The document details the embryonic development of the nephric system, which includes the pronephros, mesonephros, and metanephros stages. It explains the progression of kidney, ureter, and bladder development, as well as potential anomalies in their formation. Key processes include the ascension of the kidney and the division of the cloaca into the urogenital sinus and rectum, with various conditions linked to improper development.

1. EMBRYOLOGY OF
KIDNEY,URETER,BLADDER
M.Ch 1st
year urology

2. Nephric system
•The nephric system develops progressively as three distinct entities:
- pronephros,
- mesonephros and
- metanephros.

3. •PRONEPHROS :
- earliest nephric stage in humans,
- corresponds to the mature structure of the most primitive vertebrate.
- extends from the 4th to the 14th somites
- consists of 6–10 pairs of tubules.
- These open into a pair of primary ducts, extend caudally, and then reach and
open into the cloaca.
- Pronephros is a vestigial structure that disappears completely by the 4th week
of embryonic life.

4. •MESONEPHROS :
- Principal excretory organ during early embryonic life (4–8 weeks).
- It, gradually degenerates, although some parts become associated with
the male reproductive organs.
- Mesonephric tubules develop from intermediate mesoderm caudal to the
pronephros shortly before pronephric degeneration.
- Mesonephric tubules develop a cuplike outgrowth :
into which a knot of capillaries is pushed = Bowman’s capsule, and
tuft of capillaries = Glomerulus.

5. •MESONEPHROS :
- Mesonephric tubules extend toward and establish a connection with
the nearby primary nephric duct as it grows caudally to join the
cloaca
- Primary nephric duct is now called the mesonephric duct.
- Primordial tubules elongate and become S-shaped.
- The mesonephros, which forms early in the 4th week, reaches its
maximum size by the end of the second month.

6. •METANEPHROS :
- Metanephros - the final phase of development of the nephric system,
- originate from both the intermediate mesoderm and the mesonephric duct.
- Development begins in the 5–6-mm embryo with a budlike outgrowth from the meso-
nephric duct as it bends to join the cloaca.
- ureteral bud grows cephalad
- This mesoderm with the metanephric cap moves, with the growing ureteral bud, more
and more cephalad from its point of origin.
- During this cephalic migration, the metanephric cap becomes progressively larger,
and rapid internal differentiation takes place.

7. •METANEPHROS :
- Meanwhile, the cephalic end of the ureteral bud expands within the
growing mass of metanephrogenic tissue to form the renal pelvis.
- Outgrowths from the renal pelvic dilatation push radially and form
hollow ducts that branch and rebranch as they push toward the
periphery.
= form the primary collecting ducts of the kidney.

8. - Mesodermal cells become arranged in small vesicular masses that lie
close to the blind end of the collecting ducts.
- Each of these vesicular masses will form a uriniferous tubule
draining into the duct nearest to its point of origin.

9. •METANEPHROS :
- As kidney grows, increasing numbers of tubules are formed in its
peripheral zone.
- These vesicular masses develop a central cavity and become S-shaped.
- Oneof S coalesces -terminal portion of the collecting tubules =
continuous canal.
- Proximal portion of the S = distal and proximal convoluted tubules and
into Henle’s loop;
- the distal end becomes the glomerulus and Bowman’s capsule.

10. - Ascend of kidney .
- This ascent of the kidney is due to actual cephalic migration and
differential growth in the caudal part of the body.
- During the early period of ascent (6th–9th weeks),
- the kidney slides above the arterial bifurcation and rotates 90°.
- Convex border is now directed laterally.
- Ascent proceeds more slowly until the kidney reaches its final
position

12. ● URETER
● By 5th :
- stalk of the ureteric bud elongates to become the ureter.
- the ureter begins as a simple epithelial tube lined with cuboidal cells
and surrounded by loose mesenchyme

13. ● By 12th week :
- first signs of ureteral muscularization and development of elastic
fibers are seen
- Smooth muscle differentiation is first detected in the subserosal
region of the bladder dome and extends toward the bladder base and
urethra
- urine production begins

14. ● Kidney and the collecting system originate from interaction between the
mesonephric duct and the metanephric mesenchyme (MM).
● The uretic bud (UB) forms as an epithelial outpouching from the
mesonephric duct and invades the surrounding MM.
● Reciprocal induction between the UB and MM results in branching and
elongation of the UB from the collecting system and in condensation and
epithelial differentiation of MM around the branched tips of the UB.
● Branching of the UB occurs approximately 15 times during human renal
development, generating approximately 300,000 - 1 million nephrons per
kidney.

