The document summarizes the development of the urogenital system from the early embryonic stages through fetal development. It describes how the urinary and genital systems develop from a common ridge in the intermediate mesoderm and initially share a common cavity, the cloaca. It then details the development of the pronephros, mesonephros, and metanephros kidney systems. It discusses the development of the permanent kidney from the metanephric mesoderm and ureteric bud. It also summarizes the development of the bladder, urethra, gonads, genital ducts, and external genitalia in both males and females.

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