Thursday, January 12, 2012

Embryology- kidney and ureter

Following fertilization, a blastocyte (sphere of cells) is created, which implants into the uterine endometrium on day 6. The early embryonic disc of tissue develops a yolk sac and amniotic cavity, from which are derived ectoderm, endoderm, and mesoderm. Organ formation occurs between 3-10 weeks of gestation.Most of the genitourinary tract is derived from mesoderm.

KIDNEY & URETER (NEPHRIC SYSTEM)

  • develops progressively as 3 distinct kidneys: pronephros, mesonephros, and metanephros.
  • All develop from intermediate mesoderm.
  • Each stage contain: tubules & ducts
  • tubules at all levels appear as independent primordia unite with the duct system

Pronephros (precursor kidney; pro = (Gk) before),
  • the 1st phase of development of the nephric system. (as excret.sys in primitive vertebrate).
  • site: cervical intermediate mesoderm (4th to the 14th somites)
  • time:  till week 4
  • Structure: consists of 6-10 pairs of pronephric tubules open into a pair of pronephric ducts at the same level. ducts extend caudally open into the cloaca (terminal part of hindgut).
  • Function: no function
  • Fate: Pronephric tubules disappears completely by 4th week
         Pronephric ducts persists as mesonephric ducts.

Mesonephros (meso = (Gk) middle)
  • the 2rd phase of development of the nephric system, (as excret.sys in higher fish and amphibians).
  • origin: thoracic & upper lumbar intermediate mesoderm (caudal to pronephros)
  • time: 4-8 weeks
  • Structure: mesonephric tubules become S-shaped,
Ø med. end → cup shaped and acquire glomerulus.
Ø lat. end open to primary pronephric duct mesonephric (Wolfian) ducts
·                   Mesonephric & Paramesonephric duct (see genital duct system)
  • Fate: mesonephric tubules mostly disappears (see table).
               mesonephric ducts persists (see below & table).

Fate in male
Fate in female
Mesonephric tubules
 Appendix epididymis
Paradidymis
Efferent ductules
Epididymal head
Epoophoron (above ovary)
Paroophoron (med. to ovary)
(both in mesovarium)
Mesonephric ducts
Epididymal body & tail
Vas deferens
Seminal Vesicle
Ejaculatory duct
Ureteric bud → ut. & trigone

Gartner’s duct (rudimentary)


Ureteric bud → ut. & trigone

Paramesonephric (Mullerian) ducts from celomic epithelium (see below- genital duct system)

Metanephros (meta = (Gk) after; =permanent kidney)
  • the final (3rd) phase of development of the nephric system,
  • site: lumbosacral intermediate mesoderm caudal to mesonephros (opposite the 28th somite) (fourth lumbar segment) induced by growth of ureteric bud from mesonephric duct.
  • Structure: Excretory sys. (Parenchyma) (Nephrons) from metanephric cap
Collecting (Pelvi-calyceal) system from ureteric bud of mesonephric duct
  • Fate: form permanent kidney (with & induced by ureteric bud)

    Development of the permanent kidney
     1-  Development of Excretory & Collecting system
      ·      mesonephric ducts develop laterally & advance downwards to fuse with primitive cloaca    (hindgut).
      ·      a ureteric bud grows from posteromedial side of distal part of the mesonephric ducts
      ·      ureteric bud induces formation of the metanephros from intermediate mesoderm, at 5th    wk.
      ·      The ureteric bud acquires metanephric cap
Ø          ureteric bud Branching → forms the renal pelvis, calyces, and collecting ducts.
Ø                metanephric cap → tubules → differentiation  → Glomeruli and nephrons (S-shaped).
Ø                Collecting system joins excretory system.
      ·      Urine production starts,  10th wk.

      3- Further dvt. [shape change, ascent, rotation & revascularization] (simultaneous)
  • 9 – 12 wks
  • Shape change: lobulated surface → smooth surface.
  • Ascent: metanephros lies early in pelvis → lumbar region T12 or L1 (at term).
  • ascent of kidney is due to cephalic migration  & differential growth in the caudal part of the body.
  • Rotation: during early ascent (7-9 weeks) kidney slides above the arterial bifurcation rotates 900 90° rotation from a plane with the pelvis anterior to one with the pelvis lying medial and somewhat  posterior (convex border is directed dorsal lateral).
  • Revascularization: as each  kidney ascends and rotates, it is successively supplied by arteries which are located higher and higher on the  urogenital  ridge. 
  • early  from  iliac,  middle  sacral,  lower  aorta,  and  IMA RA from aorta  just below the superior mesenteric artery.

        Development of ureter
  • from ureteric bud of mesonephric duct
  • The urogenital sinus (see below) receives the mesonephric ducts
  • Lower part of mesonephric duct & ureteral bud is progressively absorbed into urogenital sinus) mesonephric duct and the ureteral bud have 2 different opening sites (7th wk).
·            opening of the mesonephric duct (which will become the ejaculatory duct) migrates downward and medially (lat. to Muller’s tubercle)
·            opening of the ureteral bud (which will become the ureteral orifice) migrates upward and laterally
·            absorbed mesoderm of the mesonephric duct expands trigone (island of mesodermal tissue in surrounding endoderm of urogenital sinus)
·            normal Congenital sites for ureteric constriction points are UPJ & UVJ (last sites in ureter to become patent).
·                        Chwalla's Membrane:membrane between lumen of primitive ureter and UG sinus  normally  disappears 
               ANOMALIES OF THE NEPHRIC SYSTEM (kidney & ureter)
Anomaly
Cause
Bilat. agenesis
Lack of development of both ureteral bud
Unilat. agenesis (solitary kidney) & hemitrigone:
Lack of development of a ureteral bud
Unilat. agenesis (solitary kidney), hemitrigone, absent vas and epididymis
Lack of development of mesonephric duct.
dysplasia

ectopic kidney
Failure of the metanephros to ascend  
Ø simple ectopy: ectopic kidney on proper side but low
Ø crossed ectopy: on opposite side  with or without fusion.
malrotated kidney
Failure to rotate during ascent
horseshoe kidney
Fusion of the paired metanephric masses at lower pole (kidneys pushed together during ascend due to arterial fork by umbilical arteries)
bifid ureter
early ureteric bud bifurcation
duplicated ureter (more common).
accessory ureteric bud from mesonephric duct    same metanephric mass
Complete duplex system (less common)  
accessory ureteric bud from mesonephric duct    separate metanephric mass (2 buds are near = both open to bladder)
ectopic ureter in duplex sys.
Same but the 2 ureteral buds are widely separated on the mesonephric duct (accessory bud  is higher in mesonephric duct)
ectopic ureter in single sys.
single ureteric bud higher in mesonephric duct.
PCKD
Failure of fusion between excretory & collecting system
Persistent fetal lobulation (commonest)
 Failure of change in shape
Uretrocele
Persistent Chwalla’s membrane

  • Weigert-Meyer law: "double ureters always cross" bec. the 2 ureteral buds reverse their relationship as they move from the mesonephric duct to the urogenital sinus.
1.    main ureteral bud (first to appear & most caudal on mesonephric ducts & drains lower part of kid.)reaches the bladder first move upward and laterally (more cranial on the urogenital sinus)
2.    accessory ureteral bud (appears later & more proximal on mesonephric duct & drains upper part of kid.) reaches bladder later (more caudal on urogenital sinus).

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