Functionally, the urogenital system can be divided
into two entirely different components: the urinary
system and the genital system.
Embryologically and anatomically, however, they are
Both develop from a common mesodermal ridge
(intermediate mesoderm) and the cloaca.
Initially the excretory ducts of both systems enter a
common cavity, the cloaca.
Development of Urinary System
Urinary system is derived from the intermediate
mesoderm and cloaca.
Epithelium of the urinary system is derived from the
After the folding of embryonic disc and formation of
peritoneal cavity, the intermediate mesoderm forms a
bulging on the posterior abdominal wall called
nephrogenic cords or urogenital ridge.
It is covered by epithelium lining the peritoneal cavity
It extends from cervical to sacral region.
At varying stage of development, a number of important
structures are formed in relation to nephrogenic cord as
Cloaca is subdivided into the ventral primitive
urogenital sinus and dorsal primitive rectum by the
Primitive urogenital sinus is subdivided into a cranial
part, called vesico-urethral canal and a caudal part,
called the definitive urogenital sinus.
The openings of mesonephric ducts lie at the junction
of these two subdivisions.
The definitive urogenital sinus is further divided into a
cranial pelvic part and a caudal phallic part.
Development of kidney systems
Three kidney systems are formed in a cranial-to-caudal
sequence during intrauterine life in humans:
The first of these systems is rudimentary and
nonfunctional; the second may function for a short
time during the early fetal period; the third forms the
At the beginning of the fourth week, the pronephros
is represented by 7 to 10 solid cell groups in the
These groups form vestigial excretory units,
nephrotomes, that regress before more caudal ones are
By the end of the fourth week, the pronephric system
The mesonephros and mesonephric ducts are derived from
intermediate mesoderm from upper thoracic to upper lumbar
Early in the 4th week of development, the first excretory tubules
of the mesonephros appear.
They lengthen rapidly, form an S-shaped loop, and acquire a
tuft of capillaries that will form a glomerulus at their medial
Around the glomerulus, the tubules form Bowman’s capsule,
and together these structures constitute a renal corpuscle.
Laterally, the tubule enters the longitudinal collecting
duct known as the mesonephric or wolffian duct.
While caudal tubules are still differentiating, cranial
tubules and glomeruli show degenerative changes, and
by the end of the 2nd month, the majority have
In the male, a few of the caudal tubules and the
mesonephric duct persist and participate in formation
of the genital system, but they disappear in the female.
Metanephros: The Definitive Kidney
The third urinary organ, the metanephros or permanent
kidney, appears in the 5th week.
Its excretory units develop from metanephric
mesoderm in the same manner as in the mesonephric
The development of the duct system differs from that
of the other kidney systems.
Formation of Collecting System
Collecting ducts of the permanent kidney develop from the
ureteric bud, an outgrowth of the mesonephric duct close to
its entrance to the cloaca.
The bud penetrates the metanephric tissue, which is molded over
its distal end as a cap.
The ureteric bud form a dilation called ampulla. The ampulla
First 3-5 generations of branches fuse to form pelvis of kidney.
Next division - major calyces and further divisions - minor
calyces and collecting tubules (1 to 3 millions).
These buds continue to subdivide until 12 or more
generations of tubules have formed.
During further development, collecting tubules of the
fifth and successive generations elongate considerably
and converge on the minor calyx, forming the renal
The ureteric bud gives rise to the ureter, the renal
pelvis, the major and minor calyces, and
approximately 1 to 3 million collecting tubules.
Formation of Excretory System
Each collecting tubule is covered by Metanephric cap.
Under the inductive influence of tubule, cells of tissue
cap forms renal vesicles, which in turn is converted
into ‘S’ shaped tubule.
Capillaries grow into the pocket at one end of the S and
differentiate into glomeruli.
These tubules, together with their glomeruli, form
nephrons, or excretory units.
The proximal end of each nephron forms Bowman’s
capsule, which is deeply indented by a glomerulus.
