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).
