Introduction

The kidney is a structurally complex organ essential for human survival since its embryonic development. Every cell in the renal parenchyma is highly specialized in maintaining electrolyte, volume, and waste homeostasis.[1] Renal pathologies can be grossly categorized depending on the affected segment of the nephron: the glomerulus, tubules, interstitium, or blood supply (see Figure. Renal Corpuscle Structure, Nephron Histology). Each differs in its clinical manifestations, making it vital for the clinician to consider differential diagnoses. This topic examines renal histology, kidney function, and their correlation with clinical practice.

Issues of Concern

The kidneys are the primary organs responsible for blood filtration, water and electrolyte balance, and maintenance of blood pressure. They are essential to clear bacterial components and cytokines from the blood. In addition, kidney tissue is critically susceptible to immune-mediated disorders. Renal histology is highly specialized across nephron segments, providing crucial information for diagnosing various kidney diseases.[2]

Structure

Macroscopically, the kidney is divided into 2 regions: the renal cortex (outer) and the medulla (inner). Both contain nephron structures, the functional units of the kidney. It is crucial to understand the nephron’s structure to understand kidney function. 

The nephron is comprised of a glomerulus and a complex tubular system (see Image. Histology of the Cortex of a Kidney, Nephron Histology). The glomerulus and the first portion of the tubular system, known as the proximal convoluted tubule (PCT), are located in the renal cortex. Following the PCT, the loop of Henle, a hairpin-like structure, penetrates the medulla and returns to the cortex to connect with the distal convoluted tubule (DCT). Finally, the nephron drains into the collecting duct via connecting tubules.[3]

There are 2 types of nephrons: superficial nephrons with glomeruli near the cortical surface and short loops of Henle, and juxtamedullary nephrons with glomeruli located near the cortico-medullary junction and long loops of Henle descending deeper into the renal medulla.[4]

The glomerulus filters large volumes of blood, and the tubular system converts this filtrate into urine through reabsorption and secretion of water and solutes.[1]

The Glomerulus

The glomerulus forms by a tuft of capillaries surrounded by an impervious capsule, denominated Bowman’s capsule.[3] The glomerular capillaries are flanked by 2 resistance vessels, the afferent and efferent arterioles, regulating intraglomerular pressure. These capillaries have unique characteristics that allow them to filter large volumes of blood. The filtration barrier comprises 3 structures that provide the support and specific properties required to form the primary glomerular filtrate, the ultrafiltrate.

1. Fenestrated endothelium of the glomerular capillaries: this layer confers size selectivity through fenestrae with diameters between 70 to 100 nm. 
2. Glomerular basement membrane (GBM): This is a thick structure composed of extracellular proteins, including proteoglycans, laminin, fibronectin, and type IV collagen. This layer confers charge selectivity to the filtered particles. 
3. Podocytes: a visceral epithelium of specialized cells that lines the GBM and forms the outermost layer of the filtration barrier.[5] These create a slit diaphragm through interdigitating long and thin foot processes.[3] These cells maintain the integrity of the capillary loops.[5]

Once blood is filtered, the ultrafiltrate resides between the visceral epithelium and Bowman’s capsule. From here, the ultrafiltrate flows into the PCT.[3]

The Proximal Convoluted Tubule

Bowman’s capsule gives rise to the PCT, which lies adjacent to the glomerulus in the renal cortex. The PCT forms from simple cuboidal epithelium and is dedicated to absorbing and transporting water, electrolytes, and other solutes. These cells are characterized by a brush border of microvilli that increases the surface area in contact with the glomerular ultrafiltrate, abundant long, thin mitochondria lining the basal pole of the cell, and numerous vesicles involved in transcellular transport of 60-80% of the ultrafiltrate.[3]

The peritubular capillaries surround the PCT. This capillary network supplies the tubules and recovers reabsorbed free water, ions, and other plasma constituents, such as amino acids and glucose.[3] 

The Loop of Henle

The PCT leaves the renal cortex and becomes the thin descending limb (TDL) of the loop of Henle, which penetrates the renal medulla. The tubule becomes narrower, and the cells become smaller, with few mitochondria and short microvilli that are often not discernible on light microscopy.[3]

The tubule then ascends toward the cortex, becoming the thick ascending limb (TAL). Here, the lining cells become more prominent, with more numerous microvilli and mitochondria that facilitate active sodium transport to dilute the urine.[3]

The Juxtaglomerular Apparatus and the Distal Convoluted Tubule

The juxtaglomerular apparatus is the region that regulates glomerular filtration via the tubuloglomerular feedback.[6] Histologically, this region is near the vascular pole of the glomerulus. It comprises the macula densa cells of the cortical TAL and the granular smooth muscle cells of the afferent arteriole of the glomerulus, functionally and structurally connected by glomerular mesangial cells.[7] The macula densa cells are morphologically distinct tubule cells characterized by a dense region of tall cells.[3]

