Development of Female Genital System

Overview of Female Genital System Development

• Two sequential phases

  • Indifferent stage – early embryo, male & female morphologically identical

  • Differentiated stage – starts ≈ $7^{th}$ week, XX embryos acquire female characteristics

• Determinants of pathway

  1. Chromosomal sex established at fertilisation (type of sperm: X or Y)

  2. Sex chromosomes dictate gonadal differentiation

  3. Gonad-derived hormones (or their absence) steer ducts & external genitalia

• Female pathway (XX)

  • Absence of Y → no SRY gene → no testes

  • Consequences of lacking testicular products

  • No Müllerian-inhibiting substance (MIS) → paramesonephric ducts persist

  • No testosterone → mesonephric ducts regress

  • X-linked genes (e.g., WNT4) promote ovarian differentiation

  • Controversy over hormonal need: Moore (2008) says female ductal differentiation is hormone-independent; Langman (2010) attributes a role to estrogens

Development of Gonads (Ovary)

Indifferent Stage (≈ $5^{th}$ week)

• Three embryologic sources

  • Mesothelium (coelomic epithelium)

  • Underlying mesenchyme (embryonic connective tissue)

  • Migrating primordial germ cells (PGCs)

• Steps

  1. Mesothelium & mesenchyme proliferate medially to mesonephros → gonadal (genital) ridge

  2. Mesothelium forms external cortex; mesenchyme forms internal medulla

  3. Mesothelial cells grow inward as gonadal (primitive sex) cords that trap incoming PGCs

Primordial Germ Cells (PGCs)

• Originate in epiblast, relocate to wall of yolk sac near allantois

• With embryo folding, dorsal yolk sac becomes hindgut; PGCs migrate along dorsal mesentery to gonadal ridges (arrive $6^{th}$ week)

• Become incorporated into gonadal cords

Differentiated Stage – Formation of Ovary

1. Genetic signals: presence of $XX$, absence of Y → cords & rete ovarii degenerate

2. Surface mesothelium proliferates again → cortical cords

3. Cortical cords penetrate cortex, include PGCs that mitose → oogonia

4. Cortical cords fragment → primordial follicles = 1 oogonium + flattened follicular cells (from cortical cord epithelium)

5. By $8$ weeks: cortex–medulla organisation; by $20$ weeks numerous follicles present

6. Post-natal changes

   • Surface epithelium flattens to single layer; becomes continuous with peritoneal mesothelium at hilum

   • Tunica albuginea (fibrous capsule) forms between epithelium & follicular cortex

   • Ovary detaches from regressing mesonephros and is suspended by mesovarium

Development of Genital Ducts

Indifferent Stage (both sexes, $5^{th}$ week)

• Two duct systems run in parallel

  1. Mesonephric (Wolffian) duct – drains mesonephric kidney; opens into urogenital sinus lateral to sinus tubercle

  2. Paramesonephric (Müllerian) duct – forms by longitudinal invagination of mesothelium lateral to mesonephros; cranial end funnel-shaped opening into peritoneal cavity; caudal ends grow inferomedially, cross anterior to mesonephric ducts and fuse → Y-shaped uterovaginal primordium with median septum that contacts urogenital sinus forming sinus (paramesonephric) tubercle

Mesonephric Duct in Females (Differentiated Stage)

• Regresses owing to absence of testosterone

• Vestigial remnants

  • Appendix vesiculosa (cranial tail)

  • Gartner’s duct / Gartner’s cyst (within broad ligament lateral to uterus)

• Mesonephric tubule remnants

  • Epoophoron (in mesovarium between ovary & uterine tube)

  • Paroophoron (mesosalpinx)

Paramesonephric Duct in Females (Differentiated Stage)

• Persists (no MIS)

• Outcomes

  1. Unfused cranial parts → uterine (Fallopian) tubes

  2. Fused caudal segment (uterovaginal primordium) → uterus + upper $2/3$ of vagina

• Fusion draws urogenital ridges together forming transverse peritoneal fold = broad ligament

• Broad ligament + primordium partition pelvic cavity into rectouterine & vesicouterine pouches

• Initially a septum exists between fused ducts; later resorbed to create single uterine canal

• Surrounding mesenchyme → myometrium & perimetrium

Development of Vagina

1. Dual origin

   • Upper portion – uterovaginal primordium (mesoderm-derived epithelium)

   • Lower portion – urogenital sinus (endoderm)

2. Sequence for lower part

   • Sinus tubercle thickens → paired sinovaginal bulbs (endoderm)

   • Bulbs project toward and fuse with uterovaginal primordium → solid vaginal plate

   • Canalisation of the plate creates vaginal lumen (inferior $1/3$)

3. Hymen (endodermal membrane) separates vaginal lumen from urogenital sinus; generally perforates perinatally, remnants prevail around orifice

4. Urogenital sinus lumen evolves into vestibule of vagina

Development of External Genitalia

Indifferent Stage (early $4^{th}$ week–$7^{th}$ week)

