This session discusses the development of the urogenital system and includes: 1. Development of the kidneys and ureters 2. Development of the bladder and urethra 3. Development of the gonads and genital ducts 4. Development of the external genitalia

1. • The Urogenital System
• The Kidneys
• The Bladder and Urethra
• Gonads, Genital Ducts, and External Genitalia
• Descent of the Testes and Ovaries

2.  The nephron is the functional unit of the kidney; it is composed of a glomerulus, Bowman’s capsule,
and ducts which together form the excretory unit that drains into the collecting duct

3.  Urinary and genital systems are functionally separate but are both embryologically derived from the
intermediate mesoderm, composed of a nephrogenic cord and a gonadal ridge

4.  Three kidney systems form: pronephros (pl. -nephroi), mesonephros, and metanephros
 The pronephroi are composed of segmented cell clusters in the cervical region, called nephrotomes,
that soon degenerate; their duct extends to the cloaca and persists as the mesonephric duct

5.  The mesonephros forms a large ovoid body from the upper thoracic to upper lumbar region; the
excretory tubules become S-shaped; their medial end becomes Bowman’s capsule and it acquires a
tuft of capillaries that forms the glomerulus; the lateral end connects with the mesonephric duct
(also called wolffian duct) which is the remnant of the pronephric duct
 All the mesonephric excretory units and duct degenerate in female; some of the caudal units and duct
persist in the male and contribute to the genital system

8.  The definitive kidney forms from two sources: the ureteric bud and the metanephric mesoderm
 The ureteric bud grows from the caudal end of the mesonephric duct towards the metanephric
mesoderm and begins forming generations of branches
 The ureteric bud forms the ureter, then renal pelvis, major calices, minor calices, and finally
collecting tubules; these tubules induce the metanephric tissue cap to form renal vesicles that
develop into the excretory units (Bowman’s capsule and excretory tubules)

12.  The kidneys ascend from their initial pelvic position to the lumbar region due to
• Diminution of body curvature
• Growth of body in lumbar and sacral regions
 Blood supply also changes as the kidneys ascend until they arise from aorta in the lumbar region;
kidneys become functional at the 12th week

14.  The distal end of the hindgut is dilated, forming the cloaca; the boundary between the cloaca and the
ectoderm is the site of the cloacal membrane; the anterior portion of the cloaca receives the
allantois; mesoderm from around the allantois and yolk sac forms the urorectal septum which
separates the region of the allantois from that of the rest of the hindgut
 Later the tip of the septum comes close to the cloacal membrane, though they never make contact;
the cloacal membrane ruptures at the end of the 7th week forming a ventral opening for the
urogenital sinus and a dorsal opening for the anorectal canal, separated by the urorectal septum (later
forming the perineal body)

16.  The urogenital sinus is divided into three parts:
• Upper part (vesical part): develops into the bladder
• Middle part (pelvic part): develops into urethra (entire urethra in females and prostatic and membranous parts of male urethra)
• Lower part (phallic part): grows toward the genital tubercle

17.  The urogenital sinus is divided into three parts:
• Upper part (vesical part): develops into the bladder
• Middle part (pelvic part): develops into urethra (entire urethra in females and prostatic and membranous parts of male urethra)
• Lower part (phallic part): grows toward the genital tubercle

18.  The bladder is initially continuous with the lumen of the allantois; later allantois becomes a thick
fibrous cord known as urachus; urachus is called median umbilical ligament in adults
 The caudal ends of mesonephric ducts are absorbed into the cloaca, thus the ureters obtain their
own separate entrance into the cloaca; ascent of kidneys pulls ureters cranially while mesonephric
ducts move close together to form ejaculatory ducts in male and enter prostatic urethra
 Site of incorporation of ureters and ducts is called trigone of bladder and is this formed by their
mesoderm; however, later the endoderm proliferates and replaces trigone mesoderm

19.  The bladder is initially continuous with the lumen of the allantois; later allantois becomes a thick
fibrous cord known as urachus; urachus is called median umbilical ligament in adults
 The caudal ends of mesonephric ducts are absorbed into the cloaca, thus the ureters obtain their
own separate entrance into the cloaca; ascent of kidneys pulls ureters cranially while mesonephric
ducts move close together to form ejaculatory ducts in male and enter prostatic urethra
 Site of incorporation of ureters and ducts is called trigone of bladder and is this formed by their
mesoderm; however, later the endoderm proliferates and replaces trigone mesoderm

22.  The epithelium of the male urethra is derived from endoderm except for its caudalmost end (navicular
fossa) in the glans which is derived from ectoderm
 This endoderm proliferates and gives rise to the prostate and bulbourethral glands also

23.  The entire female urethra is derived from endoderm; the endoderm proliferates and gives rise to
urethral and paraurethral glands (Skene’s glands) in addition to the greater vestibular glands

24.  The genital ridges appear medial to the mesonephroi; these develop into the gonads and are derived
from three sources:
• Somatic mesoderm (mesothelium) lining posterior abdominal wall
• Underlying mesenchyme from intermediate mesoderm; some epithelial cords break and penetrate the
mesenchyme to form primitive sex cords in both sexes (indifferent gonad)
• Primordial germ cells (spermatogonia and oogonia) derived from the yolk sac; they migrate through dorsal
mesentery of hindgut until they reach the genital ridges and penetrate them

