Flashcards in Histology: Renal Deck (49):
What are the main roles of the kidneys in homeostasis?
Water/electrolyte balance maintenance, acid-base balance maintenance, excretion of toxic products such as creatinine and urea, endocrine functions to produce renin (increase BP), EPO (stimulates erythropoiesis) and convert calcidiol to calcitriol (regulates calcium levels)
What is the renal papilla?
The pointed, central oriented part of the medullary pyramid; this is surrounded by the minor calyx
What are the papillary ducts (Ducts of Bellini)?
These are the collecting ducts that then extend back into the medulla and then merge to form the largest ducts that open into the calyces
What histological features characterise the renal cortex?
Contains renal corpuscles, PCT, DCT and medullary rays (collecting ducts)
What histological features characterise the renal medulla?
contains the straight thick and thin portions of the loop of Henle, collecting ducts and vasa recta
What are the four main branches of the renal artery as it enters the renal hilum?
Segmental branches - there is one posterior branch and 2/3 anterior branches
Interlobar arteries - these run between the medullary pyramids
Arcuate arteries - these run laterally where the renal cortex meets the medulla
Interlobular arteries - these run upwards into the cortical tissue
How do the efferent arterioles provide blood supply to the kidney parenchyma?
The efferent arterioles give rise to two secondary capillary plexi:
Vasa recta - these arise from the efferent arterioles of the glomeruli where this system runs straight down into the medulla and then drains into the veins of the corticomedullary junction to supply the renal medulla.
Cortical capillary network - these capillaries arise from the efferent arterioles in the rest of the cortex and allow exchange between the cortex and cortical tubules
Describe the blood supply from the vasa recta to the kidney parenchyma
Vasa recta arise from juxtamedullary glomeruli and run straight into the medulla and drain into veins at the corticomedullary junction to supply the medulla and generate a high osmotic pressure for the generation of concentrated urine
What is the renal corpuscle?
This consists of the Bowman's capsule and the glomerulus within it; this is the site of plasma filtration
Describe the structure of the Bowman's capsule (visceral and parietal walls)
The bowman's capsule consists of two continuous layers of epithelial cells with a urinary space between:
Visceral layer - this consists of the podocyte layer of cells
Parietal wall - this is formed by capsular simple squamous epithelium
What makes up the visceral wall of the Bowman's capsule?
What makes up the parietal wall of the Bowman's capsule?
Simple squamous epithelium
What is minimal change nephropathy?
A condition that occurs in children due to the disruption of podocyte architecture which leads to excessive proteinuria and nephrotic syndrome
Describe the structure of the glomerulus
The glomerulus is a collection of fenestrated capillary loops that are enclosed by the visceral layer (podocyte layer) of the Bowman’s capsule). The glomerulus is supported by mesangial cells.
What type of capillaries are present in the glomerulus?
What is the role of the mesangial cells in the glomerulus?
They synthesise the connective tissue matrix (mesangium), phagocytose trapped particles on the endothelial side of the filtration barrier, maintains the basement membrane and control glomerular blood flow by contracting or relaxing to make the glomerular capillaries narrowed or wider respectively.
What makes up the glomerular filtration barrier?
The podocyte layer (visceral layer of Bowman's capsule), fenestrated endothelium of the glomerulus and the basement membrane
Where does the proximal convoluted tubule originate?
At the urinary pole of the renal corpuscle
Describe the histological structure of the proximal convoluted tubule
Lined by simple cuboidal epithelial cells with prominent brush border of microvilli and basolateral interdigitations of the plasma membrane (increase surface area)
What is the role/function of the proximal convoluted tubule (PCT)?
- Sodium is reabsorbed via active transport (Na+/K+ ATPase) into the basolateral space between cells and then into the peritubular capillaries
- Reabsorption of water and negatively charged ions – this is driven by the reabsorption of sodium and 70-80% of water, sodium and chloride are reabsorbed in the PCT
- Bicarbonate ions are reabsorbed as H+ ions are secreted (exchange mechanism)
- Glucose and amino acids are reabsorbed completely here by facilitated diffusion (using carrier proteins driven by the co-transport of sodium/Na+)
- Larger proteins and carbohydrates that have managed to enter the filtrate undergo endocytosis into the PCT cells and are then degraded by lysosomal enzymes into amino acids and simple sugars which can then diffuse from the cells.
What are the four segments of the loop of Henle?
1) Pars recta (straight portion) - this is the straight portion of the proximal tubule
2) Thin descending limb
3) Thin ascending limb
4) Thick ascending limb
What type of epithelia makes up the walls of the thin ascending and descending limbs of the loop of Henle?
Simple squamous epithelia
Describe the histological structure of the thick ascending loop of Henle?
This is lined by simple cuboidal epithelia with basolateral interdigitation of the membrane and abundant mitochondria (same as the DCT)
What is the function of the loop of Henle?
