Development of the reproductive system (Differentiation of the sexual organs)

Overview of sexual differentiation

Components of sexual differentiation

Timeline of sexual differentiation

  • Weeks 1–6: embryos are sexually indifferent (no phenotypical differences)
    • The mesonephric and paramesonephric ducts develop the same in both genetically male and female embryos.
  • Week 6–7: ductal sex differentiation begins
  • ∼ Week 12: male external genitalia development is complete (somewhat later for females)
  • By week 20: phenotypic differentiation is complete

Mechanisms of sexual differentiation

Gonadal sex differentiation (and disorders)

Ductal sex differentiation (and disorders)

Ductal sex differentiation

Ductal sex differentiation
Embryonic structure General description

Males (starting the 7th week)

Females (starting the 8th week)

Mesonephric ducts (Wolffian ducts)

  • Develops from mesoderm until the 6th week
  • Requires testosterone for continued development
  • Precursors to male internal sex organs (driven by testosterone)
  • Also functions as primitive kidney during the 1st trimester (see development of the kidney)
  • Seminal vesicles
  • Ejaculatory duct
  • Epididymis (also paradidymis)
  • Ductus deferens
  • Degenerates into vestigial remnants (appendix vesiculosa and Gartner's duct)

Paramesonephric ducts (Müllerian ducts)

  • Develops from mesoderm until the end of the 8th week
  • Precursors to female internal sex organs (driven by estrogen)
  • Fallopian tubes
  • Uterus
  • Upper ⅓ of vagina

Dysfunctional ductal sexual differentiation

External genitalia and urogenital differentiation (and disorders)

External genitalia and urogenital differentiation

External genital and urogenital differentiation (starting the 9th week)
Embryonic structure

Males

- Development driven by DHT

Females

- Development driven by estrogen

Genital tubercle

- First forms primordial phallus

  • Glans clitoris
  • Vestibular bulbs

Urogenital sinus

- Develops from cloaca during the 4–6th weeks

- Precursor to bladder and urethra (both sexes)

Urogenital folds

  • Ventral penile shaft
  • Penile urethra
  • Labia minora

Labioscrotal swelling

  • Scrotum
  • Labia majora

Dysfunctional external genital and urogenital differentiation

Descent of gonads

Embryonic structure General description Male remnant Female remnant
Gubernaculum
  • Fibrous tissue that aids in the descent of the gonads
  • Develops around week 7 and exists in males until descent of the testes into the scrotum is complete (typically ∼ 9 months)
Processus vaginalis
  • Obliterated
  • If patent: canal of Nuck
last updated 10/18/2018
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