The distal end forms an open connection with the
Continuous lengthening of the excretory tubule results
in formation of the proximal convoluted tubule, loop
of Henle, and distal convoluted tubule.
Hence, the kidney develops from two sources:
Metanephric mesoderm, which provides
excretory units and
Ureteric bud, which gives rise to the collecting
Ascent of kidney
Definitive human kidney develops from the
metanephros in the sacral region.
Ascends due to the differential growth of the
Final position is in the lumbar region.
The hilum faces medially.
The metanephros, at first, receives blood supply from
lateral sacral arteries, but with its ascent, branch of
the aorta at the level of L2 take over the supply.
Anomalies in shape
Horse shoe kidney
Anomalies in position
Present in sacral region
May be in thoracic region
Congenital polycystic kidney
Development of Ureter
Derived from the part of ureteric bud that lies
between pelvis of kidney and vesico-urethral
canal i.e., intermediate mesoderm.
The connective tissue & muscles are derived
from the intermediate mesoderm of the
Ureter may be duplicated partially or completely.
Abnormal opening of ureter.
Post- caval ureter - Right sided hydronephrosis.
Development of Urinary bladder
Epithelium of the bladder – develops from the cranial part of
the vesico-urethral canal (endoderm).
Epithelium of trigone – derived from the absorbed
mesonephric ducts (mesoderm)
Muscular and serous walls – derived from splanchnopleuric
Initially, bladder is continuous with allantois, but when the
lumen of allantois is obliterated, a thick fibrous cord, the
It connects the apex of bladder to umbilicus, in adult called as
median umbilical ligament.
The caudal portions of the mesonephric ducts are absorbed into the
wall of the urinary bladder.
Consequently, the ureters, initially outgrowth from the mesonephric
ducts, enter the bladder separately.
As kidneys ascend, the orifices of the ureters move farther cranially;
those of the mesonephric ducts move close together to enter the
prostatic urethra and in males becomes the ejaculatory ducts.
Since both the mesonephric ducts and ureters originate in the
mesoderm, the mucosa of the bladder formed by incorporation of the
ducts (the trigone of the bladder) is also mesodermal.
With time, the mesodermal lining of the trigone is replaced by
endodermal epithelium, so that finally, the inside of the bladder is
completely lined with endodermal epithelium.
Urachal fistula, urachal cyst or sinus:
The lumen is persist in the urachus.
Development of Urethra
From urinary bladder upto the openings of ejaculatory ducts
– derived from caudal part of vesicourethral canal. Posterior
wall derived from absorbed mesonephric ducts.
Rest of prostatic urethra and membranous urethra derived from the pelvic part of definitive urogenital sinus.
Penile part of the urethra - derived from the phallic part of
definitive urogenital sinus.
The most terminal part - derived the ECTODERM
Derived from the caudal part of vesico-urethral
As it corresponds to the prostatic part of male urethra,
may receive slight contribution from pelvic part of
Development of Prostate
Develops from large number of buds arising from the
epithelium of the prostatic urethra, i.e. caudal part of the
vesico-urethral canal and from pelvic part of definitive urogenital
Buds arising from the mesodermal part of prostatic urethra
(posterior wall above the openings of ejaculatory ducts) form
inner glandular zone.
Buds arising from the rest of prostatic urethra (endoderm) form
the outer glandular zone.
Muscles and connective tissue are derived from surrounding
mesenchyme, which also form capsule of the gland.
Inner zone : Benign prostatic hypertrophy
Outer zone : Carcinoma
Female homologues of Prostate
Urethral gland – developed from buds arising from the
caudal part of vesico-urethral canal.
Paraurethral glands of Skene – develops from buds
arising from the urogenital sinus
Paramesonephric duct or Mullerian
The paramesonephric duct arises as a longitudinal invagination
of the coelomic epithelium of nephrogenic cord.
Cranially, the duct opens into the abdominal cavity with a funnel
Caudally, it first runs lateral to the mesonephric duct, then
crosses it ventrally to grow medially.