Acts to increase the osmotic gradient from the cortex to the tip of the renal papilla to increase water loss via the counter-current mechanism
Explain the counter-current multiplier mechanism
• Descending limb is permeable to water and urea but less permeable to sodium and chloride water moves into the surrounding tissue down the osmotic gradient and is absorbed by the vasa recta supplying the nephron
• The thin ascending limb is permeable to sodium and chloride but not to water therefore you get NaCl efflux but water cannot follow out of the tubule here due to impermeability
• The thick ascending limb actively transports NaCl out of the tubule whilst limiting diffusion of most other molecules such as water, ions and urea. This generates a high solute concentration in the medullary interstitium and the tubule fluid become hypotonic increased water loss.
Describe the histological structure of the distal convoluted tubule
Lined by simple cuboidal epithelium with basolateral interdigitations and abundant mitochondria (same as thick ascending loop of Henle)
What's the difference in the epithelium of the PCT in comparison to the DCT and thick ascending loop of Henle?
All of them have simple cuboidal epithelia with basolateral interdigitations and abdundant mitochondria, but the PCT has extensive brush border microvilli whereas the other two do not have this.
What is the function of the distal convoluted tubule?
• Reabsorbs Na+ through coupled secretion (exchange) of H+ or K+ ions into the tubular fluid in the presence of aldosterone balance
• Usually relatively impermeable to water but ADH can increase permeability to permit concentration of urine
• Secretes ammonium ions and some drugs
• Forms part of the juxtaglomerular apparatus
Describe the juxtaglomerular apparatus
This apparatus is found at the vascular pole of the renal corpuscle and consists of juxtaglomerular cells, macula dense and extraglomerular mesangial cells (lacis cells) and is important in the control of systemic blood pressure and volume.
What is the role of the juxtaglomerular cells in the juxtaglomerular apparatus?
These are modified smooth muscle cells in the wall of the afferent arterioles which can sense changes in blood pressure and secrete renin in response to low detected BP
What is the role of the macula densa in the juxtaglomerular apparatus?
This is a region of the distal tubule • that associates with the glomerulus as it loops back into the cortex. The cells of the distal tubule closest to the glomerulus become taller, more densely packed and with more prominent nuclei. The cells of the DCT in this region are thought to be able to detect Na+ concentration changes in the DCT and convey this to the juxtaglomerular cells
What is the macula densa?
A region of the distal tubule that associates with the glomerulus as it loops back into the cortex
What is the role of the extraglomerular mesangial cells (lacis cells) in the juxtaglomerular apparatus?
These cells support the complex
How can you identify the macula densa on histology?
The cells of the distal tubule closest to the glomerulus become taller, more densely packed and with more prominent nuclei
Describe the histological structure of the collecting tubules
The collecting ducts are lined by simple columnar cells with distinct lateral plasma membranes due to reduced interdigitations of these membranes with neighbouring cells.
What is the function of the collecting tubule?
• Concentrate urine in the presence of ADH
• Regulation of acid-base through secretion of H+ and reabsorption of bicarbonate
• Secretion of ammonium and some drugs
• Maintenance of interstitium solute concentration as it’s permeable to urea, which may diffuse down the gradient in concentrated urine to aid osmotic gradient production.
What does the 'lower urinary tract' consist of?
Everything below the kidneys; renal calyces, renal pelvis, ureters, bladder, uretha
What two factors make the urothelium (transitional epithelium) most suited for it's function?
The membrane folds (umbrella cells) allow for distension and the presence of tight junctions also creates urine impermeability to allow removal of concentrated urine
Outline the four layers of the lower urinary tract wall
2) Submucosa (including lamina propria)
3) Muscularis mucosa
Describe the muscular layer of the lower urinary tract wall
Consists of 2-3 layers of smooth muscle:
1) Innermost longitudinal muscle
2) Circular muscle
3) Outermost longitudinal muscle
Does the epithelium of the kidney tubules rest on a basement membrane?
All epithelia rests on a basement membrane
When is the outer layer of a wall referred to as a serosa instead of an adventitia?
Serosa is used if the wall is covered by mesothelium, if it is not the outer layer is known as adventitia
How does epithelium derive nutrition?
From the underlying connective tissue
Where is urothelium/transitional epithelium found?
Kidney calyces, renal pelvis, bladder, urethra (proximal urethra in females and prostatic urethra in males)
What two things can lead to thickening of the glomerular capillary walls?
Expansion of the basement membrane or an increase in the mesangial matrix
What does a PAS stain highlight?
PAS stains for glycoproteins, which are found in the basement membrane
How does the basement membrane play a role in the glomerular filtration barrier?
It is negatively charged and therefore acts to repel negatively charged proteins
What is membranous glomerulonephritis?
A condition characterised by thickening of the glomerular capillaries due to autoimmune reaction which leads to glomerular damage which can cause glomerular filtration barrier 'leakiness'