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You are watching: Which of the following is not a characteristic of the cortical nephrons?

StatPearls . Treasure Island (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

The kidney is a structurally facility organ essential for human survival because its embryonic development.<1> Every cabinet in the renal parenchyma is highly committed in maintaining electrolyte, volume, and also waste homeostasis. Renal pathologies deserve to be grossly categorized depending on the influenced segment the the nephron: the glomerulus, tubules, interstitium, or blood supply. Each one differs in clinical manifestations, do it vital for the clinician to incorporate differential diagnoses. This article will cover renal histology, kidney function, and their correlation through clinical medicine.


Structure

Macroscopically, the kidney divides into two sections: the renal cortex, the outer part of the kidney, and the medulla, the within section. Both contain different structures the the nephron, the functional unit the the kidney. It is vital to comprehend the nephron’s structure to recognize the to work of the kidney. 

The nephron is consisted of of a glomerulus and also a complicated tubular system. The glomerulus and also the an initial portion of the tubular system, well-known as the proximal convoluted tubule (PCT), are located in the renal cortex. Adhering to the PCT, the loop of Henle, a hairpin-like structure, penetrates the medulla and returns come the cortex to affix with the distal convoluted tubule (DCT). Finally, the nephron drains into the collecting duct via connecting tubules.<2>

There are two species of nephrons: (1) superficial nephrons through their glomeruli located near the cortical surface and short loops that Henle, and also (2) juxtamedullary nephrons with glomeruli situated near the cortico-medullary junction and long loops that Henle descending deeper into the renal medulla.<3>

The glomerulus filters big amounts of blood, which the tubular mechanism converts into urine through reabsorption and also secretion of totally free water and also solutes.<1>

The Glomerulus

The glomerulus develops by a tuft that capillaries surrounded by one impervious capsule denominated Bowman’s capsule.<2> The glomerular capillaries are flanked by 2 resistance vessels, the afferent and efferent arterioles, regulating intraglomerular pressure. These capillaries have actually unique attributes that enable them to filter big volumes that blood. The filtration barrier is composed of three structures that carry out the support and also selective properties needed for the development of the primary glomerular filtrate, the ultrafiltrate.


Fenestrated endothelium of the glomerular capillaries: this layer confers size selectivity with fenestrae with diameters between 70 to 100 nm. 
Glomerular basement membrane (GBM): this is a thick structure composed of extracellular proteins, consisting of proteoglycans, laminin, fibronectin, and form IV collagen. This great confers charge selectivity come the filtered particles. 

Once blood is filtered, the ultrafiltrate resides between the visceral epithelium and also Bowman’s capsule. From here, the ultrafiltrate flows into the PCT.<2>

The Proximal Convoluted Tubule

Bowman’s capsule offers rise come the PCT, i m sorry lies nearby to the glomerulus in the renal cortex. The PCT forms from straightforward cuboidal epithelium devoted to the absorption and transport of water, electrolytes, and other particles. This cells are characterized by a brush border of microvilli designed to increase the surface in call with the glomerular ultrafiltrate, with numerous long, slim mitochondria lining the basal pole of the cell; and also numerous vesicles connected in transcellular transport of 60 come 80% that the ultrafiltrate.<2>

The peritubular capillaries surround the PCT. This capillary network is responsible for the blood it is provided of the tubules as well as the recovery of the reabsorbed free water, ions, and other plasma constituents like amino acids and glucose.<2> 

The Loop the Henle

The PCT pipeline the renal cortex and also turns into the slim descending body (TDL) the the loop the Henle penetrating the renal medulla. The tubule becomes narrower, and also the cells end up being smaller, with few mitochondria and short microvilli often unnoticeable on irradiate microscopy.<2>

The tubule then provides a rotate upward towards the cortex, turning into the special ascending limb (TAL). Right here the lining cells become larger with an ext numerous microvilli and also mitochondria to communicate in the energetic transport of salt to dilute the urine.<2>

The Juxtaglomerular Apparatus and the Distal Convoluted Tubule

The juxtaglomerular device is the region in fee of regulating the glomerular filtration v the tubuloglomerular feedback.<5> Histologically, this an ar is near the vascular pole the the glomerulus. That is consisted of of the macula densa cells of the cortical TAL and also the granular smooth muscle cell of the afferent arteriole the the glomerulus, functionally and also structurally linked by glomerular mesangial cells.<6> The macula densa cells room morphologically unique tubule cells characterized by a dense region of high cells.<2>

The TAL turns right into the DCT after return to the renal cortex near its glomerulus that origin.<2> The DCT comprises the nephron segment in between the macula densa and also the cortical collecting tubule (CCT).<3>

