Normal male and female embryological development of external and internal genitalia

Question 1

Development of primordial germ cells?

Answer 1

Precursors of gametes

• Week 3 - line endoderm of yolk sac
• Week 5-6 - migrate to genital ridge
• Week 6- primitive sex cords develop around the germ cells in the indifferent gonad

Question 2

Development of external genitalia

Answer 2

• Around week 6 cloacal membrane folds and is separated into anterior urogenital parts and posterior anal folds
• Week 7, urogenitals becoming: genital tubercle, urogenital membrane, urogenital folds, labioscrotal swellings

Question 3

Development of sex

Answer 3

• Chromosomal determination - SRY gene on Y chromosome (and also determinants on X)
• Gonadal differentiation
• Development of either mullerian or wolffish ducts in response to gonadal differentiation
• Development of external genitalia dependent on hormones present

Question 4

Normal male development

Answer 4

• SRY gene on Y chromosome activated
• causes indifferent gonad to develop into testis
• Leydig cells secrete testosterone
• Sertoli cells secrete AMH- causes regression of mullerian ducts
• Testosterone- causes development of Wolffian ducts (later become seminal vesicles, vas deferent, epididymis)
• Testosterone- converted systemically to DHT by 5 alpha reductase
• DHT - Development of the external genitalia

Question 5

Normal female development

Answer 5

• Absence of SRY gene and genes on X cause differentiation of undifferentiated gonads to ovaries
• Absence of testosterone causes regression of wolffish ducts
• Absence of AMH results in development of mullein ducts
• Absence of DHT results in female external genitalia
• Mullerian ducts form bilaterally and then at weeks 8-10 fuse to form uterus, cervix and upper vagina
• Defect in this fusion results in mullerian abnormalities