Genital Embryology

• Week 1-6
  • embryo is sexually indifferent
  • genetically male and female embryos are phenotypically the same
  • both male and female have mesonephric (Wolffian) ducts at this stage
• Week 7
  • sexual differentiation occurs
• Week 12
  • sex of embryo can be distinguished based on external genitalia 
• Week 20
• phenotypical differentiation is complete

Male vs. Female Differentiation

• Mesonephric (wolffian) duct
  • needs to be induced to develop
    • by testosterone from the Leydig cells of testes
  • forms male internal sexual organs: Seminal vesicles, Epididymis, Ejaculatory duct, Ductus deferens
• Paramesonephric (müllerian) duct
  • default development but can be suppressed
  • forms female internal sexual organs: fallopian tube, uterus, upper 1/3 of vagina (lower 2/3 from urogenital sinus)  
• Female 
  • default phenotypic differentiation
  • mesonephric duct degenerates and paramesonephric duct develops
• Male 
  • SRY gene (Y chromosome) produces testis-determining factor which is necessary for testes development
  • testes gives rise to
    • sertoli cells: produces Müllerian inhibitory factor suppressing the development of paramesonephric ducts 
    • leydig cells: produces androgens stimulating development of mesonephric ducts 

Female Genital Abnormalities

• Unicornate Uterus
  • one paramesonephric fails to develop
• Bicornuate Uterus
  • due to incomplete/partial fusion of the paramesonephric ducts
  • associated with urinary tract abnormalities and infertility
• Uterus didelphys
  • no fusion between the two paramesonephric ducts resulting in a “double uterus”Müllerian agenesis  
  • failure of paramesonephric ducts to develop resulting in abnormalities of the upper portion of the vagina, cervix, uterus, and fallopian tubes

Male Genital Abnormalities

• Hypospadias
  • failure of urethral folds to fuse
  • resulting in opening of penile urethra on inferior side of penis
  • susceptible to UTI’s
  • surgery is recommended for treatment
• Epispadias
  • faulty positioning of genital tubercle
  • resulting in penile urethra opening on superior side of penis
  • associated with exstrophy of the bladder
• Cryptochidism
  • failure of testes to descend into scrotum (undescended testes)
  • testes can be found in inguinal canal or abdominal canal
  • can lead to infertility if both are undescended
  • observable within 3 months of birth
• Hydrocele
  • processus vaginalis remains patent allowing fluid to flow in
  • resulting in a fluid filled sac in the scrotum
  • can differentiate from tumor by shining light through testes
  • hydrocele allows light to flow through whereas a tumor does not