation of urine begins in the glomeruli, where the filtration of approximately 800 L of plasma each day results in 140 to 180 L of filtrate, most of which is reabsorbed by the tubules. Each glomerulus comprises a tuft of capillaries projecting into Bowman space ( Fig. 21.1 ). Blood enters and leaves the glomerular capillaries by arterioles, the efferent arterioles supplying blood to the peritubular capillaries. In contrast to all other systemic capillaries, hydrostatic pressure within the glomerular capillary remains high throughout its length, averaging 60 mm Hg, and thus enables efficient filtration.

The filtration barrier of the glomerular capillary wall is formed by a fenestrated endothelium, basement membrane and specialised epithelium (podocyte), and has two components.

• A charge-dependent barrier to anionic molecules, such as plasma proteins. This comprises polyanionic glycosaminoglycans, such as heparan sulphate and sialoproteins, in the basement membrane and fenestrae of the endothelial cells.

• A size-dependent barrier for large molecules which are neutral or cationic, comprising proteins of the basement membrane (collagen type IV, laminin, fibronectin) and the filtration slit diaphragm between foot processes of podocytes.

This filtration barrier allows the movement of water, electrolytes and small molecules into Bowman space whilst retaining macromolecules, such as most proteins, within the plasma. The integrity of the filtration barrier is disturbed in many glomerular diseases, particularly those associated with injury to the podocyte. The interdigitating foot processes of podocytes envelop the capillary loops. Modified adherens-type junctions (filtration slit diaphragms) occur where the foot processes meet and are essential for filtration function (see Fig. 21.1 ). The integrity of the slit diaphragm is maintained by the complex interrelationship of many proteins, including nephrin, podocin and CD2-associated protein. Other proteins, such as integrins, span the membrane and anchor the actin cytoskeleton to the basement membrane. Defects in the genes encoding proteins of the slit diaphragm result in simplification of the foot processes, and loss of selective filtration leading to proteinuria.

The glomerular capillary tufts are supported centrally by specialised pericytes, called mesangial cells, surrounded by a loose matrix. Mesangial cells contain actin filaments and are contractile. They attach to the capillary basement membrane at the point where it is reflected over the mesangial matrix, thus anchoring the capillary to the central structure. Contraction of mesangial cells pulls on the glomerular basement membrane and will alter the shape and calibre of the capillary. Mesangial cells are also phagocytic, processing immune complexes that are deposited within the glomerulus. In response to injury, activated mesangial cells secrete cytokines, proliferate and synthesise new matrix.

Glomerular filtration rate

Blood flow through the kidneys produces on average 180 L/day (125 mL/min) of ultrafiltrate, which is termed the GFR. The GFR reflects the permeability of the capillary wall, together with the hydrostatic and osmotic gradients between the capillary lumen and Bowman space. The GFR is modified by three important mechanisms, all of which are closely interrelated and involve the juxtaglomerular apparatus (JGA):

• autoregulation within the glomerulus

• tubuloglomerular feedback

• neurohormonal influences.

The JGA, situated at the hilum of the glomerulus, comprises cells in the media of the afferent arteriole that secrete renin, the juxtaglomerular cells , and modified tubular cells of the thick loop of Henle, the macula densa ( Fig. 21.2 ), and enables autoregulation and tubuloglomerular feedback. The specialised cells of the macula densa monitor the level of chloride in the tubular luminal fluid, reflecting the amount of chloride reabsorbed by the tubule. A reduced GFR leads to a fall in the luminal chloride level. This results in dilatation of the afferent arteriole, together with constriction of the efferent arteriole, resulting from the release of renin. These two changes increase the hydrostatic pressure within the glomerular capillary and restore the GFR.

Neurohormonal mechanisms involve extrarenal baroreceptors and renal sympathetic nerves. A drop in systemic arterial pressure leads to sympathetic stimulation of the juxtaglomerular cells and increased renin production. Renin acts as an enzyme, converting angiotensinogen to angiotensin I. This is converted to angiotensin II by angiotensin converting enzyme, present on capillary endothelium. Angiotensin II constricts the efferent more than the afferent arteriole, which preserves the GFR, and also causes the adrenal cortex to produce aldosterone which, in turn, leads to increased reabsorption of sodium by the distal tubular epithelium.

The glomerular filtrate, which is isotonic with the plasma, has to be substantially modified osmotically so that water and electrolytes are conserved and the waste metabolites are concentrated. This occurs as the filtrate flows through the tubules (see Fig. 21.2 ).

