Gonadal Development:
Genital ridges: pair of longitudinal ridges derived from intermediate mesoderm and overlying
epithelium
Fourth week:
Germ cells begin to migrate from the endoderm lining of the yolk sac to the genital ridges
Epithelium of the genital ridges proliferates and penetrates the intermediate mesoderm to form
the primitive sex cords
If male embryo (XY):
SRY gene, which stimulates the development of the primitive sex cords to form testis
Testis descend from abdomen to scrotum guided by gubernaculum
If female embryo (XX):
No SRY gene, primitive sex cords degenerate and do not form the testis cords
Epithelium of the gonad continues to proliferate, producing cortical cords which form ovarian
tissue
Mullerian Duct Embryology:
Fusion of the caudal part two Mullerian ducts: occurs in the 10th week
Fused Mullerian ducts form the uterine cavity and proximal vagina with CT septum in the midline
Cranial unfused part of the ducts forms the two fallopian tubes
Connective tissue septum undergoes resorption at 20 weeks
Abnormalities:
Absence of Sertoli cells or lack of Mullerian inhibitory factor: leads to the development of both
male and female internal genitalia and male external genitalia (streak gonads)
5α-reductase deficiency: inability to convert testosterone into DHT leads to male internal
genitalia and atypical external genitalia until puberty (when increased testosterone levels cause
masculinization)
In the testes:
Leydig cells lead to male (internal and external) sexual differentiation
Sertoli cells shut down female (internal) sexual differentiation
Genital ridges: pair of longitudinal ridges derived from intermediate mesoderm and overlying
epithelium
Fourth week:
Germ cells begin to migrate from the endoderm lining of the yolk sac to the genital ridges
Epithelium of the genital ridges proliferates and penetrates the intermediate mesoderm to form
the primitive sex cords
If male embryo (XY):
SRY gene, which stimulates the development of the primitive sex cords to form testis
Testis descend from abdomen to scrotum guided by gubernaculum
If female embryo (XX):
No SRY gene, primitive sex cords degenerate and do not form the testis cords
Epithelium of the gonad continues to proliferate, producing cortical cords which form ovarian
tissue
Mullerian Duct Embryology:
Fusion of the caudal part two Mullerian ducts: occurs in the 10th week
Fused Mullerian ducts form the uterine cavity and proximal vagina with CT septum in the midline
Cranial unfused part of the ducts forms the two fallopian tubes
Connective tissue septum undergoes resorption at 20 weeks
Abnormalities:
Absence of Sertoli cells or lack of Mullerian inhibitory factor: leads to the development of both
male and female internal genitalia and male external genitalia (streak gonads)
5α-reductase deficiency: inability to convert testosterone into DHT leads to male internal
genitalia and atypical external genitalia until puberty (when increased testosterone levels cause
masculinization)
In the testes:
Leydig cells lead to male (internal and external) sexual differentiation
Sertoli cells shut down female (internal) sexual differentiation
