EMBRYOLOGY

1. Urogenital system
Themba Hospital FCOG(SA) Part 1 Tutorials
By Dr N.E Manana

2. Intro
• The urogenital system can be divided into two entirely different
components: the urinary system and the genital system
• Embryologically and anatomically, however, they are intimately
interwoven
• Both develop from a common mesodermal ridge (intermediate
mesoderm)
• Initially, the excretory ducts of both systems enter a common cavity,
the cloaca

3. Kidney Systems
• Three slightly overlapping kidney systems are formed in a cranial-to-
caudal sequence
• The pronephros, mesonephros, and metanephros
• The pronephros is rudimentary and non functional; the mesonephros
function for a short time during the early fetal period; the
metanephros forms the permanent kidney.

4. Pronephros
• At the beginning of the fourth week, is represented by 7 to 10 solid
cell groups in the cervical region
• These groups form vestigial excretory units (nephrotomes), that
regress before more caudal ones are formed
• By the end of the fourth week, all indications of the pronephric
system have disappeared.

5. Mesonephros
• The mesonephros and mesonephric ducts are derived from
intermediate mesoderm from upper thoracic to upper lumbar (L3)
segments
• During regression of the pronephric system, the first excretory
tubules of the mesonephros appear
• Form an S-shaped loop, and acquire a tuft of capillaries that will form
a glomerulus at their medial extremity
• Around the glomerulus, the tubules form Bowman’s capsule

8. Mesonephros
• Laterally, the tubule enters the longitudinal collecting duct known as
the mesonephric or wolffian duct
• The mesonephros forms a large ovoid organ on each side of the
midline
• Since the developing gonad is on its medial side, the ridge formed by
both organs is known as the urogenital ridge
• Cranial tubules and glomeruli show degenerative changes, and by the
end of the second month, the majority have disappeared

9. Metanephros: The Definitive Kidney
• The third urinary organ, the metanephros or permanent kidney,
appears in the fifth week
• Its excretory units develop from metanephric mesoderm
• The development of the duct system differs from that of the other
kidney systems.

12. 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 moulded over its
distal end as a cap
• Subsequently, the bud dilates, forming the primitive renal pelvis, and splits
into cranial and caudal portions, the future major calyces
• Each calyx forms two new buds while penetrating the metanephric tissue.
These buds continue to subdivide until 12 or more generations of tubules
have formed

14. Excretory System
• Each newly formed collecting tubule is covered at its distal end by a
metanephric tissue cap
• Under the inductive influence of the tubule, cells of the tissue cap form
small vesicles, which in turn give rise to small S-shaped tubules
• Capillaries grow into the pocket at one end of the S and differentiate into
glomeruli
• These tubules, together with their glomeruli, form nephrons ( excretory
units)

17. Bladder and Urethra
• During the 4th to 7th week the cloaca divides into the urogenital sinus
anterior and the anal canal posterior
• The urorectal septum is a layer of mesoderm btn the primitive anal canal
and the urogenital sinus, The tip of the septum will form the perineal body
• 3 positions of the urogenital sinus can be distinguished: Upper and largest
part (Urinary Bladder)
• The urachus remains attached to the apex of the bladder and the umbilicus
after the allantois is obliterated (Median umbilical ligament)
• Next part is the pelvic part of the urogenital sinus (Prostatic and
membranous parts of the urethra)
• The last part is the phallic part of the urogenital sinus

19. Bladder and Urethra
• 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
• At the end of the third month, epithelium of the prostatic urethra
begins to proliferate and forms a number of outgrowths that
penetrate the surrounding mesenchyme(prostate gland, urethral and
paraurethral glands)

22. Genital system
• The key to sexual dimorphism is the Y chromosome, which contains
the testis-determining gene called the SRY (sex-determining region on
Y)
• Its found on the short arm (Yp11)
• The protein product of this gene initiates a cascade of genes that
determine the fate of the rudimentary sexual organs
• In the abscess of the SRY protein the female development is
established

23. Gonads
• The gonads do not acquire male or female morphology
characteristics until the seventh week of development
• Gonads appear initially as a pair of longitudinal ridges (Genital or
Gonadal ridges)
• They are formed by proliferation of the epithelium and a
condensation of underlying mesenchyme
• Germ cells do not appear in the genital ridges until the sixth week of
development
• If they(PGC’s) fail to reach the gonads, the gonads do not develop
• primitive sex cords