Tubular structure and function

Epithelial cells modify the filtrate by transferring electrolytes and solutes aided by a series of carrier proteins or transporters within the apical (luminal) cell membrane. Transfer from the cytoplasm to the interstitial and peritubular fluid is performed by an energy-dependent adenosine triphosphatase pump situated on the basolateral membrane of the cell. The epithelial cells are separated from each other by tight junctions that contain claudins, membrane proteins that prevent the unregulated passage of electrolytes, water and solutes through the epithelial layer between the cells.

In the proximal tubule , approximately 50% to 55% of the sodium in the filtrate is reabsorbed through selective sodium transporters, together with specific transmembrane co-transporters linked separately to glucose, phosphate or amino acids. In this way, nearly all of the glucose, phosphate and amino acids are reabsorbed by the proximal tubule, thus altering the osmolality of the tubular fluid and causing water to flow into the cytoplasm through specialised water channels termed aquaporins. Some of the sodium transporters are linked with hydrogen exchange, whereby sodium is reabsorbed and hydrogen is excreted. Consequently, 80% of all the bicarbonate filtered is reabsorbed by the proximal tubules.

The loop of Henle , situated in the medulla and doubling back on itself, is the next part of the nephron through which the now reduced volume of the filtrate must pass. The two limbs have quite different physiological properties. The descending loop is permeable to water but not to ions, whereas the ascending limb is permeable to ions but, lacking aquaporins, is impermeable to water. Thus the interstitium of the medulla becomes hypertonic. The filtrate in the loop lumen equilibrates with this, because of the permeability to water in the descending limb.

The distal tubule is continuous with the ascending limb of the loop of Henle. The epithelial cells of this segment lack aquaporins, making this segment impermeable to water. Sodium and chloride are reabsorbed by a co-transporter, the activity of which is governed by the concentration of chloride in the luminal fluid. Transport of sodium and chloride in the loop of Henle and distal convoluted tubule is flow dependent, an important concept in the context of understanding the action of loop diuretics which tend to increase the rate of flow. Calcium transport, under the influence of parathyroid hormone (PTH) and 1,25-dihydroxycholecalciferol (vitamin D ₃ ), occurs in the distal convoluted tubule and adjacent segments.

The distal convoluted tubule continues into the collecting duct which contains two main cell types:

• principal cells found mainly in the cortical collecting duct and inner medullary collecting duct are concerned with sodium and water reabsorption, both of which are influenced by hormones

• intercalated cells are found in the cortex and outer medulla and are involved with acid-base balance.

Aldosterone increases the number of open sodium channels, thus increasing the reabsorption of sodium in the event of volume depletion. The principal cells of the collecting ducts are relatively impermeable to water due to the paucity of aquaporins on the apical membrane. However, under the influence of antidiuretic hormone (ADH) produced by the pituitary, a complex sequence of changes occurs within the cell. This culminates in the fusion of intracytoplasmic vesicles containing preformed aquaporins with the apical membrane so that water can be cleared into the circulation.

The intercalated cells are concerned with hydrogen ion excretion. The excreted hydrogen combines with ammonia in the lumen to form ammonium. Ammonia, formed in the proximal tubule by the metabolism of glutamine and by diffusion from the interstitial fluid, is freely diffusible, in contrast to ammonium which is lipid insoluble and cannot pass back into the tubular cytoplasm.

The vasa recta are a delicate meshwork of capillaries that invest the tubules and are derived from the efferent glomerular arteriole. The configuration of the vascular network complements that of the tubule and plays an integral role in the functioning of the countercurrent mechanism.

The countercurrent mechanism ensures urine of variable osmolarity forms in response to a variable water intake. The hairpin configuration of the loop of Henle, the complementary vasa recta, coupled with the selective permeabilities to ions and water of the different segments of the loop, the distal tubule and collecting tubules, are all pivotal to the countercurrent mechanism. The active transport of sodium by the thick ascending limb increases the osmolarity of the interstitium. As a result of this, water diffuses from the filtrate in the lumen of the descending limb, which is permeable to water but not to ions. With progress towards the tip of the loop, osmolarity of the filtrate and interstitium increases, particularly in the longer loops derived from the juxtamedullary glomeruli. The principal cells of the collecting tubules display a variable permeability to water under the influence of ADH, achieving urine of variable osmolarity by passing through this hyperosmolar environment on the way to the papillae.

Tamm–Horsfall protein (uromodulin) is a large mucoprotein produced exclusively by the cells of the thick ascending limb of the loop of Henle. Its functions are not fully understood; it may have a role in water and electrolyte balance in the thick ascending limb and there is evidence that it protects against urinary tract infections and stone formation. It is the main constituent of tubular casts. Mutations in the gene encoding uromodulin are associated with medullary cysts, hyperuricaemia and progressive renal failure.

