Urinary System Embryology

• Intermediate mesoderm


  The basic foundation for developing the urinary system
• Gastrulation


  The process of going from a bilaminar to a trilaminar disc
• Components of the mesoderm

  • Paraxial mesoderm
  • Intermediate mesoderm
  • Lateral plate mesoderm
• Urogenital ridge


  The structure formed by condensation of the intermediate mesoderm, which will become the urinary and reproductive systems
• Nephrogenic cord


  The core of mesoderm within the urogenital ridge that will form the urinary system
• Development of the urinary system from the nephrogenic cord

  1. Pronephros forms at week 4 (duct and tubules), then degenerates

  2. Mesonephros forms from the pronephros, developing a duct and tubules

  3. Mesonephric duct connects to the cloaca (which will become the bladder and urethra)

  4. Mesonephric tubules form capsules around glomeruli derived from the aorta via angiogenesis

  5. This primitive urinary system functions until week 10
• Nephrogenic cord


  Connects to the cloaca (bladder and urethra)
• Formation of primitive urinary system

  1. Mesonephric duct

  2. Mesonephric tubule

  3. Bowman's capsule

  4. Aorta

  5. Glomerular capillaries

  6. Filtration

  7. Mesonephric duct connects to cloaca

  8. Cloaca drains urine
• Transition from mesonephros to metanephros


  1. Metanephric mesoderm/blastoma forms

  2. Metanephric blastoma releases growth factors

  3. Stimulates mesonephric duct to form ureteric bud

  4. Reciprocal induction between ureteric bud and metanephric blastoma

  5. Ureteric bud forms renal pelvis, major calyces, minor calyces, collecting tubules

  6. Ureteric bud connects to mesonephric duct to form ureter
• Ureteric bud and metanephric blastoma

  • Reciprocal induction - two-way interaction with growth factor signalling
• Ureteric bud


  Portion that invades metanephric blastoma and forms renal pelvis, calyces, collecting tubules
• Ureteric stalk


  Connects ureteric bud to mesonephric duct, becomes ureter
• Formation of nephron


  1. Metanephric mesoderm cap forms around collecting tubule

  2. Metanephric vesicle forms

  3. Metanephric tubule forms, connects to collecting tubule as distal convoluted tubule

  4. Distal convoluted tubule forms proximal convoluted tubule

  5. Proximal convoluted tubule forms loop of Henle
• Mesonephros continues to function as primitive urinary system while metanephros develops
• Distal convoluted tubule


  Structure formed from the metanephric tubule
• Formation of distal convoluted tubule

  1. Metanephric tubule connects to collecting tubule

  2. Continues to grow, replicate, coil, and become s-shaped
• Proximal convoluted tubule


  Structure formed from the distal convoluted tubule
• Formation of proximal convoluted tubule


  1. Distal convoluted tubule makes the proximal convoluted tubule

  2. Proximal convoluted tubule starts to change its conformation and configuration
• Bowman's capsule


  Cup-shaped structure formed from the proximal convoluted tubule
• Formation of glomerular capillaries


  1. Arise from the common iliac arteries

  2. Interact with the Bowman's capsule
• Formation of nephron


  1. Glomerular capillaries

  2. Bowman's capsule

  3. Proximal convoluted tubule

  4. Loop of Henle

  5. Distal convoluted tubule
• Formation of collecting system

  1. Collecting tubule

  2. Minor and major renal pelvis

  3. Ureter
• Kidneys ascend as common iliac arteries degenerate

  Renal arteries develop from lateral branches of the aorta
• Formation of bladder and urethra

  1. Urorectal septum separates cloaca into anterior urogenital sinus and posterior anal canal

  2. Urogenital sinus forms bladder proximally, urethra medially, and penile urethra distally
• Urachus


  Structure formed from the allantois that becomes the median umbilical ligament
• Mesonephric duct does not contribute to the urinary system, but rather to the reproductive system
• Kidneys


  • Two kidneys in the abdomen
  • Each kidney has millions of nephrons
  • Kidneys have an excretory portion and a collecting portion
• Excretory portion of the nephron

  1. Glomerulus

  2. Proximal convoluted tubule

  3. Loop of Henle

  4. Distal convoluted tubule
• Collecting system


  1. Collecting ducts and tubules

  2. Minor calyces

  3. Major calyces

  4. Renal pelvis

  5. Ureter
• Nephrogenic cords


  Portions of the intermediate mesoderm that condense to give rise to the kidneys
• Kidney development


  • Bilateral
  • Extend from the cervical end to the sacral end of the embryo
• Stages of kidney development

  1. Pronephros

  2. Mesonephros

  3. Metanephros
• Pronephros


  Very primitive, just some cell clusters and tubules, and the pronephric duct
• Mesonephros


  • Has mesonephric tubules, which are like primitive versions of renal tubules, and these are associated with capillaries (glomerulus)
  • Can perform filtration
  • Functional until the final kidney takes over
• Formation of the metanephros

  1. Ureteric bud grows from the lower portion of the mesonephric duct

  2. Ureteric bud induces differentiation of the metanephric blastema to form the excretory portion

  3. Metanephric vesicles form, become S-shaped, one portion associates with capillaries to form the glomerulus, the rest forms the renal tubule
• Ureteric bud


  Forms the collecting system of the kidney
• Metanephric blastema


  Forms the excretory portion of the kidney
• As gestation continues, the number of nephrons increase
• Kidney ascent


  1. Initially in the pelvic region

  2. Ascend to the abdomen due to body growth

  3. Blood supply changes from branches of the abdominal aorta to renal arteries directly off the abdominal aorta
• During foetal life, kidneys form urine that drains into the cloaca and then into the amniotic fluid