HOMOLOGUES OF EXTERNAL GENITALIA
Before 9 weeks of gestation, both sexes have identical external genitalia, characterized by a urogenital sinus. At this undifferentiated stage, the external genitalia consist of a genital tubercle above a urethral groove. Lateral to this are urethral or urogenital folds and even more lateral are the labioscrotal swellings or folds. The male and female derivatives from these structures are shown.
The bladder and genital ducts ﬁnd a common opening in the urogenital sinus. This sinus is formed from the earlier urogenital slit, which is a consequence of the perineal membrane separating the urogenital ducts from the single cloacal opening.
In male development, the genital tubercle elongates, forming a long urethral groove. The distal portion of the groove terminates in a solid epithelial plate (urethral plate) that extends into the glans penis and later canalizes. The midline fusion of the lateral urethral folds is the key step in forming a penile urethra, but this fusion only occurs after the urethral plate canalizes distally. In the female, the primitive structures do not lengthen and the urethral folds do not fuse in the midline. Instead they become the labia majora.
The vagina develops as a diverticulum of the urogenital sinus near the müllerian tubercle. It becomes contiguous with the distal end of the müllerian ducts. Roughly four-ﬁfths of the vagina originates from the urogenital sinus and one-ﬁfth is of müllerian origin. In the male, the vaginal remnant is usually extremely small, as the müllerian structures atrophy before the vaginal diverticulum develops. In intersex disorders (formerly called pseudohermaphroditism and most recently termed disorders of sexual development [DSD]) such as androgen insensitivity syndrome, however, an anatomic remnant of the vaginal diverticulum may persist as a blind vaginal pouch.
In normal female development, the vagina is pushed posteriorly by a down growth of connective tissue. By the 12th week of gestation, it acquires its own, separate opening. In female intersex disorders, the growth of this septum is incomplete, thus leading to persistence of the urogenital sinus.
Male and female external genitalia in the ﬁrst trimester of development appear remarkably similar. The principal distinctions between them are the location and size of the vaginal diverticulum, the size of the phallus, and the degree of fusion of the urethral folds and the labioscrotal swellings.
FACTORS DETERMINING DIFFERENTIATION OF THE EXTERNAL GENITALIA
Similar to the genital ducts, there is a tendency for the external genitalia to develop along female lines. Masculinization of the genital ducts is induced by androgenic hormones, principally testosterone from Leydig cells in the fetal testis during the differentiation process. More important than the source of androgens, however, is the timing and amount of hormone. Examples of this include inappropriate androgen exposure from congenital adrenal hyperplasia or from the maternal circulation, both of which can induce various degrees of masculinization of the female system characteristic of intersex disorders. By the 12th week, androgenic exposure will no longer cause fusion of the urethral and labioscrotal folds in the female, as the vagina has migrated fully posteriorly. Clitoral hypertrophy, however, may still result from such exposures at any time in fetal life or even after birth.