Embryology of the Kidney

Lecture given:


State the developmental origins of the urogenital and nephrogenic ridges

  • 4th week of development
  • intermediate mesoderm form bilateral, longitudinal urogenital ridges
    • lateral part is the nephrogenic ridge (urinary system)
    • medial part is the genital ridge (parts of the female/male reproductive systems)
    • 1.PNG

Define the terms "pronephros, mesonephros, metanephros"

  • in the 4th week, there are 3 pairs of kidneys beginning to form, only the last pair persists to become the adult kidneys (metanephros)
  • Pronephros: rudimentary tubules and ducts that degenerate by week 6. Located towards the cranial region
  • Mesonephros: develop in the lumbar segments of the nephrogenic ridge, with blood vessels from the aorta connecting to glomerulus, which filters to the excretory tubules and excretes through the mesonephric duct (Wolffian Duct)
  • 2.PNG
    • The mesonephric duct passes caudally through the nephrogenic ridge, joining the cloaca, which is a common opening for waste (eventually divided into anorectal and urinary)
    • The mesonephros are functional kidneys, until the metanephros come in
    • By the end of the second month, most mesonephros has degenerated, but some more caudal nesonephric tubules persist and contribute to the genital ducts of the male
    • The caudalmost parts of the mesonephric duct contribute significantly to further development of the urinary system
  • Metanephros: the ultimate kidneys
    • develop in the pelvic region
    • Functional by 12 weeks
    • Comes from two origins: the ureteric bud and metanephric mesoderm


State the roles of the ureteric bud and metanephric mesoderm in kidney development

  • Ureteric bud is an outgrowth of the distal end of the mesonephric duct
    • The ureteric bud meets the metanephric mesoderm, and induce surrounding cells (ie. the metanephric mesoderm) to differentiate into a metanephric cap, which induces division of the distal end of the ureteric bud.
  • Metanephric mesoderm is part of the caudal end of the nephrogenic cord
  • Together, they form the metanephros, which will develop into adult kidneys

List the parts of the kidney tubule and duct system that are derived from the ureteric bud and metanephric vesicle

  • Ureteric bud:
    • The part that penetrates the metanephric mesoderm will become the renal pelvis
    • The part outside will be the ureter
    • The dividing part (4 generations) would have been absorbed back into the renal pelvin, forming 204 major calyces
    • The next 4 generations of division become minor calyces
    • Also gives the kidney a lobulated look since the metanephric cap becomes associated with the ends of each tubule
    • More division of tubules become the collecting ducts at the 8th week
    • Eventual differentiation of the ureteric bud: ureter, renal pelvis, calyces (major & minor), collecting ducts
  • Metanephric Mesoderm
    • At the same time, mesodermal caps differentiate into metanephric vesicles which elongate into metanephric tubules and will become nephrons
    • Proximal end of each tubule (Bowman's capsule) is invaginated by blood vessels forming the glomerulus
    • Eventual differentiation of the mesodermal caps will form Bowman's capsule, proximal tubule, loop of Henle, and distal tubule
  • By three month, the fetal kidneys should be actively producing urine and add to the volume of the amniotic fluid. If amniotic fluid levels are low at this point, there may be problems with kidney development (bilateral renal atresia)

Outline the "ascent" of the kidneys and the accompanying changes in blood supply

  • Initially, the metanephros are pelvic organs
  • As the embryo lengthes, the metanephros anchored by covering fascia are pulled cranially, and the ureters stretched accordingly
  • Initial blood supply is from the branches of the internal iliac, but as they enter the abdomen, new renal arteries sprout of the aorta and the lower ones disappear
  • Ascend stops at the level of the adrenal glands, and blood supplied by a pair of renal arteries from the aorta
  • Is is not uncommon to have accessory renal arteries that failed to degenerate during the ascend
  • (Gonads are pulled caudally by the gubernaculum)

Outline the development of the bladder and urethra from the urogenital sinus

  • The urorectal septum will divide the cloaca into anorectal canal and urogenital sinus during the second month
  • Mesonephric ducts open into the urogenital sinus
  • The parts where the mesonephric ducts open to will expand to form the bladder
    • endodermal lining forms the epithelium
    • surrounding mesoderm forms connective tissue and muscle

Outline the role of the mesonephric ducts in the formation of the "trigone" of the bladder

  • the allantois eventually forms a fibrous remnant called the urachus - connecting the bladder to the umbilicus
  • As the bladder enlarges, terminal parts of the mesonephric ducts (including ureters) become incorporated into the posterior wall and forms a smooth triangular area of the bladder wall — trigone
    • In males, part of the genital duct system (from the mesonephric ducts) is absorbed into the urogenital sinus
  • The pelvic part of the urogenital sinus form part of the male, and most of the female, urethra. In males, the inferior-most part of the urogenital sinus (phallic part) also forms part of th urethra

Explain the following in terms of abnormal development of the urinary system: renal agenesis, pelvic kidney, horseshoe kidney, polycystic, ectopic kidneys, exstrophy bladder

  • Renal agenesis:
    • No ureteric bud
    • Failure of the ureteric bud to reach the metanephric mesoderm
    • Can be compensated if unilateral, but almost fatal if bilateral
  • Pelvic kidney (ectopic kidney):
    • Failure to ascend
    • Retains its primitive blood supply from the internal iliac
  • Horseshoe kidney
    • Early in development, the lower 2 poles of the kidneys are close together, and can fuse
    • The fused part is trapped under the inferior mesenteric artery brand, and so the ascend of the kidneys are restricted
  • Polycystic kidneys
    • Large numbers of small/large cysts
    • Most due to dilation or ballooning of specific parts of the developing nephron
  • Exstrophy bladder:
    • Failure of the ventral abdominal wall to close during folding and results in lumen of the bladder being exposed to the outside through the anterior abdominal wall
    • Sometimes, parts of the allantois may persist, instead of fibrosing, and forms a urachal sinus. Since the allantois connects to the umbilical cord, the urachal sinus will cause urine to come out of the umbilicus


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