HIST - Urinary System

Question 1

KIDNEY MORPHOLOGY

Answer 1

• Urinary system → kidney and urinary tract
• Kidney Functions
  
  • Filtration – kidney glomerulus
  • Selective reabsorption and excretion – kidney tubular system
  • Secrete renin – regulate BP
  • Secrete erythropoietin – regulate RBC production in response to decreased [O₂] in the kidney interstitial fibroblast
  • Activate vitamin D3
• Kidney Structure
  
  • Encapsulated
  • Cortex and Medulla (separated by the acurate blood vessels)
    
    • Cortex
      
      • Renal corpuscles (glomeruli)
      • Medullary ray (400-500 per lobe in humans)
      • Cortical labyrinth
        
        • Interstitial cells
        • Peritubular capillary system
        • PCT/DCT
      • Renal column – portion of the cortex that extends down through the medulla
    • Medulla
      
      • 15-20 renal pyramids or lobes
        
        • Base of pyramids make up the corticomedullary border
        • Rounded ends of each pyramid = renal papilla
          
          • Each papilla drains up to 20 collecting ducts or ducts of Bellini
          • Area cribrosa = openings of the collecting ducts in the renal papilla
  • Lobe – medullary pyramid + overlying cortex
    
    • Lobule – space between interlobular arteries with the medullary ray (collecting ducts) as its axis containing a group of nephrons emptying into the same collecting duct
  • Nephron – glomeruli and kidney tubular system (PCT, loop of Henle, DCT)
    
    • Each kidney has more than a million nephrons
    • Function in filtration, secretion, and reabsorption
    • Cortical nephrons – shorter loop of Henle and renal corpuscle completely contained in the renal cortex
    • Juxtamedullary nephrons – renal corpuscle located in the cortex; loop of Henle located in the medulla and much longer
  • Uriniferous tubule = functional unit of kidney composed of the nephron plus the collecting duct

Question 2

BLOOD SUPPLY TO GLOMERULUS

Answer 2

• Arterial portal system = kidneys
  
  • Afferent arterial → capillary bed (O₂) → efferent arteriole → capillary bed (de-O₂) → venule
• Venous portal system
  
  • Arteriole → capillary bed (O₂) → vein → capillary bed (de-O₂) → vein
• Function of portal system is to control blood flow
  
  • Kidneys – maintain constant flow for filtration
  • Liver/Pituitary – maintain flow so substances can be secreted into circulation
• Blood flows through the glomeruli/capillary beds and dumps into the interlobular vein
  
  • Cortical nephron – peritubular capillary beds
  • Juxtamedullary nephron – vasa recta; fxn in creating counter current exchange

Question 3

GLOMERULAR STRUCTURE

Answer 3

• Fenestrated capillary endothelium – derived from mesoderm
• Basal lamina → selective filtration
  
  • Formed by the fusion of BL from the capillary endothelium and visceral podocytes
  • Three layers:
    
    • Lamina rara interna (capillary side)
    • Lamina densa (thick)
    • Lamina rara externa (visceral side)
• Visceral layer of Bowman’s capsule
  
  • Lines capillaries
  • Made up of podocyte end processes
    
    • Primary processes
    • Secondary processes = pedicels
      
      • Lie on the lamina rara externa
      • Interdigitate → filtration slits covered by a thick (6nm) diaphragm
• Mesangium = stroma around capillaries composed of mesangial cells and their matrix
• Integlomerular mesangial cells
  
  • CT layer derived from mesoderm
  • Synthesize collagen and matrix
  • Capable of proliferation
  • Phagocytic activity → controls BL turnover
  • Contractile → regulate blood Q
  • Also secrete Pgl’s (vasodilation) and endothelins (vasoconstriction)

Question 4

GLOMERULAR FILTRATION

Answer 4

• Filtration barrier components:
  
  • Capillary endothelium – fenestrated
    
    • Permeable for H20, urea, Na, glucose, and small proteins
    • Coated with heparin sulfate to impede large anionic proteins
  • Basal lamina
    
    • Collagen IV, fibronectin, laminin, proteoglycans
    • Lamina densa traps large molecules
    • Lamina rara interna coated with heparin sulfate impedes passage of negatively charged molecules and molecules that cannot transform
  • Pedicels
    
    • Interdigiated processes, coated with negative glycoproteins
    • Linked by a diaphragm made of nephrin (a CAM from Ig family)
      
      • Anchored to actin in pedicel by CDA2P
      • Mutation in nephrin gene → congenital nephrotic syndrome
        
        • Massive proteinuria (albumin in urine) and edema
    • Spaces between processes = filtration slits
• Trapped molecules + BL is phagocytized by interglomerular mesangial cells
• BL is then replenished by podocytes and capillary endothelium
• The fluid that is filtered from the capillary through the BL into the bowman’s space is called glomerular ultrafiltrate

Question 5

TUBULE SYSTEM

Answer 5

• Proximal Convoluted Tubule (PCT)
  
  • Eosinophilic
  • Thick brush border/microvilli
  • Simple (low) cuboidal epithelium
  • Reabsorbs about 70% of water through TJs and interdigitating basal ridges that make filtration slits
  • NaCl and glucose (via a transporter) uptake to create an osmotic gradient
  • Apical tubulovesicles internalize peptides to be degraded by lysosomes
• Thin Limbs of the Loop of Henle
  
