3. The Urogenital System
The urogenital system develops from the intermediate mesenchyme
(mesoderm) – Urogenital Ridge.
The part of the urogenital ridge giving rise to the urinary system is the
nephrogenic cord and the part giving rise to the genital system is
the gonadal ridge
4. Development of Kidneys and Ureters
· Three sets of successive kidneys develop in
human embryos.
The first set — pronephrons— is
rudimentary and nonfunctional
The second set — mesonephrons —
is well developed and functions
briefly during the early fetal
period.
The third set — metanephrons
forms the permanent kidneys.
5.
6. Pronephron
These bilateral structures appear early in the fourth week.
They are represented by a few cell clusters and tubular
structures in the neck region
The pronephric ducts run caudally and open into the Cloaca .
The pronephron soon degenerate; however, most parts of the
pronephric ducts persist and are used by the next set of kidneys.
7. Mesonephron
These large, elongated excretory organs appear late in the fourth week,
caudal to the pronephron.
The mesonephron are well developed and function as interim kidneys for
approximately 4 weeks, until the permanent kidneys develop and function.
The mesonephric kidneys consist of glomeruli (10 –50 per kidney) and tubules.
The mesonephric tubules open into bilateral mesonephric ducts which open
into the cloaca .
The mesonephron degenerate toward the end of the first trimester.
8. Metanephron
Metanephron - permanent kidneys —begin to dev
approximately 4 weeks later.
The permanent kidneys develop from two sources:
(1) The ureteric bud ( metanephric diverticulum)
(2) The metanephrogenic blastema (metanephric mass of
mesenchyme)
9. · The ureteric bud is a
diverticulum
(outgrowth) from the
mesonephric duct
near its entrance into
the cloaca.
· The metanephrogenic
blastema is derived
from the caudal part
of the nephrogenic
cord.
· As the ureteric bud
elongates, it
penetrates the
metanephrogenic
blastema —a
metanephric mass of
mesenchyme
10. Development of Kidneys and Ureters
· The stalk of the ureteric bud becomes the ureter.
· The cranial part of the bud undergoes repetitive branching,
forming branches which differentiate into the collecting
tubules of the metanephrons.
· The first four generations of tubules enlarge and become
confluent to form the major calices, and the second four
generations coalesce to form the minor calices.
· The end of each arched collecting tubule induces clusters of
mesenchymal cells in the metanephrogenic blastema to form
small metanephric vesicles.
11.
12. A uriniferous tubule
consists of two
embryologically
different parts:
A nephron derived
from the
metanephrogenic
blastema
A collecting tubule
derived from the
ureteric bud
13. · Between the 10th and 18th weeks, the number of glomeruli
increases gradually and then increases rapidly until the 32nd week.
· The increase in kidney size after birth results mainly from the
elongation of the proximal convoluted tubules as well as an
increase of interstitial tissue.
· Nephron formation is complete at birth except in premature
infants. Although glomerular filtration begins at approximately the
ninth fetal week, functional maturation of the kidneys and
increasing rates of filtration occur after birth .
14. Positional Changes of Kidneys
Initially the primordial permanent kidneys lie close to each other in the pelvis.
As the abdomen and pelvis grow, the kidneys gradually relocate to the abdomen and
move farther apart. They attain their adult position by the ninth week (contact with the
suprarenal glands).
This “ascent” results mainly from the growth of the embryo’s body caudal to the kidneys
Initially the hilum of each kidney, where blood vessels, the ureter, and nerves enter and
leave, face ventrally; however, as the kidneys relocate (“ascend”), they rotate medially
almost 90 degrees.
By the ninth week, the hila are directed anteromedially. Eventually the kidneys become
retroperitoneal (external to the peritoneum) on the posterior abdominal wall.
15.
16. Changes in Blood Supply of Kidneys
During the changes in the kidneys position, they receive their blood supply
from vessels that are close to them:
Initially, the renal arteries are branches of the common iliac arteries.
Later, the kidneys receive their blood supply from the distal end of the aorta.
When they are located at a higher level, they receive new branches from the
aorta.
Normally the caudal branches of the renal vessels undergo involution and
disappear.
The kidneys receive their most cranial arterial branches from the abdominal
aorta; these branches become the permanent renal arteries.
