Download 06. urinary embryo

Development of Urinary System
The urogenital system develops from the
intermediate mesoderm which extends
along the dorsal body wall of the embryo
During folding of the embryo in the
horizontal plane this mesoderm is carried
ventrally and loses its connection with
somites.
A longitudinal elevation of mesoderm forms
on each side of the dorsal aorta – The
Urogenital Ridge. This ridge gives rise to parts
of the urinary system which is the nephrogenic
cord or ridge.
Development of the Kidneys
3 sets are formed. The first set is the
pronephroi:
These transitory, nonfunctional structures
appear in the 4th week.
The pronephroi 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 rudimentary pronephroi
soon degenerate. Most of the
pronephric ducts persist and are
utilized by the next set of the
kidneys.
Mesonephroi
These large; elongated;
excretory organs appear late in
the 4th week caudal to the
rudimentary pronephroi.
They are well developed and
function as inteim kidneys for
about 4 weeks.
It consists of glomeruli and
mesonephric tubules. These
tubules open into the
mesonephric ducts.
The mesonephric ducts open
into the cloaca.
The mesonephroi degenerate
toward the end of the first
trimester.
In male, the tubules become the
Efferent ductules of the testes.
In female, it become the
Epoophoron and Paroophoron.
The duct gives rise in male to
paradidymis; appendix of
epididymis; duct of epididymis;
ductus deferens; ureter; pelvis;
calices; collecting tubules;
ejaculatory duct and seminal
gland.
The duct in female gives rise to
appendix vesiculosa; duct of
epoophron; duct of Gartner;
ureter ; pelvis; calices and
collecting tubules.
From 5th to 8th week
Metanephroi
The primordia of permanent kidneys begin to develop early in the 5th week and start to
function about 4 weeks later. The permanent kidneys develop from 2 sources, the
metanephreic diverticulum ( ureteric bud ) and the metanephric mass of intermediate
mesoderm ( metanephrogenic blastema ).
The metanephric diverticulum is an outgrowth from the mesonephric duct near its entrance
into the cloaca. The metanephric mass of the intermediate mesoderm is derived from the
caudal part of the nephrogenic cord. Both primordia of the metanephros are mesodermal
origin.
Metanephric diverticulum;
It is the primordium of the ureter; renal pelvis; calices and collecting tubules. As it
elongates, it penetrates the metanephric mass of the intermediate mesoderm. The stalk of it
becomes the ureter and its expanded cranial end forms the renal pelvis.
The straight collecting tubules undergo repeated branching, forming generations of tubules.
The first 4 generations enlarge and become confluent to form the major calices. The 2nd four
generations coalesce to form the minor calices. The remaining generations of tubules form
the collecting tubules.
The end of each arched collecting tubule induces clusters of mesenchymal cells in the
metanephric mass of mesoderm to form metanephric vesicles. These vesicles elongate and
become metanephric tubules.
The proximal ends of these tubules are invaginated by glomeruli. Between the 10th & 18th
weeks of gestation, the number of the glomeruli increases until the 32 week, when an upper
limit is reached. Their filtration begins around the 9th fetal week and the rate of filtration
increases after birth.
The renal corpuscle ( glomerulus and glomerular capsule ) and its proximal convoluted
tubules; nephron loop ( of Henle ) and distal convoluted tubule constitute a nephron.
Each distal convoluted tubules contacts an arched collecting tubule.
A uriniferous tubule:
It consists of 2 different parts
1.Nephron from the metanephron mass of intermediate mesoderm. The nephron
formation is complete at birth.
2- Collecting tubule from the metanephric diverticulum.
The metanephric diverticulum and the metanephric mass of intermediate mesoderm interact
and induce each other by a process known as reciprocal induction to form the
permanent kidneys.
Transformation of the metanephric mesenchyme to epithelial cells of the nephon
( mesenchymal- epithelial transition ) is regulated by factors secreted by the
metanephric diverticulum.
Fetal kidneys
They are divided into lobes. This
lobulation diminishes toward the end of
fetal period, but the lobes are still
inducated in the kidneys of a newborn
infant. These lobulation disappears by
the end of the first postnatal year during
infancy as the nephrons increase and
grow.
At term, each kidney contains 800,000 to
1000,000 nephrons. 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.
Functional maturation of the kidneys
occurs after birth.
Positional Changes of Kidneys
Initially the metanephric kidneys lie close to each other in the pelvis ventral to the
sacrum.
As the abdomen and pelvis grow, caudal to the kidneys, so that they occupy more
cranial level. They gradually come to lie in the abdomen and move farther apart
and attain their adult position by 9th week.
Initially the hilum faces ventrally, however, as the kidney ascends it rotates
medially 90 degrees. By the 9th week the hilum is directed anteromedially.
Changes in Blood Supply of kidneys
Initially, the renal arteries are branches of the common iliac arteries. As they
ascend, they receive their blood supply from the distal end of the aorta. Then,
they receive new branches from the aorta. When they come into contact with the
suprarenal glands in the 9th week their ascend stops.
Normally, the caudal branches undergo involution and disappear. The right renal
artery is longer and often more superior.