The collecting ducts open onto the surface of the renal papillae , projecting into the calyces. The shape of the duct orifice is relevant to the development of intrarenal reflux of urine and ascending bacterial infection (pyelonephritis). Two patterns have been described.

• In the mid-zone papillae , the ducts open obliquely onto the surface. In the event of urinary reflux from the bladder, these duct orifices will close under the increased pressure in the pelvicalyceal system, acting effectively as a one-way valve.

• In contrast, the polar papillae are more frequently compound. These are formed as a result of fusion of lobes of renal parenchyma during fetal development. They have a flattened or slightly depressed summit. The collecting ducts in this central area open vertically onto the surface of the papilla; they have no valve effect and remain widely patent, thus allowing the refluxed urine and any bacteria within it to flow into the kidney.

Clinical manifestations of renal disease

Damage to the kidneys will affect one or more of the functions described above, depending on the target of the injury, resulting in predictable clinical effects, as shown in Table 21.2 .

The clinical symptoms of loss of kidney function depend on (1) the severity of renal failure and (2) the rate of decline of renal function. Acute renal failure is a life-threatening emergency as a result of biochemical derangements, including hyperkalaemia and metabolic acidosis. A reduced urine output may be noted, with fluid retention producing oedema. The metabolic acidosis results in an increased respiratory rate.

In chronic kidney disease (CKD), there is time for compensatory mechanisms to limit these biochemical abnormalities, and clinical features reflect those conditions that develop slowly, over months and years, including anaemia, bone disease and complications of hypertension. In mild CKD with a GFR of more than 15% of normal, there are usually no symptoms. With further loss of function the severity of symptoms increases. When GFR reaches less than 5% of normal, there is severe illness and death without treatment.

Investigation of renal disease

Investigations that are frequently performed in renal disease are summarised in Table 21.3 . These include biochemistry, haematology, immunology, imaging, urinalysis and histology.

Renal excretory function : a common measure of renal excretory function is serum levels of urea and creatinine. An elevated level of waste products as a result of renal failure is known as uraemia . However, there are confounding factors that make simple measurement of serum levels of these molecules unreliable indicators of renal function. For example, urea is low in liver disease and elevated in dehydration. Creatinine is a product of muscle cells and is raised in individuals with muscular hypertrophy or following muscle injury. Conversely, in individuals with significant renal impairment, creatinine may be normal if there is also muscle wasting. Creatinine clearance (urine concentration × volume of urine/plasma concentration) is therefore a better measure of GFR. However, it is difficult to perform accurately and therefore GFR is usually estimated from the serum creatinine level using a formula taking into account sex, age and race.

Urinalysis : many renal parenchymal diseases result in blood and protein in the urine. The presence of red cell casts (formed in the renal tubules) distinguishes glomerular haematuria from lower urinary tract bleeding. Leucocytes, nitrites and bacteria are indicative of urinary tract infection.

Imaging : renal ultrasound is a rapid noninvasive investigation that can provide information on the size and position of the kidneys, and on the presence of scarring, stones, obstruction and tumours. Other imaging modalities used less frequently are listed in Table 21.3 .

Renal biopsy : renal histology is essential for the diagnosis of many renal parenchymal diseases. The first systematic account of using transcutaneous needle biopsy in the investigation of kidney disease was in 1951. Following its introduction, there was a rapid increase in our understanding of renal histopathology and the relationship between morphological changes and clinical disease. Much of the following account of renal pathology is based on information derived from renal biopsies. Their examination requires use of a number of techniques.

• Light microscopy (LM) : in addition to routine haematoxylin and eosin (H&E), other stains are routinely used to reveal histological details. For example, periodic acid–Schiff (PAS) and methenamine silver stains highlight basement membrane abnormalities, trichrome stains demonstrate interstitial fibrosis and Congo red stain is used to demonstrate amyloid deposits.

• Immunohistology : immune deposits and other molecules may be detected in tissue sections by applying antiimmunoglobulin (anti-Ig) or anticomplement antibodies labelled with a fluorescent marker (immunofluorescence, IF) or revealed by an enzymatic reaction with a coloured product (immunohistochemistry, IH). In most conditions, these techniques demonstrate granular positivity within immune deposits, an exception being antiglomerular basement membrane (anti-GBM) disease, in which an autoantibody to collagen type IV is revealed by linear positivity of the basement membrane on IF using a fluorescein-labelled anti-IgG ( Fig. 21.3 ).