  • Three regions: thin descending limb, loop of Henle, thin ascending limb
  • Simple squamous epithelium
    
    • Difficult to distinguish from capillaries
    • No microvilli
  • Between loops and vasa recta = interstitial cells
• Distal Convoluted Tubule (DCT)
  
  • Not permeable to water, utilizes symport to uptake NaCl
  • Location of macula densa cells of JGA that monitor NaCl in filtrate
    
    • Linear row of cells, look like teeth
  • Sensitive to aldosterone levels
  • Brush border, lysosomes, tubulovesicles are not prominent
  • Simple cuboidal cells
  • Abundant mitochondria in infolds of basolateral membrane
  • Associated with the vascular (arterial) pole and JGA
• Collecting Tubule
  
  • Tubule = high cuboidal epithelium
  • Distinct lateral cell borders
  • Found in the medullary rays/medulla
  • Lined by two types of epithelium
    
    • Principle cells
    • Intercalated cells

Question 6

JUXTAGLOMERULAR APPARATUS (JGA)

Answer 6

1. Macula densa cells of the DCT
2. Juxtaglomerular cells mainly in afferent arteriole (also efferent)
   
   1. Modified SM cells of tunica media
   2. Sympathetically innervated, stimulated by NE/D
   3. Secrete renin
   4. Communicate with macula densa cells
3. Extraglomerular mesangial cells between the arterioles
   
   1. Connected to each other and JG cells by TJs
   2. Stimulated to contract by angiotensin II

Question 7

URINARY PASSAGE

Answer 7

• Collecting tubules – high cuboidal epithelium
• Medullary collecting tubules
  
  • Combined cortical collecting tubules, larger
  • High cuboidal epithelium
• Renal Papilla
  
  • Papillary collecting tubules = ducts of Bellini = collecting ducts
  • Combined medullary collecting tubules
  • Tall, columnar epithelium principle cells only
• Minor calyxes – true transitional epithelium
  
  • Domed, binucleated
• Major calyxes – transitional epithelium
• Renal pelvis
• In renal sinus between hilum and pelvis = perirenal fat

Question 8

URETER

Answer 8

• Mucosa
  
  • Stellate lumen lined by transitional epithelium (urothelium) (can distend and relax with organ)
  • Fibroelastic lamina propria – stretchable
  • Muscularis externa
    
    • Upper 2/3 = ILOC
    • Lower 1/3 = ILMCOL
• Adventitia
  
  • Route for vascularization and nerve supply
• Oblique course through the urinary bladder wall causes lumen to compress when bladder is distended
• No mucosal/submucosal glands
• No muscularis and mucosae
• No submucosa

Question 9

URINARY BLADDER

Answer 9

• Mucosa
  
  • Transitional epithelium (urothelium) – distends and relaxes via apical plaques that form bridges which open and close; like a Chinese finger trap
    
    • Apical plaques = intramembranous proteins anchored to cytoskeletal proteins
  • Fibroelastic LP
• Muscularis - LMCOL
• Adventitia – fibroelastic CT
  
  • Superior bladder (fundus) covered by serous peritoneum
  • All retroperitoneal structures are covered by a CT adventitia that anchors the organ to surrounding tissue and provides a route for nerves and vascularization

Question 10

URETHRA

Answer 10

• Mucosa
  
  • Vascular fibroelastic LP
  • Urethral glands – keep urethra patent; secrete a slippery substance
    
    • In males called glands of Littre
  • Males:
    
    • Prostatic urethra = transitional epithelium
    • Membranous urethra = pseudostratified/stratified columnar
    • Penile urethra = transition from pseudostrat/strat to stratified squamous, non-keratinzing
  • Females
    
    • Near bladder → transitional epithelium → pseudostratified/stratified columnar → stratified squamous non-keratinizing at distal end
• Muscularis
  
  • Internal sphincter = ILOC
  • Sphincter has SkM near UG diaphragm to allow voluntary control
• Male urethra shared with reproductive system and longer
• Female urethra is unique to the urinary system and there is less distance between urethra opening and anus for bacteria to move and cause infection
  
  • UTI’s more common
  • Dangerous because can travel retrograde and cause a bladder infection where the urine provides glucose supply for bacterial growth
• Diabetics = high risk of UTI because of high glucose content of urine

Question 12

RENAL CORPUSCLE STRUCTURE

Answer 12

• Tufts of capillaries that grow into the blind end of a nephron
  
  • Capillaries are lined by fenestrated endothelium
  • Glomerular BM = fused BL’s of endothelium + podocytes
    
    • Doesn’t completely surround each capillary
    • One side left open where endothelial cell is in direct contact with mesangial cells of the mesangium.
• Surrounded by a visceral layer of podocytes
  
  • Production of glomerular filtrate and supportive mesangial cells
• Parietal layer = Bowman’s capsule
  
  • Simple squamous epithelium (may be mesothelium)
• Space in between layers = urinary space, contains glomerular ultrafiltrate (not yet considered urine)
  
  • Urinary pole → continuous with the PCT
  • Vascular pole → contains afferent/efferent arterioles and juxtaglomerular apparatus which has 3 components:

1. Macula densa cells - epithelial cells on the DCT-thick loop junction
2. Extraglomerular mesangial cells - between afferent and efferent arterioles
3. Juxtaglomerular cells - line afferent (mostly) and efferent arterioles
   
   • Modified SM cells that secrete renin
   • Sympathetically innervated in the afferent arteriole