The DCT cells space tall cells noteworthy for containing the largest number of mitochondria amongst other cells in the nephron. They have comprehensive basolateral amplification enclosing multiple mitochondria, producing a palisading illustration in the basal part of the cells.<3><7> Intercalated cells start to appear in the latter segment that the DCT and remain transparent the connecting and collecting tubules.<3>

Connecting and Collecting Tubules

The final part of the nephron is the connecting tubules, whereby the last fine-tuning the the to pee occurs. This tubules have actually two types of cells; the intercalated cells and the connecting tubule (CNT) cells. The intercalated cells appear dense top top electron microscopy and do not have the basolateral amplification properties of the DCT cells. This cells regulate hydrogen and also bicarbonate secretion. The connecting tubule cells additionally have basolateral amplification, however they possess fewer mitochondria 보다 DCT cells.<3>

The illustration of primary cells marks the shift into the collecting tubules and also the finish of the nephron. In cortical nephrons, the CNT leader to the collecting tubule, which drains to a collecting duct. The connecting tubules of juxtamedullary nephrons join and kind an arcade that drains right into a mutual collecting duct.<3><7>


Function

The kidneys space responsible for several critical functions, including electrolyte and volume regulation, excretion of waste products, acid-base balance, synthesis of hormone such together erythropoietin, and also metabolism of short molecular weight proteins. 

The Glomerulus

The kidneys obtain from 20 to 25% of the cardiac output; this is around 1,200 ml/min of renal blood flow or 600 ml/min of renal plasma circulation (RPF). The filtration portion (FF) to represent the relationship of the RPF the passes into the renal tubules and also is typically 20%; this means that the glomerular filtration price (GFR) is 120 ml/min (180 L every day) in an typical 60 kg person.<4>

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

GFR  = Kf (ΔP)

where ΔP to represent the amount of the Starling forces throughout all capillary beds, and also Kf is determined by the surface ar area obtainable for filtration and also the hydraulic conductivity that the glomerular capillary wall. Sport in any kind of of the mentioned contents may change the GFR. 

In short, filtration at a single glomerulus occurs due to the fact that of four major components: (1) Kf; (2) hydraulic pressure gradient, favoring passage of water and molecules; (3) transcapillary oncotic pressure, favoring intravascular maintenance of free water and also solutes; and also (4) the glomerular circulation rate.<4>

The Proximal Convoluted Tubule

From the 160 to 180 together of ultrafiltrate developed per day, just 1.5 to 2 together of to pee is excreted. Reabsorption that 60 to 65% of complimentary water and NaCl occurs in the PCT. Additionally, many potassium, phosphate, and HCO3, and nearly all nutrients, such together glucose and amino acids, space reabsorbed in this segment. The solute and water reabsorption in the proximal tubule is isotonic, with a minimum change in luminal osmolarity. This site of the nephron is also responsible for active solute secretion, hormone production, and also renal gluconeogenesis.<8>

The Loop of Henle

Reabsorption the 30 to 40% of sodium occurs in this segment through important changes in to pee osmolarity. The loop the Henle divides right into three segments: (1) the slim descending limb (TDL), (2)  the ascending thin limb (ATL), and (3) the special ascending limb (TAL).<9>

The TDL is permeable to water and little solutes. In contrast, the ATL and TAL space impervious come water but permeable to solutes. The furosemide-sensitive Na+-K+-2CL- cotransporter (NKCC2) is situated in the apical membrane that the TAL cells of juxtamedullary nephrons. These solutes space reabsorbed from the tubular fluid into the interstitium, raising its osmolarity. This hypertonicity contributes come the flow of cost-free water native the TDL into the renal interstitium. This procedure is well-known as the countercurrent mechanism. Urine becomes hypertonic as it passes with the TDL and hypotonic in the TAL, the diluting segment that the nephron. The reabsorbed water returns to the circulation along the vasa recta.<10>

The Distal Nephron

The DCT is responsible because that the fine-tuning the urine. It contributes 5 to 10% in the reabsorption of filtered sodium and chloride, as well as with K+ secretion. Just as the loop that Henle, the DCT is water-impermeable, more diluting the urine.<7> The complying with cluster the transporters perfect solute reabsorption:  


Tissue Preparation

A kidney biopsy is thought about the gold typical for diagnosing controlling multiple diseases. Because renal illness may be an additional to noticeable causes and also renal biopsy is an invasive test, the indications space limited. Ultrasound-guided percutaneous renal biopsy (PRB) is the many accepted and also commonly used method to carry out a renal biopsy.<13>

The appropriate sample because that microscopy have to contain 20 glomeruli because that a native kidney biopsy and also at the very least ten glomeruli in a transplant kidney biopsy because that diagnosis. The kidney cortex consists of glomeruli, and also the medulla primarily has tubules. For this reason it is essential to acquire renal cortical organization for analysis. However, medullary tissue is useful in diagnosis choose BK virus nephropathy and antibody-mediated denial in a transplanted kidney in rare circumstances.

