GENITOURINARY EMBRYOLOGY
- Begin to develop during 4th week of gestation.
- Include URINARY SYSTEM+GENITAL SYSTEM.
Develop from:
A) Intermediate mesoderm:
- Nephrostomes: Takes part in the formation of non-functioning kidneys.
Urogenital ridge:
- Caudal to segmental nephrostomes.
- The intermediate mesoderm forms urogenital ridge, divided into :
1. Nephrogenic cord (metanephric blastema or metanephric mesoderm)
- formation of excretory part of kidney.
2. Gonadal ridge:
- development of genital system.
B) Cloaca (endodermal cloaca):
- Dilated post-allantoic part of hindgut.
Divided into two parts by urorectal septum:
- Primitive rectum (dorsal part): Forms rectum (lower part) and upper part of anal canal.
- Primitive urogenital sinus (ventral part) : plays important role in development of urogenital sinus.
C) Coelomic mesothelium:
- Proliferates to form genital ridge from which develops male and female gonads.
O/B of embryonic phases discussed under 5 subdivisions:
- Nephric system
- Vesicourethral unit
- The gonads
- Genital duct system
- External genitalia
1. NEPHRIC SYSTEM
- Developed as:
– Pronephros – Earliest stage: Vestigial structure
– Mesonephros– Principal excretory organ during early embryonic life (4-8 weeks)
– Metanephros – Final phase
- KIDNEY develop from:
METANEPHROS (Metanephric mesoderm or Blastema) derived from intermediate mesoderm.
- Forms :
– GLOMERULUS
– PROXIMAL CONVOLUTED TUBULE
– LOOP OF HENLE
– DISTAL CONVOLUTED TUBULE
URETERIC BUD (MESONEPHROS) arises from lower part of mesonephric duct.
- Forms:
– PELVIS
– MAJOR CALYCES
– COLLECTING TUBULES
– URETER
2. VESICOURETHRAL UNIT
- URINARY BLADDER (except TRIGONE) is derived from:
– Vesicourethral canal
– Cranial part of urogenital sinus (endodermal).
TRIGONE is derived from:
- absorbed part of Wolffian duct/ mesonephric duct (mesodermal).
Development of URETHRA :
A) PROSTATIC URETHRA & MEMBRANOUS URETHRA
- Anterior wall of prostatic urethra above the opening of prostatic utricle→ from vesicouretral canal (endoderm).
- Anterior wall of prostatic urethra below the opening of prostatic utricle & whole of membranous urethra→ from definite urogenital sinus(endoderm).
- Posterior wall of prostatic urethra → from mesonephric/wolffian duct(mesodermal).
B) PENILE URETHRA
- Upto the glans → from phallic part of Urogenital sinus (endoderm).
- In the glans → from ectodermal urethral groove (ectoderm).
3. GONADS (Testis & Ovaries)
- Derived from 3 sources:
– Mesothelium: (mesoderm epithelium)
– Underlying mesenchyme: (embryonic connective tissue)
Primordial germ cells:
- Developed in the 4th week by proliferation of endodermal cells of dorsal wall of hindgut (part of yolk sac).
- Primordium germ cells migrate into genital ridge, where proliferation of both germinal & nongerminal cells leads to formation of gonads.
- At the 6th week, the gonad consists of:
– superficial germinal epithelium and internal blastema.
- Differentiation of genital ridge into female or male gonads (ovary & testes) occurs at 6-10 weeks.
- By 12 weeks this differentiation has occured in all fetuses.
Development of testes
1. Genital ridge
- covered by germinal epithelium (previous coelomic epithelium)
- proliferates and forms sex cords (prirnitive seminiferous cords).
2. Sertoli cells
- derived from sex cords.
3. Lyedig cells
- derived from sex cord, which derived from germinal epithelium.
4. Spermatocytes (sperms)
- Derived from primordial germ cells
- Develop from endodermal cells of hind-gut (part of yolk sac).
Development of ovary
- Coelomic epithelium on medial side of the mesonephros becomes thickened to form genital ridge, the site where ovary develops.
- Genital ridge covered by germinal epithelium.
- From these germinal epithelium, cords of cells (sex cords or medullary cords) proliferate and grow into the underlying mesoderm.
- Primordial germ cells migrate to region of developing ovary (genital ridge area) and give rise to oocytes.
- The sex cords become broken up into small masses.
- The cells of each mass surround one oocyte to form primordial follicles.
- At birth each ovary contains about 2 million primary follicles.
4. GENITAL DUCT SYSTEM
- During 5th & 6th weeks, both male & females have two genital duct systems.
Derived from mesoderm:
- WOLFFIAN DUCT – mesonephric duct ; male genital system
- MULLERIAN DUCT – paramesonephric duct ; female genital system
5. EXTERNAL GENITALIA
| Embryological structure | Fate in FEMALE | Fate in MALE |
| Genital ridge | Ovary | Testis |
| Genital swelling | Labia majora | Scrotum |
| Genital fold | Labia minora | Ventral aspect nof penis, penile urethra |
| Genital tubercle | Clitoris | Glans penis |
Exam Important
- METANEPHROS forms EXCRETORY UNIT of kidney i.e. Glomerulus , proximal convoluted tubule , loop of henle , distal convoluted tubule.
- URETERIC BUD forms COLLECTING PART of kidney i.e. pelvis , Major calyces, Minor calyces, Collecting tubules, Ureter.
- URINARY BLADDER (except TRIGONE) is derived from Vesicourethral canal, cranial part of urogenital sinus(endodermal).
- TRIGONE is derived from absorbed part of Wolffian duct/ mesonephric duct(mesodermal).
- Upper 2/3 or 3/4 of vagina is derived from paramesonephric (mullerian duct)- mesodermal.
- Lower l/3 or l/4 of vagina is derived from urogenital sinus-endodermal.
- Differentiation of genital ridge into female or male gonads (ovary & testes) occurs at 6-10 weeks.
- Germ cells in the OVARY are derived from yolk sac endoderm.
- Leyding cells, develop from mesoderm of the gonadal ridge and are located between sex cords.
- Cowden’s syndrome associated with GENITOURINARY SYSTEM.
- Malformations of genitourinary system of foetus are found to be m/c associated with single umbilical artery.
- In newborns and infants, kidneys have a larger medullary and a smaller cortical volume than in later life.
- Most common site of genitourinary tuberculosis is – KIDNEY.
- In Genitourinary TB, Sterile pyuria is consistent finding.
- Pipe stem ureter appearance is seen in patients with Genitourinary TB.
- Cystic fibrosis affects GENITOURINARY SYSTEM.
- Horse shoe kidney is the most common congenital renal anomaly.
- Congenital anomalies : Renal agenesis (unilateral/ bilateral) failure of the ureteric bud to contact the ipsilateral metanephric blastema.
Associated anomalies –
– in females: Unicornuate uterus, Uterus didelphys, Mayer- Rokitansky-Küster-Hauser(MRKH), Gartner duct cyst
– In males: Seminal vesicle cysts, Absence of the Vas deferen, Cystic dysplasia of the rete testis
- Incomplete closure of urachus results in Urachal cyst, sinus, fistula.
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