EMBRYO - Urinary System Development

Question 1

URINARY SYSTEM DEVELOPMENT

Answer 1

• Kidney, ureter, and urethra develop from intermediate mesoderm
• Three stages of kidney development
  
  • Kidney #1 → Pronephros
    
    • Exists for one week at week 4
    • Nonfunctional
    • Located in cervical region
  • Kidney #2 → Mesonephros
    
    • Exists from week 4 to end of 2^nd month
    • Mesonephric ducts persist in males, disappear in females
    • Functional
    • Located in thoracolumbar region
  • Kidney #3 → Metanephros
    
    • Appears at week 5
    • Definitive kidney
• Each stage of kidney development induces the next stage

Question 2

MESONEPHROS

Answer 2

• Development of tubular system, each tubule induces formation of next
• Excretory tubules → renal corpuscle (glomerulus + Bowman’s capsule) → mesonephric/paramesonephric ducts → ureteric bud
• By end of 2^nd month: urogenital ridge has developed → future gonads
• Mesonephric duct (Wolffian duct) disappears in females and persists in males to become the male reproductive tracts
• Paramesonephric duct disappears in males and persists in females to become the female reproductive tracts
• 2^nd kidney development extremely important without it, no development of neither the definitive kidney nor its collecting system will occur

Question 3

METANEPHROS

Answer 3

• Ureteric bud outgrowth from mesonephric ducts penetrates the metanephric blastema tissue in the intermediate mesoderm and induces its development into the definitive kidney
• Ureteric bud itself develops into the collecting system of the kidney
  
  • Ureter → renal pelvis → major/minor calyces → collecting tubules
• Completion of collecting tubule development induces development of the filtration system at the metanephric tissue caps
  
  • Nephron: DCT, limbs and loop of Henle, PCT, Bowman’s capsule
  • Glomerulus develops into Bowman’s space
• Under paracrine regulation of two cell populations
  
  • Collecting tubule and mesenchymal tissue cells each release signals that stimulate the other

Question 4

ASCENT OF THE KIDNEY

Answer 4

• Starts in pelvis → decreased curvature of body with growth of lumbar and sacral causes ascent
• Mesonephric system degenerates (except ducts associated with reproductive tracts) and gonads descend

Question 5

CONGENITAL DEFECTS

Answer 5

• Polycystic Kidney Disease
  
  • Two forms arising from mutations in different genes
  • Autosomal recessive PKD: Cysts develop in collecting tubules over time and lead to renal failure within infancy or childhood
  • Autosomal Dominant PKD: Cysts develop throughout nephron but do not lead to renal failure until adulthood
• Duplication of the Ureter
  
  • Caused by early splitting of the ureteric bud
  • Can be partial or complete
  • Creates an ectopic ureter opening in the vagina, urethra or vestibule
• Horseshoe Kidney
  
  • Kidney becomes trapped under inferior mesenteric artery during ascent and remains as one structure

Question 6

BLADDER FORMATION

Answer 6

• Ureter grows upward to connect with bladder
• Cloacal membrane divides to form (anteriorly) the urogenital sinus and (posteriorly) the anal canal near the urorectal septum
• Urogenital sinus
  
  • Upper portion is the largest, becomes the bladder
    
    • Initially connect to umbilicus by allantois, later called the urachus (eventually umbilicus → medial umbilical ligament in adults)
  • Pelvic portion → prostatic gland and membranous tissue of the urethra
  • Phallic portion → differs between sexes
• Development of bladder and definitive sinus
  
  • Ureter (initially an outgrowth of mesonephric ducts) enter bladder farther up and grows into it cranially (creates oblique angle)
  • Mesonephric ducts are absorbed into bladder wall to form the mucosa of the bladder and grow into lower area by prostatic gland to form the ejaculatory ducts of males
  • Ducts also develop into the trigone area between ducts and ureter where they empty into the urethra
    
    • Original mesodermally lined because everything originated by the intermediate mesoderm, but endoderm penetrates and endothelial lining replaces it
    • If this doesn’t occur, undifferentiated endothelial cells are trapped outside and can develop into cancer