Then the two ducts meet and fuse in the mid line to form Uterovaginal canal (uterine canal).
The caudal end of this canal projects into the posterior wall of
definitive urogenital sinus.
Development of Uterus and Uterine tubes
Epithelium of the uterus : develops from the fused paramesonephric
ducts ( utero-vaginal canal).
Myometrium : derived from surrounding mesoderm.
Fundus of uterus : formed by partial merging of the unfused
horizontal parts of the two paramesonephric ducts within the
Uterine tubes : develop from unfused parts of paramesonephric
The point of invagination into coelomic epithelium remains as the
abdominal opening, where fimbria are formed.
Complete duplication of uterus (uterus didelphys) or
partial duplication (bicornuate uterus)
Absent on one or both sides
Partially or completely duplicated on one or both sides
Atresia of the tubes
Development of Vagina
Shortly after the solid tip of the paramesonephric ducts
reaches the urogenital sinus, two endodermal swellings called
sinovaginal bulbs, grow out from the pelvic part of the sinus.
These sinovaginal bulbs, proliferate and fuse to form a solid
By the fifth month, the vaginal outgrowth is entirely canalized.
The wing-like expansions of the vagina around the end of the
uterus, the vaginal fornices, are of paramesonephric origin.
Thus, the vagina has a dual origin, with the upper portion
derived from the uterine canal and the lower portion derived
from the urogenital sinus.
The lumen of the vagina remains separated from that of the
urogenital sinus by a thin tissue plate, the hymen.
Both surfaces of hymen are lined by endoderm.
Duplication of vagina
Rectovaginal fistula or
Paramesonephric ducts in Male
Remains rudimentary in male.
Greater part disappears.
Cranial end of each duct persists as a small rounded body
attached to the testis called Appendix of testis.
It has been considered that the prostatic urticle represents the
utero-vaginal canal and therefore homologous of the uterus,
however, it is now believed to correspond mainly to vagina
( and possibly part of uterus).
Development of external genitalia
In 3rd week of development, mesenchymal cells from primitive
streak migrate around the coacal membrane to form a pair of
elevated cloacal folds.
Cranial to the cloacal membrane the folds unite to form genital
Caudally, the folds are subdivided into urethral folds anteriorly
and anal folds posteriorly.
Meantime, another pair of elevations the genital swellings
appear on each side of urethral folds.
Development of Male external genitalia
Genital tubercle becomes cylindrical, which is now called the
phallus. Undergoes enlargement to form penis.
As the phallus grows, the glans become visible by the appearance of
Prepuce is formed from ectoderm.
During this elongation, the phallus pulls the urethral folds forward so
that they form the lateral wall of the urethral groove.
This groove extends along the undersurface of the elongated phallus
but does not reach the most distal part, glans.
The epithelial lining of the groove which originates in the
endoderm, forms the urethral plate.
At 3rd month, the two urethral folds close over the urethral
plate, forming the penile urethra.
This canal does not extend to the tip of the phallus.
Most distal end of the urethra is formed from the ectodermal
cells from the tip of glans, forming external urethal meatus.
The genital swellings fuse in the midline to form scrotal sac
into which the testis later descend.
Development of Female external genitalia
Genital tubercles : clitoris.
Genital swellings : labia majora
Urethral folds : labia minora
Urogenital membrane breaks down and establishes
continuity between urogenital sinus and the exterior.
Clitoris may be absent, bifid or double.
Labia minora may show partial fusion.
Urethra may open into anterior wall of vagina,
equivalent to hypospadias.
Priomordial Germ Cells & Gonads
Priomordial germ cells are formed in the epiblast during the 2nd week of
development, then move to wall of the Yolk sac.
During 4th week they move to developing gonads.
Gonads do not develop as long as primordial germ cells do not reach them.
These cells have an inducing effect on the gonad.
Shortly before and during arrival of primordial germ cells, the epithelium of
the genital ridge proliferates, and epithelial cells penetrate the underlying
Here they form a number of irregularly shaped cords, the primitive sex cords.
