Chapter 107. Development of The Female External Genital Organs - Review of Medical Embryology Book

The indifferent stage: see Section 98 for description
Definitive development: in the absence of androgens, feminization of the indifferent external genitalia takes place

THE CLOACAL MEMBRANE, by week 3, is very extensive, and its anterior end is level with the base of the umbilical cord

It is at this stage that the cloacal membrane is bordered laterally by 2 mesenchymal projections, covered by ectoderm, which are the paired primordia of the genital tubercle
At week 4, the anterior end of the cloacal membrane retracts from the base of the umbilical cord, permitting formation of the anterior body wall below (caudal to) the umbilicus, and the paired primordia of the genital tubercle come together in the midline to form the genital tubercle
The cloacal fold, which surrounds the cloacal membrane, further prolongs the genital tubercle, and about this time new swellings appear, the genital swellings, on either side of the cloacal folds and surround the genital tubercle and the cloacal folds
At week 7, the cloacal membrane is divided into the urogenital membrane (anteriorly) and the anal membrane (posteriorly), separated by the perineal body (perineum)

The cloacal fold is divided into the genital fold, which surrounds the urogenital membrane, and the anal fold, which circumscribes the ectodermal depression of the proctodeum covered by the anal membrane

THE UROGENITAL MEMBRANE, in week 9, disappears, thus, the phallic segment of the urogenital sinus becomes open to the exterior
DIFFERENTIATION OF THE EXTERNAL GENITALIA takes place during month 3 and closely follows the pattern of the primitive structures

The genital tubercle elongates only slightly and forms the clitoris, where erectile tissues develop
The urogenital sinus remains open with the urethra opening anteriorly and the vagina posteriorly, within the interior of the vestibule portion of the sinus
The vestibule is bordered laterally by the genital folds, which become the labia minora
The genital (labioscrotal) swellings largely remain unfused and form the labia majora

They do fuse posteriorly to form the posterior labial commissure and anteriorly to form the elevation known as the mons pubis