The TAL transitions to the DCT upon returning to the renal cortex near its glomerulus of origin.[3] The DCT comprises the nephron segment between the macula densa and the cortical collecting tubule (CCT).[4]

The DCT cells are tall and are noted for containing the highest number of mitochondria among other cells in the nephron. They have an extensive basolateral amplification that encloses multiple mitochondria, creating a palisading appearance at the basal pole of the cells.[4][8] Intercalated cells are present in the distal segment of the DCT and persist along the connecting and collecting tubules.[4]

Connecting and Collecting Tubules

The final part of the nephron is the connecting tubules, where the last fine-tuning of the urine occurs. These tubules contain 2 cell types: intercalated cells and connecting tubule (CNT) cells. The intercalated cells appear dense on electron microscopy and lack the basolateral amplification characteristic of DCT cells. These cells regulate hydrogen and bicarbonate secretion. The connecting tubule cells also have basolateral amplification but possess fewer mitochondria than DCT cells.[4]

The appearance of principal cells marks the transition into the collecting tubules and the end of the nephron. In cortical nephrons, the CNT leads to the collecting tubule, which drains to a collecting duct. The connecting tubules of juxtamedullary nephrons converge to form an arcade that drains into a common collecting duct.[4][8]

Function

The kidneys are responsible for several vital functions, including electrolyte and volume regulation, excretion of waste products, acid-base balance, synthesis of hormones such as erythropoietin, and metabolism of low-molecular-weight proteins. 

The Glomerulus

The kidneys receive approximately 20-25% of cardiac output, corresponding to 1,200 mL/min of renal blood flow (RBF) or 600 mL/min of renal plasma flow (RPF). The filtration fraction (FF) is the proportion of the RPF that passes into the renal tubules and is typically 20%. This means that the glomerular filtration rate (GFR) is 120 ml/min (180 L per day) in an average 60 kg person.[5]

The GFR is the product of the Ultrafiltration Coefficient (Kf) and the net filtration pressure (the change in P),

GFR = Kf (ΔP)

where ΔP represents the sum of the Starling forces across all capillary beds, and Kf is determined by the surface area available for filtration and the hydraulic conductivity of the glomerular capillary wall. Variation in any of the mentioned components may alter the GFR. 

In short, filtration at a single glomerulus occurs because of 4 major components: Kf; hydraulic pressure gradient, favoring passage of water and molecules; transcapillary oncotic pressure, favoring intravascular maintenance of free water and solutes; and the glomerular flow rate.[5]

The Proximal Convoluted Tubule

From the 160 to 180 L of ultrafiltrate produced per day, only 1.5 to 2 L of urine is excreted. Reabsorption of 60 to 65% of free water and NaCl occurs in the PCT. Additionally, most potassium, phosphate, and bicarbonate (HCO₃), as well as nearly all nutrients, such as glucose and amino acids, are reabsorbed in this segment. Solute and water reabsorption in the proximal tubule are isotonic, with minimal change in luminal osmolarity. This nephron site is also responsible for active solute secretion, hormone production, and renal gluconeogenesis.[9]

The Loop of Henle

Reabsorption of 30-40% of sodium occurs in this segment, with concomitant changes in urine osmolarity. The loop of Henle divides into 3 parts: the thin descending limb (TDL), the thin ascending limb (ATL), and the thick ascending limb (TAL).[10]

The TDL is permeable to water and small solutes. In contrast, the ATL and TAL are impervious to water but permeable to solutes. The furosemide-sensitive Na⁺-K⁺-2CL^- cotransporter (NKCC2) is located in the apical membrane of the TAL cells of juxtamedullary nephrons. These solutes are reabsorbed from the tubular fluid into the interstitium, increasing its osmolarity. This hypertonicity contributes to the free flow of water from the TDL into the renal interstitium. This process is known as the countercurrent mechanism. Urine becomes hypertonic as it passes through the TDL and hypotonic in the TAL, the diluting segment of the nephron. The reabsorbed water returns to circulation along the renal vasa recta.[11]

The Distal Nephron

The DCT is responsible for the fine-tuning of urine. It contributes 5 to 10% to the reabsorption of filtered sodium and chloride, as well as with K⁺ secretion. Like the loop of Henle, the DCT is impermeable to water, thereby further diluting the urine.[8] The following cluster of transporters accomplishes solute reabsorption: 