27.  In addition to the mesonephric or wolffian ducts, a pair of paramesonephric or mullerian ducts form
by invagination of the epithelium on the anterolateral surface of the genital ridge;
 The duct opens into the abdominal cavity above, runs down lateral to the mesonephric duct then
crosses it ventrally and comes in contact with the paramesonephric duct of the opposite site; initially
separated by a septum, later they fuse to form the uterine canal; they open into the urogenital sinus
at the paramesonephric or mullerian tubercle

31.  The Y chromosome carries the SRY gene which directs development of male ducts; this causes the
primitive sex cords to proliferate and penetrate deeper into the medulla to form testis or medullary
cords; toward the hilum they form a thin network called rete testis; later a dense layer of fibrous
connective tissue called tunica albuginea separates the testis cords from the surface epithelium
 Testis cords are solid cords composed of primordial germ cells and Sertoli cells; interstitial cells of
Leydig develop from the mesenchyme and begin secreting testosterone at the 8th week, allowing the
testis to influence genital ducts and external genitalia

32.  At puberty, testis cords obtain a lumen and become seminiferous tubules which are continuous with
the rete testis; rete testis connect with the excretory units of the mesonephric system in this area,
which are called epigenital tubules and later become the ductuli efferentes (efferent ductules);
 These efferent ductules are by default connected to the mesonephric duct which becomes highly
convoluted below the efferent ductules to form the epididymis; from the tail of the epididymis until
urogenital sinus, the mesonephric duct forms the vas deference (ductus deference)

33.  Near its entrance to the urogenital sinus, the vas deference forms the outgrowth of the seminal
vesicle; from this site to the urogenital sinus, the vas is known as the ejaculatory duct
 Paramesonephric ducts degenerate in males and their only derivatives are the prostatic utricle and
the appendix testis; the paramesonephric tubercle becomes the seminal colliculus; other derivatives
of the mesonephric ducts include the appendix epididymis and the paradidymis

37.  In females, absence of Y chromosome causes the primitive sex cords in the medulla to degenerate
and form ovarian medulla (stroma); however, a second generation of cords form which penetrate the
mesenchyme but remain close to the surface and each surrounds one or more primordial germ cells;
the secondary sex cords form follicular cells while the primordial germ cells differentiate into oogonia

38.  The paramesonephric ducts have three parts:
• A cranial vertical part which opens into the abdominal cavity
• A middle horizontal part which crosses the mesonephric duct
• A caudal vertical part which fuses with that of the opposite duct
 The first two parts form the uterine tube while the caudal ones form the uterus
 The mesonephric excretory units degenerate in females except for remnants like epoophoron and
paroophoron in the mesovarium; remnants of mesonephric duct are the duct of epoophoron and an
occasional Gartner’s cyst in the vaginal wall

39.  The second part of the paramesonephric ducts moves mediocaudally causing the entire urogenital
ridge to lie in a transverse plane; as a result, the peritoneal linings of both sides fuse, forming the
broad ligament of the uterus extending from the sides of the uterus to the lateral pelvic wall
 This ligament holds the uterine tube in its upper border and the ovary on its posterior border by the
mesovarium; the ligament divides the pelvic cavity into the uterorrectal and uterovesical pouches

42.  The entrance of the paramesonephric ducts into the urogenital sinus is called the mullerian or
paramesonephric tubercle; at this site, two solid evaginations grow cranially, increasing the distance
between uterus and urogenital sinus; these are sinuvaginal bulbs and fuse to form a vaginal plate
 Later the plate completely canalizes except for a thin tissue plate at its caudal end called the hymen;
it develops a small opening in perinatal life; the wing-like vaginal fornices near the uterus are derived
from paramesonephric ducts; thus the vagina has dual origin: urogenital sinus (endoderm) and
paramesonephric ducts (intermediate mesoderm)

43.  The entrance of the paramesonephric ducts into the urogenital sinus is called the mullerian or
paramesonephric tubercle; at this site, two solid evaginations grow cranially, increasing the distance
between uterus and urogenital sinus; these are sinuvaginal bulbs and fuse to form a vaginal plate
 Later the plate completely canalizes except for a thin tissue plate at its caudal end called the hymen;
it develops a small opening in perinatal life; the wing-like vaginal fornices near the uterus are derived
from paramesonephric ducts; thus the vagina has dual origin: urogenital sinus (endoderm) and
paramesonephric ducts (intermediate mesoderm)

45.  Mesenchyme on either side of the cloacal membrane proliferates and forms cloacal folds; lateral to
the cloacal folds, labioscrotal swellings also form; the cloacal folds fuse at the cranial end of the
cloacal membrane to form a genital tubercle; once the urorectal septum arrives, the folds are called
urethral folds ventrally and anal folds dorsally

46.  In the male, the genital tubercle (phallus) enlarges rapidly and pulls the urethral folds forward; the
urethral folds surround a lining of endoderm derived from elongation of the phallic part of urogenital
sinus, and is called the urethral plate; later the two folds fuse, forming the penile urethra
 Penile urethra doesn’t reach the tip of the glans; ingrowth of ectodermal cells forms a solid epithelial
cord which connects with penile urethra and later canalizes to form the navicular fossa and external
urethral meatus
 Labioscrotal swellings are called scrotal swellings in the male; they fuse to form the two sides of the
scrotum and are separated by a scrotal septum

47.  In the female, the genital tubercle doesn’t enlarge and forms the clitoris; the urethral folds don’t fuse
and remain separate as the labia minora; the labioscrotal swellings (called labial swellings in females)
remain separate and form the labia majora; the urethral groove becomes the vestibule and receives
the openings for both the urethra and the vagina