15. • ANOMALIES OF NEPHRIC SYSTEM
● Failure of the metanephros to ascend leads to an ectopic kidney.
● Ectopic kidney may be on the proper side but low (simple ectopy) or
on the opposite side (crossed ectopy) with or without fusion.
● Failure to rotate during ascent causes a malrotated kidney.
● Fusion of the paired metanephric masses leads to various anomalies—
most commonly a “horseshoe” kidney.
● The ureteral bud from the mesonephric duct may bifurcate, causing a
bifid ureter at various levels depending on the time of the bud’s
subdivision.

16. ● An accessory ureteral bud may develop from the mesonephric duct,
thereby forming a duplicated ureter, usually meeting the same
metanephric mass.
● Rarely, each bud has a separate metanephric mass, resulting in
supernumerary kidneys.

17. duplicated ureters always cross (Weigert–Meyer law).
● If the two ureteral buds are widely separated on the mesonephric
duct,
- the accessory bud appears more proximal on the mesonephric duct
and
- therefore ends in the bladder more distal than usual,
- with an ectopic orifice lower than the normal one.

18. ● Lack of development of a ureteral bud results in a solitary kidney and a
hemitrigone.
● The ureteral bud may also develop or migrate into the bladder,
abnormally leading to a ureterocele.
● A ureterocele is a cystic dilation of the distal intramural ureter and is
most commonly associated with an accessory ureteral bud in a
duplicated system, but it also can be seen in a single system.

19. ● At the 4-mm stage, starting at the cephalic portion of cloaca
where the allantois and gut meet,
cloaca progressively divides into two compartments by the caudal
growth of a crescentic fold, the urorectal fold.
● The two limbs of the fold bulge into the lumen of the cloaca from either
side, eventually meeting and fusing.
● The division of the cloaca into :
- a ventral portion (urogenital sinus) and
- dorsal portion (rectum)
is completed during the 7th week.

20. ● Division of the cloaca is completed before the cloacal membrane
ruptures, and its two parts therefore have separate openings.
● Ventral part is the primitive urogenital sinus,
- which has the shape of an elongated cylinder and
- continuous cranially with the allantois;
- its external opening is the urogenital ostium.
● Dorsal part is the rectum, and its external opening is the anus.

21. ● The Wolffian (mesonephric) ducts fuse with the cloaca just before its
subdivision by the urorectal septum
● Entrance of the mesonephric ducts into the primitive UGS serves as a
landmark distinguishing the cephalad vesicourethral canal from the
caudal UGS.

22. ● Vesicourethral canal gives rise to the bladder and pelvic urethra.
● Early in development, the bladder is continuous with the allantois.
● The caudal portion of the UGS forms
- phallic urethra in males and
- urethra and vaginal vestibule in females

23. BLADDER
● By 10th week of gestation,
- bladder is a cylindrical tube lined by a single layer of cuboidal cells
- surrounded by loose mesenchymal tissue.
- allantois becomes the urachus that connects the fetal bladder to the
yolk sac, which is rudimentary in humans and probably
nonfunctional.

24. - Apex of the bladder ultimately tapers as the urachus, and
- By the 12th week the urachus involutes to become a fibrous cord, the
median umbilical ligament.

25. ● By 7th - 12th week :
- Bladder epithelium consists of bilayered cuboidal cells and acquires
mature urothelial characteristics between weeks 13th and 17th.
- the surrounding connective tissues condense, and smooth muscle
fibers begin to appear, first at the region of the bladder dome and
later proceeding toward the bladder base.

26. ● By the 21st week :
- bladder epithelium becomes four to five cell layers thick and
demonstrates ,features similar to fully differentiated urothelium.

27. ● TRIGONE :
- At fifth or sixth week of gestation,
- the common excretory ducts (the portion of mesonephric ducts distal
to the origin of ureteric buds) dilate and connect to the UGS.
- Formation of these final connections involves apoptosis, which
enables the ureters to disconnect from the mesonephric ducts and
fuse to the bladder.

28. •THANK YOU