  

• Electron microscopy (EM): this is required to demonstrate ultrastructural details, such as the precise location of immune deposits, cytopathic changes and basement membrane abnormalities. Using EM, immune complexes appear as electron-dense deposits. These may be within the mesangium, between the endothelial cell and the basement membrane (subendothelial) or between the podocyte and basement membrane (subepithelial). The type and location of these deposits are frequently diagnostic.

Glomerulonephritis

This is a term used to describe a group of conditions in which glomerular injury is mediated by immune responses or abnormalities of the complement system. Despite its name (the suffix -itis means inflammation), there may be little or no evidence of glomerular inflammation on histology. The glomerulonephritides are diverse in their aetiology, histology and clinical features. There are, however, links between the target of injury, histological changes and clinical characteristics ( Fig. 21.4 ).

Glomerulonephritis can be divided into three broad groups.

• Conditions that specifically damage the glomerular permeability barrier (e.g. minimal change disease [MCD], membranous glomerulonephritis [MGN]). These produce heavy proteinuria and the nephrotic syndrome, with little or no evidence of inflammation on histology.

• Conditions that produce severe necrotising glomerular injury (e.g. vasculitis, anti-GBM disease). These cause leakage of blood and protein into the urine and a rapid reduction in GFR, resulting in acute renal failure (RPGN). The morphology in these cases is of a crescentic glomerulonephritis, the crescents (a proliferation of cells within Bowman space) being a response to rupture of capillaries with exudation of fibrin and cytokines. The common causes of ‘crescentic glomerulonephritis’, and their diagnostic histological features, are summarised in Table 21.6 .

  Table 21.6

  Differential diagnosis and features of crescentic glomerulonephritis

  	Light microscopy	Immunofluorescence	Serology
  Anti-GBM disease	Global glomerular necrosis with synchronous crescents	Linear IgG and C3 in glomerular basement membranes	Anti-GBM antibody
  Vasculitis	Focal segmental necrosis with no proliferation away from the segmental lesions, ± arteritis	Negative or scanty (pauci-immune)	ANCA
  Immune complex GN	Mesangial and endocapillary hypercellularity	Various, e.g. full house in lupus, IgA in IgA nephropathy	Various, e.g. lupus serology, cryoglobulins

  ANCA , Antineutrophil cytoplasmic antibody; anti-GBM , antiglomerular basement membrane; GN , glomerulonephritis; Ig , immunoglobulin.

• Conditions associated with glomerular inflammation, usually in association with mesangial and/or subendothelial immune deposits (e.g. lupus nephritis, IgA nephropathy [IgAN]). The clinical and morphological features of this group are highly varied, depending on the severity and site of inflammation. Glomerular inflammation typically manifests clinically as the nephritic syndrome but, if severe, may cause acute renal failure with glomerular crescents seen on histology. These conditions may also damage the permeability barrier, producing heavy proteinuria and the nephrotic syndrome.

Whatever the initial insult, if injury is severe or persistent, there is irreversible damage to the glomerular tuft with a healing response resulting in sclerosis. Once a glomerulus is sclerosed, the remainder of the nephron undergoes atrophy. The clinical consequence of progressive glomerulosclerosis and tubular atrophy is chronic renal failure.

What is a diagnosis and what is simply a morphological abnormality seen on renal biopsy? For glomerulonephritis, the answer is that there is a spectrum from a nonspecific morphological label at one end, to a true diagnosis at the other. Most glomerular ‘diagnoses’ lie somewhere along the spectrum. Confusingly, one term, such as ‘focal segmental glomerulosclerosis (FSGS)’, may be used simply as a morphology that is seen in many conditions, but also, in the correct clinical context, as a diagnosis. For purposes of patient management, the diagnosis may be less important than the morphology on renal biopsy. For example, the decision whether or not to give immunosuppressive therapy is frequently informed by the renal biopsy findings. In such instances, the renal pathologist is asked to provide quantitative information on ‘active’ inflammatory lesions that may respond to immunosuppression, and ‘chronic’ sclerosing lesions that will not.

Mechanisms of glomerular injury

Glomeruli can be damaged by immunological or nonimmunological mechanisms.

Glossary of terminology

• Distribution of glomerular lesions:

  • Diffuse : involving most (>50%) of glomeruli

  • Focal : involving a minority (<50%) of glomeruli

  • Global : most or all of a glomerulus involved. Definitions vary from involving most (>50%) of a glomerular tuft for cellular lesions, or the entire glomerular tuft (100%) for sclerosing lesions

  • Segmental : part of a glomerulus involved. Definitions vary from involving less than 50% of a glomerulus for cellular lesions, or less than 100% of a glomerulus for sclerosing lesions.