In both male and female embryos, these cords are connected to surface
epithelium, and it is impossible to differentiate between the male and female
gonad. Hence, the gonad is known as the indifferent gonad.
Development of testis
Each testis develops from the coelomic epithelium.
If the embryo is genetically male, the primordial germ cells
carry an XY sex chromosome complex.
The primitive sex cords continue to proliferate and penetrate
deep into the medulla to form the testis or medullary cords.
Toward the hilum of the gland, the cords break up into a network
of tiny cell strands that later give rise to tubules of the rete testis.
A dense layer of fibrous connective tissue, the tunica
albuginea, separates the testis cords from the surface epithelium.
In the 4th month, testis cords are now composed of primitive
germ cells and sustentacular cells of Sertoli derived from the
surface epithelium of the gland.
Interstitial cells of Leydig, derived from the original
mesenchyme of the gonadal ridge, lie between the testis cords.
Testis cords remain solid until puberty, when they acquire a
lumen, thus forming the seminiferous tubules.
Duct system of testes
We know most of the mesonephric tubules
Some of these that lie near the testis persist and, along
with the mesonephric duct, form the duct system of
The ends of seminiferous tubules anastomose with
one another to form the rete testes.
Rete testes, in turn, establishes contact with persisting
mesonephric tubules which form the vasa
Cranial part of the mesonephric duct becomes highly
coiled on itself to form the epididymis.
Its distal part forms the ductus deferens.
Seminal vesicle arise on either side as a diverticulum from
the lower end of the mesonephric duct.
The part of the mesonephric duct that lies between its
opening into the prostatic urethtra and the origin of this
diverticulum, forms the ejaculatory duct.
Absent on one or both sides.
Two testes may be fused together
Ectopia : Femoral canal, Perineum behind the scrotum, Under
the skin of lower part of abdomen.
Duct system of testis
Seminiferous tubules fail to connect with vasa efferentia
Ductus deferens may be absent
Descent of Testes
Testes develop in relation to lumbar region of the
posterior abdominal wall.
During fetal life they gradually descend to the scrotum.
Reach iliac fossa during 3rd month.
Lie at the site of deep inguinal ring up to 7th month.
Pass through the inguinal canal during 7th month.
Normally in the scrotum by the end of 8th month.
Factors controlling the descent of testes
Not entirely clear.
Differential growth of body wall.
Hormones secreted by pars anterior of the hypophysis
Development of ovary
The primitive sex cords dissociates into small masses.
The cells of each mass surrounds one primordial germ cell or
oocyte, to form a primordial follicle.
These sex cords then undergo regression and replaced by a new set
of cortical cords arising from the coelomic epithelium.
Interstitial gland cells differentiates from mesenchyme of the
As no tunica albuginea is formed, the germinal epithelium may
contribute to the ovary even in the postnatal life.
Absent on one or both sides
May descend in to the inguinal canal or even into
Descent of ovary
Fate of Mesonephric duct (Wolffian duct) and
tubules in the Male
Mesonephric ducts (Wolffian ducts) gices
rise to :
Ureteric buds from which the ureters, pelces, calyces and
collecting tubules of the kidneys.
Tigone of the bladder
Posterior wall of the part of the prostatic urethra, cranial to the
openings of the ejaculatory ducts.
Mesodermal part of prostate
Appendix of epididymis
Remnants of mesonephric tubules
Superior aberrant ductules
Inferior aberrant ductules
Fate of Mesonephric duct (Wolffian duct) and
tubules in the Female
Vestigial structures like :
Control of differentiation of genital organs
The factors that determine whether a male or female genital
system will develop are as follow:
Sex chromosome i.e., XX for female and XY for male
Y chromosome contains testis determining gene called the SRY
(sex determining region on Y) gene on its short arm.
SRY protein is the testis determining factor, under its influence
male development occurs.
In its absence, female development is established.