Cystic kidney Disease
Polycystic kidney is an autosomal recessive disorder ( ARPKD ) that is
diagnosed at birth or in utero by ultrasonography. Both kidneys contains
many hundred small cysts which results in renal insufficiency. Gene
mutations have been implicated.
Death of the infant usually occurs shortly after birth. These infants may
survive by using postnatal dialysis and kidney transplantation.
Multicystic dysplastic kidney disease (MDK) results from dysmorphology
during development of the renal system. The outcome for children with
MKD is generally good since, in 75% of cases, the disease is unilateral. In
it, fewer cysts are seen than in ARPKD and can range in size from a few
millimeters to many centimeters in the same kidney.
Now, it is believed that the cystic structures are wide dilations of parts of
the otherwise continuous nephrons, particularly the nephron loops
( of Henle) .
5 week
Development of Urinary Bladder
Division of the cloaca by urorectal septum into dorsal rectum and a ventral urogenital sinus.
The urogenital sinus is divided into 3 parts:
1- A cranial vesical part that forms most of the bladder and is continuous with the allantois.
2- A middle pelvic part that becomes the urethra in the bladder neck and the prostatic part of
urethra in male and the entire urethra in females.
3- A caudal phallic part that grows toward the genital tubercle.
5 week
Development of Urinary Bladder
The bladder develops mainly from the vesical part of the urogenital sinus, but the trigon is
derived from the caudal ends of the mesonephric ducts.
The epithelium of the entire 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
As the bladder enlarges, the distal parts of the mesonephric ducts are incorporated into its
dorsal wall. These ducts contribute to the formation of the connective tissue in the trigone
of the bladder.
As the caudal ends mesonephric ducts are absorbed, the ureters come to open separately
into the urinary bladder.
18 week
12 weeks
Initially the bladder is continuous with the allantois. The allantois become constricted and
form a thick fibrous cord, the urachus which extends from the apex of the bladder to the
umbilicus.
In the adult the urachus is represented by the median umbilical ligament which is attached
to the apex of the bladder and extends along the posterior surface of the anterior abdominal
wall .
The median umbilical ligament lies between the medial umbilical ligaments which are the
fibrous remnants of the umbilical arteries.
Due to traction which is exerted by the kidneys during their ascent the orifices of
the ureters move superolaterally and the ureters enter oblique through the base of
the bladder.
The distal ends of the mesonephric ducts in females degenerate.
The caudal ends of mesonephric ducts become the ejaculatory ducts. The orifices
of these ducts move close together and enter the prostatic part of the urethra.
In infants and children the urinary bladder, is in the abdomen when it is empty. It
begins to enter the greater pelvis at about 6 years of age. It enters lesser pelvis
and become a pelvic organ at or even after puberty.
Urachal Anomalies
A- Urachal cysts
Remnants of the epithelial lining of the
urachus may give rise to cysts. These
cysts can be observed in about one- third
of cadavers, but are not usually detected
on living persons unless they become
infected.
B- Urachal Sinus
A remnant of the lumen usually persists
in the inferior part of the urachus and in
about 50 % of cases the lumen is
continuous with the cavity of the bladder.
The lumen in the superior part of the
urachus may remain patent and form a
urachal sinus that opens at the umbilicus.
C- Urachal Fistula
Very rarely the entire urachus remains
patent and form a urachal fistula that
allows urine to escape from its umbilical
orifice.
11 weeks
12 weeks
14 weeks
Development of Urethra
The epithelium of most of the male urethra is derived from the endoderm of the phallic part
of the urogenital sinus. The epithelium of the terminal part of the urethra is derived from the
surface ectoderm.
The entire female urethra is derived from endoderm of the urogenital sinus.
The distal part of the spongy urethra in the glans of the penis is derived from a solid cord of
ectodermal cells which is derived from the surface ectoderm. These cells grow from the tip
of the glans and joins the rest of the spongy urethra. So, the spongy urethra has a dual
origin.
The connective tissue and smooth muscle of the urethra in both sexes are derived from
splanchnic mesenchyme.
Development of the Suprarenal Gland
The medulla develops from neural crest cells that derived from an adjacent
sympathetic ganglion.
The cortex develops from mesodermal cells which is first indicated during the 6th
weeks by an aggregation of mesenchymal cells on each side, between the root of
the dorsal mesentery and the developing gonad.
Initially the neural crest cells form a mass on the medial side of the fetal cortex.
As they are surrounded by the fetal cortex, these cells differentiate into the
secretory cells of the medulla.
Later more mesenchymal cells arise from the mesothelium lining the posterior
abdominal wall and enclose the fetal cortex. These cells give rise to the
permanent cortex.
Differentiation of the suprarenal cortical zones begin during the late fetal period.
The zona glomerulosa and the zona fasciculata are pressent at birth, but the zona
reticularis is not recognizable until the end of the third year.
54 days
28 weeks
The suprarenal glands of the human fetus are 10 to 20 times larger than the adult
glands relative to body weight and are large compared with the kidneys. These
large glands result from the extensive size of the fetal cortex.
The medulla remains relatively small until after birth.
The suprarenal glands rapidly become smaller as the fetal cortex regresses
during the first year.
The glands lose about one- third to about half of their weight during the first 2 or 3
weeks after birth and do not regain their original weight until the end of the 2nd
year.