25. Testis
• Under influence of the SRY gene on the Y chromosome, which encodes the
testis determining factor,
• The primitive sex cords continue to proliferate and penetrate deep into the
medulla to form the testis or medullary cords
• The cords break up into a network of tiny cell strands that later give rise to
tubules of the rete testis
• During further development, a dense layer of fibrous connective tissue, the
tunica albuginea, separates the testis cords from the surface epithelium
• Testis cords are now composed of primitive germ cells and sustentacular
cells of Sertoli derived from the surface epithelium of the gland

27. Testis
• Interstitial cells of Leydig, derived from the original mesenchyme of
the gonadal ridge, lie between the testis cords.
• By the eighth week of gestation, Leydig cells begin production of
testosterone and the testis is able to influence sexual differentiation
of the genital ducts and external genitalia
• Testis cords remain solid until puberty, when they acquire a lumen,
thus forming the seminiferous tubules, they join the rete testis
tubules, which in turn enter the ductuli efferences
• They link the rete testis and the mesonephric or wolffian duct, which
becomes the ductus deferens

29. Ovary
• No Y chromosome, primitive sex cords dissociate into irregular cell
clusters
• Later, they disappear and are replaced by a vascular stroma that
forms the ovarian medulla
• The surface epithelium of the female gonad, unlike that of the male,
continues to proliferate
• In the seventh week, it gives rise to a second generation of cords,
cortical cords, which penetrate the underlying mesenchyme but
remain close to the surface
• Cell clusters form Follicular cells (Primordial follicle)

31. Genital Ducts
• The paramesonephric duct arises as a longitudinal invagination of the
epithelium on the anterolateral surface of the urogenital ridge
Cranially, the duct opens into the abdominal cavity with a funnel-like
structure
• Caudally, it first runs lateral to the mesonephric duct, then crosses it
ventrally to grow caudomedially
• The two ducts are initially separated by a septum but later fuse to form the
uterine canal
• The combined ducts projects into the posterior wall of the urogenital sinus,
where it causes a small swelling, the paramesonephric or müllerian
tubercle

38. External genitalia
• In the third week , mesenchyme cells originating from the primitive streak
migrate around the cloacal membrane to form cloacal folds
• Cranial to the cloacal membrane the folds unite to form the genital
tubercle
• Caudally the are subdivided into urethral folds anteriorly and anal folds
posteriorly
• Another pair of elevations, the genital swellings become visible on each
side of the urethral folds, they later from scrotum or labia majora
• At the end of the sixth week it is still impossible to distinguish between the
two sexes

40. External Genitalia in the Male
• Influence of androgens secreted by the fetal testes
• Characterized by rapid elongation of the genital tubercle, which is now
called the phallus
• During this elongation, the phallus pulls the urethral folds forward so that
they form the lateral walls of the urethral groove
• The epithelial lining of the groove (Endoderm) forms the urethral plate
• At the end of the 3rd month the two urethral folds close over the urethral
plate, forming a penile urethra, does not extend to the tip of the phallus
• Scrotal swellings/genital swellings arise from the inguinal region, move
medially were they are only separated by the scrotal septum

42. External Female Genitalia
• Under the influence of estrogens
• The genital tubercle elongates only slightly (Clitoris), but it is larger
than in males during the early stages of development(3rd and 4th
month)
• Urethral folds do not fuse (Labia minora)
• Genital swellings enlarge (Labia majora)
• The open urogenital groove forms the vestibule

44. Descent of the Testes
• Toward the end of the second month, the urogenital mesentery attaches the
testis and mesonephros to the posterior abdominal wall
• The mesonephros degenerates to form a mesentery for the Gonad
• Caudally it becomes the caudal genital ligament
• Also extending from the caudal pole of the testis is condensed mesenchymal
tissue (Gubernaculum), which terminates in the inguinal region
• As the testis begin to descend the towards the internal ring , an extra-abdominal
portion forms, from the inguinal region to the scrotal swellings
• Factors causing descent are unclear but could be increased intra abdominal
pressure, Hormones (androgens and MIS)
• Testis are at the inguinal region around 12 weeks and reach the scrotum by
33weeks

47. Thank you