19. Renal Agenesis
· Renal Agenesis (absence): Can
be unilateral or bilateral
· Renal agenesis results when
the ureteric buds do not
develop or the primordia
(stalks of buds) of the ureters
degenerate.
· Failure of the ureteric buds to
penetrate the
metanephrogenic blastema
results in failure of kidney
development
20. Unilateral Renal Agenesis
· Unilateral renal agenesis (absence)
occurs approximately once in every
1000 neonates.
· Males are affected more often than
females and the left kidney is usually
the one that is absent.
· Unilateral renal agenesis often causes
no symptoms and is usually not
discovered during infancy because the
other kidney usually undergoes
compensatory hypertrophy and
performs the function of the missing
kidney.
· Unilateral renal agenesis should be
suspected in infants with a single
umbilical artery
21. Bilateral renal agenesis
· Bilateral renal agenesis is associated with
oligohydramnios (small amount of amniotic fluid)
because little or no urine is excreted into the
amniotic cavity.
· This condition occurs approximately once in 3000
births, and is incompatible with postnatal life.
· Infants with bilateral renal agenesis usually die
shortly after birth.
22. Malrotated Kidney
· If a kidney fails to
rotate, the hilum
faces anteriorly ; that
is, the fetal kidney
retains its embryonic
position
· Abnormal rotation of
the kidneys is often
associated with
ectopic kidneys.
23. Ectopic Kidneys
· One or both kidneys may
be in an abnormal
position.
· Most ectopic kidneys are
located in the pelvis, but
some lie in the inferior
part of the abdomen.
· Pelvic kidneys and other
forms of ectopia result
from failure of the kidneys
to ascend.
24.
25. Horseshoe Kidney
· In 0.2% of the population, the poles of the
kidneys are fused ; usually it is the inferior
poles that fuse.
· The large U - shaped kidney usually lies in
the pubic region.
· Normal ascent of the fused kidneys is
prevented because they are held down by
the root of the inferior mesenteric artery.
· A horseshoe kidney usually produces no
symptoms because its collecting system
develops normally and the ureters enter
the bladder.
· Approximately 7% of persons with Turner
syndrome have horseshoe kidneys.
26. Duplications of Urinary Tract
· Duplications of the
abdominal part of
the ureter and the
renal pelvis are
common.
· These defects result
from abnormal
division of the
ureteric bud.
27. · A supernumerary kidney with its own ureter, which is rare,
probably results from the formation of two ureteric buds.
28. Ectopic Ureter
· An ectopic ureter does not enter the urinary bladder.
· In males, the ureter usually opens into the neck of the bladder or
the prostatic part of the urethra. They may also enter the ductus
deferens, prostatic utricle, or seminal gland.
· In females, the ureter opens into the neck of the bladder, or the
urethra, vagina, or vestibule of vagina.
· Incontinence is the common complaint resulting from an ectopic
ureter because the urine flowing from the orifice does not enter
the bladder; instead it continually dribbles from the urethra in
males and the urethra and/or vagina in females .
29. Cystic Kidney Diseases
· In autosomal recessive polycystic kidney disease both kidneys
contain many small which result in renal insufficiency.
· Death of the infant usually occurs shortly after birth.
· Multicystic dysplastic kidney disease results from
dysmorphology during development of the renal system.
· In this kidney disease, fewer cysts are seen than in autosomal
recessive polycystic kidney disease and they range in size from
a few millimeters to many centimeters in the same kidney.
· The cystic structures are wide dilations of parts of the
otherwise continuous nephrons, particularly the nephron loops
(of Henle ).
30. Development of Urinary Bladder
· The urogenital sinus is divided into three parts
(imaginary):
A vesical part that forms most of the urinary bladder and is
continuous with the allantois.
A pelvic part that becomes the urethra in the neck of the bladder, the
prostatic part of the urethra in males, and the entire urethra in
females
A phallic part that grows toward the genital tubercle ( primordium of
the penis or clitoris
31. Development of Urinary Bladder
· The bladder develops mainly from the vesical part of
the urogenital sinus but its trigone (triangular area at
the base of the bladder between the openings of the
ureters) is derived from the caudal ends of the
mesonephric ducts.
· The entire epithelium of the bladder is derived from
the endoderm of the vesical part of the urogenital
sinus.
· The other layers of its wall develop from adjacent
splanchnic mesenchyme .