• Na⁺-K⁺ ATPase: It is expressed at the basolateral membrane of the distal nephron. It contributes to Na⁺ reabsorption in 2 ways: it maintains a low intracellular Na+ concentration and a high K+ concentration, and it generates an electronegative gradient across the cell membrane. The DCT is the nephron segment with the highest Na⁺ -K⁺ ATPase activity.[4]
• The thiazide-sensitive Na-Cl cotransporter (NCC) mediates the majority of Na⁺ and Cl^- reabsorption. The expression of NCC is limited to the DCT.[12]
• Amiloride-sensitive Na⁺ transporter (ENaC) and Renal outer medullary potassium channel (ROMK): ENaC generates an electrogenic gradient that mediates potassium secretion through ROMK. The more sodium is reabsorbed through ENaC, the more potassium is excreted through ROMK. Aldosterone, an adrenal hormone stimulated by hyperkalemia and hypovolemia, favors this process.[8]
• Acid-base ionic channels: H⁺ and HCO₃ are secreted by intercalated cells of types A and B, respectively, located in the collecting duct.[13]

Tissue Preparation

A kidney biopsy is the gold standard for diagnosing and managing multiple diseases. Since renal diseases may be secondary to evident causes and renal biopsy is an invasive test, its indications are limited. Ultrasound-guided percutaneous renal biopsy (PRB) is the most widely accepted and commonly used technique for renal biopsy.[14] The ideal sample for microscopy should contain 20 glomeruli in a native kidney biopsy and at least 10 glomeruli in a transplant kidney biopsy for diagnostic purposes. The kidney cortex contains glomeruli, whereas the medulla primarily consists of tubules. Hence, it is essential to obtain renal cortical tissue for analysis. However, in rare circumstances, medullary tissue can aid in the diagnosis of conditions such as BK virus nephropathy and antibody-mediated rejection in a transplanted kidney.[15] After obtaining kidney tissue, it is fixed and embedded in paraffin, then sectioned on a microtome for light microscopy, immunofluorescence, and electron microscopy.[16]

Microscopy, Light

There are 3 primary microscopy modalities of clinical relevance: light microscopy, immunofluorescence, and electron microscopy.[16]

Light microscopy  is the essential modality used on all tissue samples and provides descriptive information on existing lesions across different renal parenchyma segments; this aids clinicians in determining differential diagnoses, particularly in pathologies affecting renal glomeruli. Histologic description of glomerular pathologies includes terms such as “proliferative” when there is an increase in cell number, “sclerosing” when there is scarring, and “necrotizing” when there are areas of cellular death. Lesions are further described as diffuse or focal if more or fewer than 50% of all glomeruli are involved, respectively. Within an individual glomerulus, the process is considered global or segmental if more than 50% or less than 50% of the glomerular tuft is involved, respectively. Light microscopy can be further characterized by the stains used; below is a brief description of common light microscopy stains.

1. Haematoxylin-eosin staining for general evaluation.
2. Periodic Acid-Schiff stain (PAS) is widely used to evaluate glycogen storage disorders after kidney transplants to display tissue rejection (see Image. Normal Glomerulus, Periodic Acid-Schiff Stain).[17] 
3. Masson’s trichrome stain for the determination of renal fibrosis.[18]
4. Methenamine-silver stain (Jones) for better visualization of glomerular basement membranes.[19]

Fluorescence microscopy: As fluorescent dye-associated antibodies developed, immunofluorescence has revolutionized clinical nephrology and is particularly useful in determining the primary physiopathological mechanism generating a given renal lesion. This modality has helped guide the diagnosis of immune-mediated pathologies, as mentioned in other sections.

Microscopy, Electron

Electron microscopy has been widely used to diagnose common and uncommon nephrologic diseases, including minimal change disease, hereditary nephritis, fibrillary glomerulonephritis, and certain classes of lupus nephritis. This is 1 of the few medical disciplines in which electron microscopy plays an active role in clinical practice, and its findings are necessary for the final diagnosis. Therefore, it is generally recommended that renal biopsies preserve a portion of the specimen in an appropriate fixative for electron microscopy.[20][21]

Pathophysiology

Nephron pathologies are as complex as their structure. Each section of the nephron is susceptible to different forms of damage; for instance, glomerular diseases are often immunologically mediated, whereas tubular and interstitial disorders are more likely to be caused by toxic or infectious agents. However, a single disease can affect multiple structures, and the interdependence among kidney structures affects other components when only 1 part is damaged. Immune disorders affecting glomeruli can be either 1) mediated by antibodies against glomerular antigens, 2) mediated by complement, or 3) pauci-immune. The clinical manifestations and the microscopic appearance of the glomerulus depend on the mechanism of damage.[22][2]

Clinical Significance

Diseases affecting the glomerulus generally divide into 2 different entities according to the clinical presentation: 

• Nephrotic syndrome: This syndrome is characterized by proteinuria >3.5g per 24 hours or a protein-to-creatinine ratio >3000 mg/g, hypoalbuminemia <3g/dL, edema, and hyperlipidemia.[23]
• Glomerulonephritis or nephritic syndrome occurs when the patient presents with hypertension, hematuria, proteinuria (usually sub-nephrotic), and rapidly progressive azotemia.[24] 

Nephrotic Syndrome

Nephrotic syndrome may be primary or secondary. Primary nephrotic syndrome occurs when the kidney is the primary or sole affected organ. Secondary nephrotic syndrome is when systemic immunologic, metabolic, or vascular diseases affect the glomeruli. Out of these 2, secondary nephrotic syndrome is the most common.[25] 

Primary Nephrotic Syndrome

The following are some of the most common causes of primary nephrotic syndrome. 

