Introduction

Sexual differentiaton by default follows a pathways for development of female internal and external genitalia, requiring no active intervention. Endocrine activity of the testes, production of Androgens by Leydig cells and Mullerian Inhibiting hormone (MIH) by Sertoli cells, is required for sexual differentiation to be diverted down the male genitalia development pathway.

Internal Genitalia

From the primitive reproductive tracts the Internal genitalia can only develop into one gender and so are termed unipotent.

Male: The primitive male reproductive tract The Wolffian Duct developes from the Mesonephric Duct
Female: The primitive female reproductive tract Mullarian Ducts develop from the Paramesonephric Duct
This is dependent on appropriate hormonal influence.

Male

MIH from Sertoli cells and Androgens from Leydig cells prevent spontaneous development of female internal genitalia.
Androgens actively maintain Wolffian Ducts, which give rise to male internal genitalia:
- Epididymis
- Vas Deferens
- Vesicular Glands/ Seminal Vesicles
MIH causes the primitive female reproductive tract Mullerian Ducts to regress.

Female

Wolffian Ducts spontaneously regress in the absence of Androgens.
Mullarian Ducts persist to give rise to female internal genitalia:
- Oviducts
- Uterus
- Cervix
- Cranial Vagina
Normal Ovarian activity is NOT essential for the development of normal female internal genitalia as this is the default development pathway.

External Genitalia

External genitalia have the potential to be either gender and so are bipotential.

Male

Potential to develop male or female external genitalia. Testosterone is required to develop male external genitalia.
Androgen secretion from the Testes induces:
- Urethral folds to fuse to allow enclosure of the urethral tube. This, together with the cells from the genital swelling, forms the shaft of the Penis.
- Genital swellings fuse in the midline to allow formation of the Scrotum
- Genital Tubercle expands to give rise to the Glans Penis

Female

Development is ensured by the absence of Testosterone, it is independent of Ovarian endocrine activity.
Urethral folds and Genital Swellings remain separate to form the Labia Minora and Majora.
Genital tubercle forms the Clitoris

Exposure of the female tract to Androgens will masculinise external genitalia.
Failure of proper endocrine communication between the gonads, internal and external genitalia can lead to dissociation of gonadal and genital sex.