• Mesenchyme surrounding cloacal membrane proliferates forming cloacal folds; folds unite cranially → genital tubercle which elongates forming phallus

• Urorectal septum partitions cloacal membrane into:

  • Anterior urethral (urogenital) membrane & folds

  • Posterior anal membrane & folds

• Rupture of urethral membrane → urethral orifice opens to amniotic cavity

• Genital (labioscrotal) swellings appear lateral to urethral folds

Female Differentiation (by $12$ weeks)

• Phallus → clitoris (relative growth limited vs. penis)

• Urethral folds → labia minora; posterior fusion forms frenulum of labia minora

• Labioscrotal swellings → labia majora; fuse posteriorly (posterior labial commissure) and anteriorly (anterior commissure + mons pubis)

• Perineal raphe absent in females; external genitalia complete by $12^{th}$ week

Development of Inguinal Canal & Descent of Ovary

Formation of Inguinal Canal (both sexes)

• Mesenchyme at caudal pole of gonad condenses as gubernaculum, extending obliquely through anterior abdominal wall to genital swelling

• Peritoneum evaginates ventral to gubernaculum → processus vaginalis that drags layers of abdominal wall ahead, creating inguinal canal

Descent of Ovary

• Ovaries descend from posterior abdominal wall to pelvis; do NOT enter canal

• Gubernaculum crosses paramesonephric duct, attaching to uterus near uterine tube → divided into

  1. Cranial segment → ovarian ligament (ovary to uterus)

  2. Caudal segment → round ligament of uterus (uterus through inguinal canal → labia majora)

• Processus vaginalis in female usually obliterates; persistence = canal of Nuck (potential for indirect inguinal hernia or hydrocele)

Development of Auxiliary Genital Glands (Female)

• Urethral & para-urethral (Skene) glands – epithelial outgrowths from urethra into surrounding mesenchyme

• Greater vestibular (Bartholin) glands – epithelial buds from urogenital sinus into vestibular mesenchyme

Clinical Correlations

• Disorders of Sex Development (DSD)

  • Gonadal dysgenesis (e.g., Turner syndrome, 45,X): streak ovaries, primary amenorrhoea, short stature, cardiac anomalies, webbed neck

  • Female pseudohermaphroditism (46,XX with virilisation) – usually congenital adrenal hyperplasia; excess androgens → clitoromegaly, labial fusion

  • True hermaphrodites (ovotesticular DSD) rare, most karyotype $46,XX$

• Müllerian duct anomalies (uterus & vagina)

  • Causes: incomplete fusion, unilateral development failure, septum persistence, partial agenesis

  • Types

  • Double uterus with double vagina (uterus didelphys)

  • Bicornuate uterus (± rudimentary horn)

  • Septate uterus

  • Unicornuate uterus

  • Single vagina + double uterus

  • Hymenal anomalies: microperforate, cribriform, septate, imperforate hymen

• Vaginal Atresia

  • Failure of canalisation of vaginal plate → blockage; imperforate hymen = failure of inferior end to perforate

• Vaginal Agenesis (Müllerian agenesis; Mayer–Rokitansky–Küster–Hauser)

  • Failure of sinovaginal bulbs to form vaginal plate

  • Usually accompanies uterine agenesis because uterovaginal primordium induces bulb formation

Genital & Urogenital Primordia – Female vs. Male Summary

• Genital tubercle → clitoris (female) / penis (male)

• Urogenital folds → labia minora / ventral penis & penile urethra

• Labioscrotal folds → labia majora / scrotum

• Gubernaculum → ovarian ligament + round ligament / gubernaculum testis

• Mesonephric duct derivatives → vestigial (epoophoron, paroophoron, Gartner structures) / epididymis, vas deferens, seminal vesicle etc.

• Paramesonephric duct derivatives → uterine tube, uterus, upper vagina / vestigial appendix of testis, prostatic utricle

• Urogenital sinus derivatives → bladder, urethra, lower vagina, vestibular & para-urethral glands / bladder, most urethra, prostate, bulbourethral glands

Ethical & Practical Implications

• Early identification of DSD and Müllerian anomalies essential for psychosocial well-being, fertility counselling and surgical planning

• Awareness of canal of Nuck in female inguinal swellings prevents misdiagnosis

Key Timelines & Numerical References

• $4^{th}$ week: formation of cloacal folds & genital tubercle

• $5^{th}$ week: appearance of gonadal ridges & paired genital ducts

• $6^{th}$ week: PGCs enter gonad; sinovaginal bulbs not yet present

• $7^{th}$ week: onset of sex-specific differentiation

• $8^{th}$ week: distinct ovarian cortex & medulla

• $9−12$ weeks: canalisation of vagina; external genitalia differentiation complete by $12^{th}$ week

• $20$ weeks: abundant ovarian follicles

Remember

• Female differentiation is the "default" pathway – active factors (SRY, MIS, testosterone) are required to divert embryo to male fate; absence of these factors = female phenotype

• Multiple structures share common embryologic origins; maldevelopment in one often predicts anomalies in another (e.g., vaginal & uterine agenesis)