• Minimal Changes are the most common cause of primary nephrotic syndrome in children. It is idiopathic in most cases, but it can also be associated with neoplasms, recent infections, or vaccination. It is characterized by normal glomerular appearance on H&E staining, negative immunofluorescence, absence of immune complex deposits, and effacement of foot processes on electron microscopy.[25] 
• Focal Segmental Glomerulosclerosis (FSGS) is the most common cause of primary nephrotic syndrome in Hispanics and African American adults. It is usually idiopathic (primary) but can be associated with HIV, sickle cell disease, or heroin use (secondary). Focal and segmental sclerosis on H&E stain, effacement of foot processes on electron microscope, and negative immunofluorescence characterize it. This type of glomerulopathy often progresses to chronic renal failure.[25] 
• Membranous nephropathy is the most common etiology of primary nephrotic syndrome in White adults. It is usually idiopathic (primary) but can be related to hepatitis, rheumatic diseases, neoplasias, or drugs (secondary). Microscopic characteristics include glomerular basement membrane thickening on H&E staining, subepithelial immune complex deposition with a “spike and dome” appearance on electron microscopy, and granular deposits on immunofluorescence. Similar to FSGS, membranous nephropathy often progresses to chronic renal failure.[25]

Secondary Nephrotic Syndrome

Secondary nephrotic syndrome can result from systemic immunologic diseases, such as systemic lupus erythematosus or vasculitis, metabolic diseases like diabetes, or vascular diseases like hypertension. The most common cause of secondary nephrotic syndrome is diabetes mellitus. In diabetes, hyperglycemia leads to glycosylation of the vascular basement membrane, causing hyaline arteriolosclerosis. The efferent arteriole is most commonly affected, thereby increasing glomerular filtration pressure and promoting hyperfiltration. This state eventually progresses to albuminuria, one of the earliest clinical markers of renal dysfunction. Histologically, it typically demonstrates mesangial sclerosis and Kimmelstiel-Wilson nodules.[25]

Nephritic Syndrome

As with nephrotic syndrome, nephritic syndromes can also be primary or secondary. Some of the most common causes of nephritic syndrome are post-infectious glomerulonephritis, IgA nephropathy, and lupus nephritis.

• Poststreptococcal nephritic syndrome arises 2 to 3 weeks after group A B-hemolytic nephritogenic streptococcal infection of the skin (impetigo) or pharynx. It usually occurs in children but can also occur in adults. No characteristic finding on H&E stain, but granular immunofluorescence pattern and subepithelial immune complex deposition (“humps”) on electron microscopy.[24] 
• IgA is the most common nephropathy worldwide. IgA immune complex deposition in the mesangium of glomeruli characterizes the glomeruli. It commonly presents with hematuria after mucosal infections, particularly gastroenteritis.[24]

A distinct presentation of nephritic syndrome is rapidly progressive nephritic syndrome. In this clinical scenario, patients progress to renal failure in weeks to months. It presents with characteristic crescents in the Bowman space on H&E staining. Crescents are an extra capillary proliferation of macrophages, fibroblasts, and epithelial cells, as well as fibrin deposition due to a rupture of the glomerular membrane, indicating a severe injury to the glomerular capillary wall. The differential diagnosis can be made based on histologic and immunofluorescence patterns on renal biopsy. Immunofluorescence patterns help identify the etiology. 

• Linear pattern: caused by anti-basement membrane antibodies, as is characteristic of Goodpasture syndrome.[24]
• Granular pattern: caused by immune complex deposition. This pattern can occur in post-streptococcal glomerulonephritis or diffuse proliferative glomerulonephritis.[24]
• Negative immunofluorescence: in some diseases, such as Wegener granulomatosis, microscopic polyangiitis, and Churg-Strauss syndrome, the tissue is negative to immunofluorescence. This condition, known as pauci-immune glomerulonephritis, is caused by antineutrophil cytoplasmic antibodies (ANCA).[24]

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Disclosure: Antonio Madrazo-Ibarra declares no relevant financial relationships with ineligible companies.

Disclosure: Pradeep Vaitla declares no relevant financial relationships with ineligible companies.