• Proliferative lesions:

  • Mesangial proliferation : increased mesangial cellularity (>3 mesangial cells/mesangial area) with patent capillary loops

  • Endocapillary proliferation : increased cellularity within glomerular capillaries, due to marginating neutrophils or monocytes, or endothelial cell proliferation

  • Extracapillary proliferation (cellular crescent) : increased cells within Bowman space, usually due to epithelial cell proliferation and infiltrating macrophages.

• Sclerosing lesions:

  • Glomerulosclerosis : obliteration of capillaries by matrix (subdivided as above into global and segmental sclerosis)

  • Mesangial sclerosis : an increase in mesangial matrix with patent capillary loops

  • Nodular sclerosis : a nodular increase in mesangial matrix with patent capillary loops.

Minimal change disease

MCD is a term that reflects the subtle abnormalities of glomerular morphology in this condition. At LM, glomeruli appear normal, with changes only evident at the ultrastructural level.

Clinical features : MCD is the most common cause of nephrotic syndrome in children, but may occur at any age. The proteinuria is typically massive, abrupt in onset and selective, with urinary loss of albumin but not larger globulins. The patient is typically well before onset of nephrotic syndrome, but infrequently there is an association with Hodgkin disease and sensitivity reactions to drugs and venom. Over 90% of patients respond to steroid therapy, although relapses of nephrotic syndrome following discontinuation of therapy are common. Some patients become steroid-dependent but progression to chronic renal failure is very infrequent, and those that do progress usually suffer from FSGS, the diagnostic lesions being absent in the original biopsy. The diagnosis of MCD is frequently assumed in children presenting with the above clinical features and they are treated with steroids without renal biopsy. The clinical term steroid sensitive nephrotic syndrome is then used to describe this condition.

Pathogenesis : the pathogenesis of podocyte injury in MCD is uncertain. There is some evidence that permeability factors, including angiopoietin-like-4, mediate proteinuria.

Morphology : glomeruli are normal by LM. Proximal tubules show resorption droplets of proteins and lipids that are present in the glomerular filtrate. This is the basis of the old name for this condition, lipoid nephrosis. At EM, glomerular podocytes show diffuse effacement of foot processes, and microvillous change. The extent of foot process effacement matches the severity of proteinuria; clinical response to steroid therapy is accompanied by resolution of the podocyte changes.

Focal segmental glomerulosclerosis

FSGS is a term used to describe a histological abnormality, focal segmental sclerosis, which is seen in many glomerular diseases (e.g. IgAN, lupus nephritis, vasculitic glomerulonephritis) and is also associated with hyperfiltration injury to glomeruli. However, as a diagnosis, it also refers to conditions in which the primary injury is to the podocyte, the podocytopathies. In this context, it is a clinicopathological diagnosis requiring: (1) heavy proteinuria/nephrotic syndrome; (2) focal segmental sclerosing lesions on LM; (3) absence of immune deposits on IF; and (4) evidence of podocyte injury on EM. It is FSGS defined in this way that is described in this section.

Clinical features : FSGS presents with heavy proteinuria/nephrotic syndrome. It may occur at any age. In contrast to MCD, proteinuria is unselective. Response to steroid therapy is less frequent than in MCD and most patients develop renal failure, although rate of progression is variable. Renal survival is 50% at 7 years following presentation. Initial response of proteinuria to steroids is the best predictor of long-term outcome. An aggressive form of FSGS, collapsing glomerulopathy, is seen particularly in patients of African ethnicity and in association with HIV infection.

Pathogenesis : in approximately 80% of patients (‘primary FSGS’), the mechanism of podocyte injury is uncertain. There is, however, experimental evidence for a plasma factor that induces greater permeability of the glomerular filtration barrier to macromolecules. A candidate permeability factor is soluble urokinase receptor that interacts with β ₃ integrin on podocyte foot processes. In 20% of patients with FSGS, there are other identifiable underlying causes for the injury. These include drugs, infections and mutations of the proteins of the podocyte slit diaphragm ( Fig. 21.5 ). Variants of the APOL1 gene, which encodes apolipoprotein L1, is a major risk factor for FSGS in individuals of black African descent. The effect of carrying two APOL1 risk alleles explains 18% of FSGS and 35% of HIV-associated nephropathy.

Morphology : there is segmental obliteration of glomerular capillary tufts by sclerosis, frequently accompanied by endocapillary foamy macrophages and hyalinosis ( Fig. 21.6 ). Podocytes may be prominent over the segmental lesions and contain protein resorption droplets, particularly in collapsing glomerulopathy that is characteristic of HIV-associated nephropathy (see Fig. 21.6 ). IF is negative other than nonspecific trapping of IgM and C3 in the segmental lesions. EM shows podocyte injury with foot process effacement.