After obtaining kidney tissue, that is fixed and processed v a microtome right into thin sections and processed for light microscopy (LM), immunofluorescence (IF), and electron microscopy (EM).


Microscopy Light

As previously mentioned, many pathologic procedures can impact distinct areas of the nephron come a differing extent. As such, knowledge of the varied lesion trends that room observable in renal organization through various microscopic methods is of great importance.

There are three major microscopy modalities of clinical relevance are light microscopy (LM), immunofluorescence (IF), and electron microscopy (EM)


Light microscopy: It is the vital modality used on all tissue samples and provides descriptive information concerning existing lesions in various renal parenchyma segments—this aids clinicians in determining differential diagnoses, specifically in pathologies influence renal glomeruli. Histologic summary of glomerular pathologies has terms such together “proliferative” once there is an increase in the variety of cells, “sclerosing” as soon as there is scarring, and “necrotizing” when there are locations of moving death. Lesions space further defined as diffusive or focal if much more or less than 50% of all glomeruli room involved, respectively. In an individual glomerulus, the procedure is considered global or segmental if more or much less than 50% of the glomerular tuft is involved. LM can be further characterized based on the stains used, listed below is a brief description of miscellaneous LM stains.
Fluorescence microscopy (IF): as the development of fluorescent dye-associated antitoxin expanded, IF has revolutionized clinical nephrology and also is specifically useful in identify the main physiopathological mechanism generating a offered renal lesion. This modality has been useful to guide the diagnosis that immune-mediated pathologies, as pointed out in further sections.

Microscopy Electron

Though rarely easily accessible outside the academic/specialized centers, electron microscopy (EM) is necessary for diagnosing many common and uncommon conditions (e.g., slim basement membrane nephropathy) in nephrology. This is just one of the few medical self-controls in i beg your pardon EM has an active duty in clinical practice.


Pathophysiology

Nephron pathologies room as facility as that is structure. Each particular section the the nephron is prone to different mechanisms that damage; because that instance, glomerular illness are frequently immunologically mediated, conversely, tubular and interstitial obstacle are more likely to be brought about by toxic or infectious agents. However, an ext than one structure have the right to be affected by a single disease, and also the interdependence of structures in the kidney provides other components be affected when only one part is damaged. 

As pointed out earlier, glomerular pathologies are greatly immune. Immune disorders can be one of two people 1) mediated through antibodies versus glomerular antigens, 2) mediated by complement, or 3) pauci-immune. The clinical manifestations, as well as the microscopic appearance the the glomerulus, will certainly be dependant top top the device of damage. 


Clinical Significance

Diseases affecting the glomerulus usually divide into two various entities follow to the clinical presentation: 

Nephrotic Syndrome

Nephrotic syndrome might be primary or secondary. Primary nephrotic syndrome occurs as soon as the kidney is the main or sole impacted organ. As soon as systemic immunologic, metabolic, or vascular diseases affect the glomeruli, it is termed second nephrotic syndrome. Out of this two, an additional nephrotic syndrome is many common.<19> 

Primary Nephrotic Syndrome

The adhering to are some of the many common reasons of main nephrotic syndrome. 

Secondary Nephrotic Syndrome

Secondary nephrotic syndrome can result from systemic immunologic diseases, such together systemic lupus erythematosus or vasculitis, metabolic conditions like diabetes, or vascular diseases such together hypertension. The many common reason of second nephrotic syndrome is diabetes mellitus. In diabetes, hyperglycemia leader to glycosylation that the vascular basement membrane, bring about hyaline arteriolosclerosis. The efferent arteriole is most frequently affected, increasing the glomerular filtration pressure and hyperfiltration. This state eventually progresses to albuminuria, among the very first clinical markers of changed renal function. Histologically, that characteristically demonstrates mesangium sclerosis and the development of Kimmelstiel Wilson nodules.<19>

Nephritic Syndrome

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

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A one-of-a-kind presentation that the nephritic syndrome is the Rapidly progressive Nephritic Syndrome. In this clinical scenario, patients progress to renal failure in weeks to months. It presents v the characteristic crescents in the Bowman an are on the H&E stain. Crescents space an extra capillary proliferation that macrophages, fibroblasts, and epithelial cells, and also the deposition the fibrin due to a rupture of the glomerular membrane, denote a severe injury come the glomerular capillary wall. Immunofluorescence patterns help identify the etiology. The differential diagnosis is feasible through histologic immunofluorescence trends on renal biopsy. 


Linear pattern: caused by anti-basement membrane antibodies, as is characteristics of Goodpasture syndrome.